Managing Pests

In Texas, the greatest challenge to vegetable producers is in the area of pest control. The mild climate prevalent in most of the state is extremely favorable for all forms of crop pest to flourish (weeds, insects and diseases). Limited tools to combat these problems is one of the major contributing factors to the high risk associated with vegetable production in general. Consequently, high level of management is required to successfully control pests’ problems. Successful production of organically grown vegetables requires an even higher level of management for profitability to occur because even fewer control tools are available to producers using this production system. Organic producers have to rely heavily upon them minor details of vegetable culture in order to avoid problems. Pest control is often a result of the cumulative effects of many production practices. Emphasis is placed on prevention rather than control. This often is difficult to achieve with current technology. Therefore, a grower must develop pest control strategies. To do so a grower must have an understanding of the pest and the beneficials that usually occur in a given crop, and, he should also have the ability to anticipate pest problems and manipulate practices that minimize pests and maximize beneficials (10).

Weed Control

In a recent cooperative survey of cucurbit growers from Texas and Oklahoma, Scientists from Texas A&M University and Oklahoma State University found that weeds and the lack of effective control measures were their most serious pest problems. This may or may not be the case for all vegetable growers but it certainly does illustrate the difficulty in controlling weeds even with the use of a limited number of herbicides. It also points out one of the major challenges to be faced by organic growers who do not have the capability of using herbicides.

Controlling weeds is essential in order to obtain optimum yield and quality of vegetables. These unwanted plants complete with the cash crop for water, nutrients and sunlight. The first and largest plant to occupy a given location usually has the competitive advantage over others. Therefore, any practice that allows a copy to become established, prior to the establishment of weeds, give the advantage to the crop. As such one weed control goal in organic production should be to establish the crop in manner as to again the competitive advantage over any potential weeds population. A well-established crop will be able to out compete emerging weed seedlings. To be able to accomplish this in the absence of herbicides a system utilizing a combination of techniques or strategies will be required. Ideally, a zero weed population would be the most advantageous to a crop. Realistically, this is not often possible. Instead, another goal of a weed control systems should reducing weed populations directly by removal or death, and, indirectly by reducing or preventing future populations by preventing weed seed and perennial propagule development in existing weed populations (26). Research has shown that the most critical period during the cropping cycle for weed control is the first 40 days most crops are in the field (22). Consequently, development of effective weed control strategies should target this period.

There are a number of options available to organic producers to assist them in achieving the above weed control goals. The use of cover crops and crop rotation, timely cultivation, mulching, solarization, proper water management, flaming, crop competition, immediate crop destruction, and biological control to name a few. Although none of these options are totally effective alone, when properly used in combinations they can be highly effective.

Competition/crop rotation

Densely planted cover or rotation crops such as small grains can be highly effective in reducing future populations of certain weed species. These crops are very vigorous and out compete most weed species for nutrients etc. They also shade out many problem weeds, and, can have an allopathic effect on certain species. Alleopathy is a term given to the phenomenon whereby one plant species can kill or inhibit growth of another through toxic exudates emitted from their roots or by toxic chemical produced during the decomposition process occurring after their death. In addition, when small grain cover or rotation crops are used in combination with no or low till highly effective weed control can be achieved because the cover out competes the weeds and the no till system prevents or reduces new weed seeds from being brought to the surface for germination (22).

Flaming

This technique utilizes high temperature (1300 + F) disruption of weed plant cells. With flaming, propane fueled flamers are tractor mounted and carried through a field. Flames are directed toward the emerging weed seedlings of a fallow field or in fields with slow germinating crops such as onion and pepper. Flaming can also be used on tougher crops such as corn by directing the flame toward the base of the corn plant. Avoid direct flame contact to the stalk because plant injury and yield reductions can result. The efficiency of this technique is increases when employed in the absence of wind, when no moisture is present on the surface of the weeds, and, with tractor speeds of 3 – 5 mph. The economy of this method is directly related to propane fuel costs (26).

Cultivation

Cultivation is perhaps the oldest method of weed control practiced by man. It still is one of the most effective and economical means of weed control. Although the concept of cultivation has not changed over time, implements used to cultivate have undergone considerable improvement. There are many excellent cultivating implements available today. The choice of implement is largely based on soil type, crop grown and grower preference. As stated earlier, weeds do the most damage during the first 40 days after crop seed germination. Consequently, one, timely cultivation often is sufficient to achieve adequate weed control. Depending upon the crop being grown, up to 80 % of a plant bed can be easily cultivated. If improperly preformed, cultivation can cause more crop injury than the presence of weeds. Most vegetable crops are shallow rooted having their roots concentrated in the top 12” of the soil. As a result, maximum cultivation depth should be no greater than two inches. Deeper cultivation will cause roots system disruption, plant injury and even death. It is a good practice to adjust cultivating equipment prior to each use or with each change of field or crop to be cultivated. Periodic checking of the cultivation depth is also advisable to insure root pruning is kept to a minimum.

Irrigation

In addition to its benefit to germination of seed and subsequent growth of a cash crop, irrigation water also provides similar benefit to weeds. Therefore, anything that can be done to restrict water to the area of the crop root zone should be practiced. Drip irrigation can serve as one of the tools to manage weeds in an organic farming enterprise as well as an efficient means of supplying supplemental water to crops. When properly operated and maintained, drip irrigation systems will supply water only to a precise location around the crop rooting zone. No water is applied to the area in the furrows or between the plant beds. As a result, this helps to reduce the potential for weed seed germination in these areas.

Pre-plant irrigation/cultivation

Combining pre-plant irrigation with cultivation can be an effective means of reducing weed and weed seed buildup. After seed beds are established, a pre-plant irrigation is applied to encourage weed seed germination is applied well in advance of the anticipated crop planting date. Soon after weed seedlings emerged, the field is cultivated, killing emerged weeds. When a large seeded crops such as sweet corn or green beans are to be planted, the cultivated beds can be allowed to dry down to 2 – 3 inches to form a dust mulch. The seeds are then sown into the moisture below the dust mulch.

Crop Competition

With some species, the cash itself can become an effective means of reducing weed populations in a field. Cucurbit species such as cantaloupe, watermelon and pickling cucumbers are low growing vining plants. Under good management practices and high-density plantings their vines form a very dense ground cover which shades out weeds and out competes them for water, nutrients and sunlight. Similar, rotation crops such as small grains can serve a similar purpose when they are rotated with vegetable species. To maximize the benefits from dense planting and/or rotation cover crops, immediate crop destruction is advisable. This will help to prevent late emerging weed species from emerging through the diminishing cover resulting from crop maturity and harvest.

Mulching

Perhaps the most effective and widely used means of weed control in organic plantings is mulching. Mulches can control weeds by either a shading effect as previously described or by forming a barrier to the emergence of the weed seedling. Basically there are two category of mulches, organic or inorganic. Organic mulches can be any organic material. Examples of organic mulches are hay or straw, pine needles, shredded bark and sawdust (2). Fresh material such as saw dust can rob N from the crop being mulched. To offset this, supplemental N applications may be required. Nitrogen is required by organisms which breakdown the organic materials. The addition will assist in the deterioration process and aid the return of N to the soil. When using organic, avoid applying the material too early because these materials can delay soil warming and reduce seed germination, seedling emergence and crop development. It is best to apply organic mulches in the Spring when soil temperature has reached 650 F at a 4 “ depth. Organic mulches such as hay and grass clippings can also serve to introduce weed seed into the mulched field. If the hay crop contains weed species, or, if it has gone to seed prior to harvesting, it can serve as a carrier for unwanted seeds. Commercially produced hay can also pose a danger of transporting unwanted herbicides contained in the hay to the field being mulched. When organic mulches are used they should be placed 2 – 3 “ away from the seedlings or plant stems (2). Occasionally additional mulch materials will be needed if weed seedlings begin to emerge through the mulch layer. Products such as these are better suited for small acreage situations.

Certain organic mulches can be grown in place or in the desired area for mulching. These mulches, termed living mulches, are usually crops such as small grains, clover, vetch, etc., grown specifically as a mulch material. The cash crop is planted into the living mulch. The living mulch must be killed once it has produced sufficient biomes to be an effective mulch or it will become a serious competitor to the cash crop (26).

The most commonly used inorganic mulches are plastic films. Most of these films are made from petroleum based products. Plastic mulches are very effective in preventing weed seedlings from emerging.

These films are widely used in large acreage planting because they are relatively inexpensive and their application is totally mechanized. The successful use of plastic film mulches is depended upon proper application. To properly apply films, a well pulverized and firm seed bed, free of clods, debris and plant residue is required. Such beds will enable film to be tightly stretched across the seed bed surface with good soil to film contact, and, enhance good mulch anchorage. In areas prone to windy conditions, it is advisable to use windbreaks in combination with plastic mulch to reduce the incidence of the film being blown from the bed surface. The ability of the mulch to act as a barrier to emerging weeds can also pose a problem with moisture from rainfall or sprinkler irrigation penetrating into the mulch beds. Therefore, beds to be mulched should have a full profile of moisture prior to mulching.

Although plastic mulches can provide good weed control, certain weed species such as nutsedge have the ability to penetrate the plastic films and cause real problems to a producer.

Solarization

Solarization is a process, which takes advantage of the soil heating ability of clear plastic mulch films. Clear plastic films can cause excessive heat buildup in the top 3 – 4 inches of a seed bed below the mulched surface. As a result of the resulting high temperatures, germinated weed seeds are killed. For solarization to be effective, seedbeds must contain sufficient moisture in order to germinate weed seeds, and, should be accomplished during the hottest part of the summer. Therefore, in the absence of good soil moisture, beds to be solarized should be irrigated prior to applying the clear film. Normally, 4 – 6 weeks are required to properly solarize soil. Once the clear plastic mulched is removed, minimal soil disturbance should be practiced. This process is better suited to fall planting because less time is available for recontamination of the beds by weeds than available between solarization and spring planting. A list of weeds and their susceptibility to solarization is presented in the following table.

Table 14. Relative Susceptibility of Selected Weed Species to Soil Solarization.

Weed Species Relative susceptibility
Common name Scientific name
Annual bluegrass
Barnyardgrass
Bermudagrass
Bermuda buttercup
Black nightshade
Cheeseweed
Coclebur
Common chickweed
Common groundsel
Common sowthistle
Field bindweed

Goosegrass
Hairy nightshade
Henbit
Jimsonweed
Johnsongrass
Lambsquarters
Large crabgrass
Nettleleaf goosefoot
Purslane
Redroot pigweed
Shepherdspurse
Wild oat
Yellow nutsedge

Poa annua
Echinochloa crus-galli
Cynodon dactylon
Oxalis pes-caprae
Solanum nigrum
Malva parviflora
Xanthium spinosum
Stellaria media
Senecio vulgaris
Sonchus oleracea
Convolvuls arvensis
Eleusine indica
Solanum sarachoides
Lamium amplexicaule
Datura stramonium
Sorgham halepense
Chenopodium album
Digitaria sangauinalis
Chenopodium murale
Portulaca oleracea
Anagallis retroflexus
Capsella bursa-pastoris
Avena alba
Cyperus esculentus
S
S
MS
S
S
S
S
S
S
S
S(from seed)
MR(established)
MS
S
S
S
S(from seed)
S
MS
S
MS
S
S
MS
MR

Source: Pullman DeVay, Elmore and Hart. 1984. Soil Solarization. UC ANR Leaflet 21377
S = Susceptible – may control all of this weed during the current cropping season if soil is undisturbed
MS = Reduction, but may not achieve total control
MR = Reduction, but regrowth occurs or reduction is limited to the surface 2 ”

Beneficial organisms (Geese)

Geese(Chinese weeders) have been successfully used to control weeds in some crop production situations. Their main effect is through their feeding on weed seeds. However, they seem to do a better job at eating grass seeds than broadleaf seeds. Geese should not be used in corn crops or when tomatoes are beginning to turn color. It should be remembered that geese are living organism and require water, shade and protection from predators (26).

Chemical control

Although no conventional herbicide materials are allowed in organic production, there are a limited number of chemical compounds approved for use. Examples of these materials are; acetic acid, citric acid, solutions of sodium nitrate, and, a pre-emergence material, corn gluten (suppresses weeds as they germinate). There are also soaps touted to kill weeds. Currently, the efficacy of these products is marginal at best (26).

 

Insect Control

Insect control can be very troublesome for organic producers in Texas. Unlike disease pest, very few vegetable varieties contain resistance to very many insects. Consequently, a producer must rely on his knowledge of insect problems affecting his crops, insect life cycles and environmental conditions favoring insect emergence and buildup, insect feeding habits, the relationship between harmful and beneficial insect, and, have the ability to accurately identify pest in a given crop in order to devise and effective control strategy. Therefore, the primary insect control goal in organic production is to develop a strategy to prevent or reduce insect population buildup. A good control strategy will include a number of cultural practices or tactics in combination with the use of a limited number of organically derived pesticides. A few of the more effective control tactics available to organic producers are: field scouting and insect trapping; avoidance; trap cropping; biologicals, soaps, oils, diatomaceous earth and precise record keeping (22).

Field scouting-

In order to develop and effective insect control strategy, a producer must first know what pests are posing a problem and to what extent is the damage that they are causing. Therefore, the key to accurately assessing potential pest problems is field scouting. For scouting to be of value it must be done routinely and in a systematic manner. If scouting is done properly, the initial emergence of insects in a field and the peak population occurrence will enable the development of threshold levels for significant damage by a given insect. Insect population thresholds are the populations above which significant crop injury will occur (22). Threshold levels can also be useful in determining when to implement a control program if available.

 

Record keeping-

Record keeping goes hand in hand with field scouting. Record should include the date of first observation of an insect or the emergence date; the location in the field first observed; the presence of hot spots or the uniformity of population distribution within a field; and, the type and severity of damage caused by insects. Such information is necessary in devising a control strategy or which tactics to be included within a control strategy and when it should be implemented. Accurate weather data will also be helpful in devising and implementing an effective control strategy.

Insect trapping-

Insect trapping is another good tools to assist a grower in monitoring the insect pressure within a given field or crop. Devices such as black lights and pheromone traps are very effective means of monitoring insect emergence and threshold levels. These devices lure insects into traps. By checking the traps on a regular basis (preferably daily) valuable information on the potential damage to a crop can be obtained. Black light traps have an advantage of not being specie specific. This can also become a disadvantage in that the vast majority of insects trapped by these devices are neither harmful or beneficial to a crop yet considerable time must be expended sorting and identifying trapped insects.

Pheromone traps offer the advantage of being species specific. Pheromones are sex attractants normally secreted by insects to enhance their opportunity to mate. Therefore, only the insect attracted to a given pheromone will be present in these traps (30). Unfortunately, they’re not many pheromone specific for very many insect pest of vegetables. Table 15 list common vegetables for which pheromones are commercially available, Pheromones can also be used to confuse insects by making them mates are in the area. This can help to reduce populations of the next generation (9, 10).

Avoidance-

Simply stated, avoidance is the use of production practices that enables a crop to miss a potential insect infestation or lessen the damage caused by a peak population buildup. An example of an avoidance tactic is adjusting planting dates to allow a crop to mature before or after an anticipated insect outbreak. For this tactic to be effective a good understanding of what pest infests the crop to be grown, the life cycles and the influence of environment on their emergence and development, and, the insect feeding habits or damage caused. Information of this type in combination with knowledge of growth development, planting dates can be selected which enable the crop to avoid the potential insect problem. Unfortunately, This practice can have a negative effect if environmental conditions occur which slow up or hasten the emergence and development to coincide with the maturity of the crop. Also, adjusting the planting date to miss an anticipated insect infestation can potentially cause a grower to miss a profitable market window if the adjusted date causes the crop to come off too early or too late.

 

Table 15. Commercially available pheromones for specific insects on selected vegetable crops.

Caterpillar Crop
Black cutworm, Agrotis ipsiolon General vegetables
Tomato pin worm, Keiferia Llycopersicella Tomato
Beet armyworm, Spodoptera exigaua General vegetables
Cabbage looper, Trichoplusia ni General vegetables
Diamondback moth, Plutella xylostella Cole crops
Variegated cutworms, Peridroma saucia General vegetables
Artichoke plume moth, Platyptilia carduidactyla Artichokes
Potato tuberworm, Phtorimaea operculella Potato, tomato

Source: Pest of garden and small farm: A grower’s guide to using less pesticides. UC Davis Small Farms program

Properly located a crop in reference to other crops can also serve as an avoidance tactic (22). For instance, a cabbage crop grown next to a small grain crop can experience an increased incidence of onion thrip. Fall planted cucurbit crops planted near a cotton crop can have a increased incidence of white fly once cotton defoliation is begun. Also, vegetable planted next to hay crops or into a site previously in pasture for a relatively long often experience high worm populations and be plagued by soil insects. Therefore, the damage caused by the pest in the above examples can be prevented or reduced by avoiding planting crops in close proximity to these crop.

Physical barriers-

Physical barriers can also serve as an avoidance tactic by preventing insects from coming into contact with the crop (22), thus avoiding potential crop damage caused by insects. Row covers are a good example of physical barriers that prevent insect damage. Row covers can also serve to enhance growth and early maturity, and can serve to prevent frost injury to a crop in certain situations. In some instances, plastic mulch can serves to prevent soil insects, such as wire worms, from boring in to fruit, such as cantaloupe, which come directly into contact with the soil surface. Above ground insects such as aphids can become confused by reflective mulch surfaces and not land on the mulched plants.

Trap cropping and crop rotation-

Trap cropping is a practice in which a crop or species is specifically planted next to another crop or species for the sole purpose of luring insects from the desired crop to the other and thereby reduce feeding damage within the desired crop (22). This is an effective method of minimizing pest problems when used in combination with proper timing of crop planting (5).

A similar practice, inter-copping or companion planting, utilizes the same concept with the exception that two or more crops are planted in alternating portions of a field. The logic being that insect preference of one crop over the other will come into play. That is, a given pest may prefer to feed on one species to the other. As long as that species is in abundant supply the pest will tend to continue to feed on it an give an opportunity for the other crop to mature and be harvested with minimal injury (9). Some research has shown that preference may not hold true in all situations and with all insect species and crops. They have found the companion planting technique offers better control in situations were a given insect attacks only one species but does not work well where general insect feeders are involved. In these cases, the general feeding insect population may be increased. Others have claimed that some plant species can be inter-copped and serve to repel certain insects within a field (9). Marigolds inter-planted with beans to repel Mexican bean beetle and Basil inter-planted with cabbage to repel worms are good examples of these practices. Unfortunately little data is available to confirm these claims.

Biologicals-

Biologicals are controls such as non-genetically engineered microbes and insects. Biological controls include but are not limited to resistant varieties, viruses, bacteria, protozoa, fungi, insects, nematodes, plant and animals (5). The concept of biological control entails the direct or indirect use of the above to hold pest populations at a low level in order to avoid economic loss (15). The effectiveness of a biological is related to its ability to find a host when pest numbers are small and its ability to survive under all kinds of conditions which may occur in a cropping season. When a biological is introduced into a cropping system, it must be done in a timely manner similar to conventional control in a conventional cropping systems. The better the timing of their introduction the more effective will be the pest control achieved. If they are released prior to the buildup of an adequate pest population to support their feeding, they will leave the field in search for an adequate food supply.

There are three commonly used methods to achieve insect control with biologicals: introduction of natural enemies of the harmful insect; enlarge the existing population of the pest’s natural enemies; and, conservation of beneficial insects. Under certain conditions, multiple release of biologicals may be required (10). The use of these types of insect control agents may be the most cost effective means of control during the transition from conventional to organic production.

Another important consideration to the successful use of biological controls is the continuous monitoring of their populations within a given field. Consequently, correct identification of the pest and beneficials is required as is the ability to recognize the various stages in their life cycles (10). For instance, there are many predator insects in nature that can control harmful caterpillars such as spiders, bigeyed bugs, pirate bugs, lacewing larvae, ground beetles, damsel bugs and assassin bugs. These predators can control caterpillar populations by consuming their eggs and eating small caterpillars. Certain species can even consume large caterpillars. Therefore, proper identification of the beneficial insects and their life cycles is essential to organic farming operations.

Mites are a common pest of many vegetable crops (9). Although related to insects they are actually members of the spider family. Mites are tiny organisms and are often difficult to see. Usually plant damage occurs before their populations become readily observable. Sometime these pest are long gone before their damage is apparent. Hand lens are useful tools in finding mites on plant foliage. Fortunately, there are many natural enemies of mites, several of which are actually predatory mites. The most commonly released predatory mite is Metaseialus occidentalis. This mite is most effective at high temperatures (900 + F). Phytoseiulus persimilis is another effective predator and performs best under mild humid conditions up to 80 0 F (a few high temperature strains are available). Amblyseius (Euseius) species are another mite predators but are not as aggressive as the above species and will not normally enter areas heavily webbed by spider mites. Greenhouse growers have found the species Amblyseius californicus to be especially useful if temperatures in the houses are maintained under 85 0 F (9).

Mite populations can be increased with the use of pesticides and hot dry dusty conditions. Therefore, restrict the use of approved pesticides as much as possible when mites become evident in a crop. Some benefits have been claimed by the frequent wetting or sodding of dirt road around field to reduce dust build up on the plants (9).

Aphid, another severe insect pest of vegetable crops, also have several naturally occurring enemies. The most popular of which are ladybug beetle adults. Others are the pink potato aphid, parasitic wasp such as Diaeretiella rapae and Trichograma sp., and certain species of ants.

Chinese Praying Mantid is another insect predator claimed to be useful in insect control. However, these insects are unreliable and have actually been know to eat each other as well as other beneficials.

Worm pest have been successfully controlled with the use of microbial insecticides such as BT (Bacillus thuringiensis). Several formulation of BT are available, some are more effective than others on differing worm species.

Pesticides, Soaps and Oils-

There are many readily available insecticidal soaps on the market today. These tend to be more against mites and whiteflies but also have been found to reduce aphid and thrip populations as well. Although these soap products have little negative effect on the environment, they can be toxic to some plants, especially those with dull surfaces and those having hairy leaf surfaces. Seedlings and small immature plants can also can be effected adversely by these products under certain conditions (9).

Vegetable oils (mineral oils) are plant oil derivatives used to control aphids, scales and mites. These product must coat the entire leaf surface to be completely effective. Therefore, their use should be limited to slow growing vegetables.

Other plant derivatives, botanical pesticides, such as neem, pyrethrum, quassia, rotenone, ryania and sabadilla have also been successful used in organic production to control insects (20). However, these are restricted use pesticides must be used in conjunction with a biorational pest management program, and cannot be the primary method of control. They must be used in the least ecologically disruptive manner as possible. When using botanical pesticide products such as pyrethrum and neem, always read and follow labeled use instructions and only on crops listed on the label (16).

Diatomaceous earth-

Diatomaceous earth is a sorpitive dust derived from skeletons of microscopic marine organisms. As a desiccant, they work better as a household insecticide because they tend to loose their effectiveness when they become wet.

Herbal sprays-

Herbal sprays are made from purees, teas and / or oils of plants such as garlic or hot peppers. These products have been claimed to be effective in controlling worms and other soft-bodied pest.

The following table lists a few of the more commonly used pest management products used to control insects.

Table 16. Allowed insecticidal products.

Product Insect Controlled Active ingredient Supplier
Bacillus thuringiensis(BT’s)
Lepidopterous larvae (worms)
BT aizawi,
kurstaki,
hurstaki,
Thermo Trilogy
Valent
Bonide Products
Dragon Products
Dow AgroSciences
Cillus Tech. Inc.
Agroneem
Ecozin
Neemix 4.5
Numerous insects
Azadirachtin Agro Logistic Sys.
AMVAC Chem
Thermo Trilogy
Bonide Colorado Potato
Beetle Beater
Colorado Potato Beetle
BT san diego Bonide Products, Inc.
Envirepel
Garlic Barrier AG
Insect repellent
Garlic oil Cal Crop USA
Garlic Res. Lab.
Hot pepper wax
Insect repellent
Capsaicin Hot Pepper Wax, Inc.
Golden Natur’l Spray Oil
Numerous insects
Soybean oil Stoller Enterprises
M-Pede
Safer Soap
Soft-bodied insects
Insecticidal soaps
(Potassium salts of fatty acid)
Dow AgroSciences
Safer, Inc.
Novodor
Beetle larvae
BT tenebrionis Valent
Naturalis-L
Numerous insects
Beauveria bassiana Troy Biosciences
Surround WP Crop Protectant
Flea beetles, leafhoppers, others
Kaolin Engelhard Crop.
Rotenone dust
Numerous insects
Rotenone Ortho, Bonide, Agway

Insecticides in IPM Internet:nysaes.cornell

Disease Control

Of the major pests that plague organic vegetable production (weeds, insects and diseases), diseases are by bar the most devastating in Texas. Unlike other pests the casual organisms for diseases are microscopic in nature and not visible to the naked eye. As a result, growers often rely on the occurrence of disease symptoms to detect their presence. The most common symptoms include; leaf scorch, leaf spots, dying or dead plant parts, and distorted, blemished or decayed fruit. Unfortunately, once disease symptoms are evident it is often too late to implement a program that will effectively control the disease. Adequate disease control is further complicated by weather conditions. If certain weather patterns persist after infection has occurred, spread is often rapid and crop loss is eminent.

Reliance on symptoms to detect can cause other problems because many of the symptoms listed above can be the result of non-pathogenic causes. Injury form salts, wind desiccation, sand basting, spray drift air pollution and nutrient deficiencies or toxicity often produce similar symptoms. Misdiagnosis can result in costly needless input by producers attempting to control a pest that does not exist. In many instances only professionally trained individuals or laboratory analysis can adequately determine if the problem seen is pathological or not. Needless input can be avoided if a producer is aware of the factors needed for disease development to occur. Irregardless of the disease, the same four conditions are needed before infection will occur: a susceptible host; s source of inoculum; a favorable weather pattern; and, a means of pathogen distribution. If any one of these conditions is absent a disease can not develop, Figure 1. Understandably, a specific disease will have its own set of limits within each of these conditions.

Figure 1 shows how four conditions are need for disease

Figure 1. Conditions needed for disease occurance.

Management Strategies-

In many instances, conventional producers tend to rely on fungicides as the primary line of defense against diseases. With the proper use of effective fungicides they usually can successfully control most diseases in most situations. Since the use of fungicides is a limited option for organic producers, they must rely on a disease control strategy that is geared to preventing diseases from occurring rather than controlling them after they occur (22). To achieve this, many techniques must be used. The most successful disease management strategies consist of three major components; genetic resistance, avoidance techniques, and approved fungicidal products. Unfortunately, all three of these components are not always available for all crops and all diseases. For example, genetic resistance is only available to a limited number of disease organisms in a limited number of crops. However, organic producers should always strive to incorporate as many of the components as available into his or her disease management strategy. The more of the components used to develop the strategy the more effective will be the control achieved.

COMPONENT I – Genetic Resistance:

Variety and/or crop selection

As indicated previously, one of the essential conditions for disease infection initiation is a susceptible host. Genetic resistance removes the susceptible host from the infection. Therefore, the best and most economical method of control is through the use of genetic resistance whenever it is available. Consequently proper variety selection is one of the keys to the development of a successful disease management strategy. Given a choice the organic grower should always chose a variety that is highly resistant to the major disease(s) of an area but possesses only acceptable yield and quality over a high yielding, high quality variety with no resistance to the major disease(s) of the area. A list of suggested varieties for Texas is given in Table 2 of the appendix. As many varieties as are adaptable to Texas which contain resistance to the important diseases of the state are on this list. The suggested variety list is not an all-inclusive listing of varieties having good disease resistance. There are other equally as resistant and as productive varieties available as those listed. The varieties on this list have been shown to be highly adapted to Texas and are provided as a guide to assist growers in their variety selection. It should be mentioned that many of these varieties do not have the level of resistance to be grown in Texas without some means of disease control under high disease pressure situations. In the event that an intended crop to be grown does mot have resistant varieties to major diseases of an area are available, it is advisable for an organic grower to consider selecting another crop to be grown. In many instances, use of genetic resistance not only affords the highest level of disease control but also serves to reduce input costs required to protect a non-resistant variety.

COMPONENT II – Avoidance:

Simply stated, avoidance is the production of a crop utilizing practices that helps a crop avoid contact with a disease causal organism or with conditions that favor disease development. It can be comprised of one or more practices such as crop rotation, mulching, water management, trap cropping etc. The greater the number of practices utilized in an avoidance component of a management strategy the greater will be the incidence of achieving good disease control. The more commonly used avoidance practices are discussed below.

Crop rotation-

Crops belonging to the same botanical family are likely to be susceptible to similar diseases (23). For example, cantaloupe and watermelon are both members of the Cucurbitaceae family. As such they are susceptible to the same wilt diseases such as Fusarium and gummy stem blight. The causal agent for these diseases can survive on plant residue and debris in the soil for years (14). Planting cantaloupe in a field following a watermelon crop that had been infected with fusarium should be avoided because the incidence for fusarium infecting the cantaloupe is extremely high. Crop rotation is the ultimate form of disease avoidance. Table 17 presents grouping of vegetable crops based on susceptibility to similar diseases.

 

Table 17. Vegetables grouped based on susceptibility to similar diseases.

Group A
(Cucurbits)
Group B
(Brassicas)
Group C
(Solanums)
Group D
(Chenopodiacea)
Group E
(Legumes)
Group F
(Alliums)
Group G
(Grasses)
Watermelon
Cucumber
Squash
Cantaloupe
Honeydew
Pumpkin
Cushaw
Cabbage
Cauliflower
Broccoli
Brussels sprouts
Mustard
Turnip
Radish
Collards
Chinese cabbage
Peppers
Tomato
Irish Potato
Eggplant
Table beets
Swish chard
Spinach
Beans
Peas
Cowpea
Onion
Shallot
Garlic
Leek
Sweet corn

Source: Texas Vegetable Growers Handbook

Sanitation-

Sanitation can serve as a means of removing the source of inoculum needed to infect another crop. Sanitation practices includes the removal of diseased plants from a field, immediate crop destruction and plowing under excess plant residue after the last harvest, clearing fence rows and other areas of undesirable plants and cleaning tractors and implements after each use or when moving from field to field. Although removal of dead or infected plants from a field is often mentioned as a control technique, it can be of limited value in reality without due consideration of whether or not such a means is practical or effective (31). For example, removal of cabbage affected with clubroot or yellows would be of no value because more of the persistent pathogen in each case is left in the roots that remain in the field. Thus, this means of sanitation is only feasible when the life history of the organism concerned permits a rational, effective and economical procedure.

Immediate crop destruction and plowing under of residue remaining in the field after harvest can offer real benefits in a disease control strategy. This technique is especially effective when done prior to sporulation or development of disease propagule developing in the plant material remaining in the field or before the excess plants infected by diseases vectored by insects are visited by those insects. This technique can function in a dual manner. It can remove a source of inoculum as well as remove a susceptible hosts in the case where non-resistant varieties are grown. The longer excess plants or residue is allowed to remain in the field the greater the incidence of them becoming infected. Generally care of these plants are abandoned with the last harvest.

Infected plant debris an soil containing disease propagules can adhere to tractor tires, implements harvest containers etc. When these items are moved into a new clean field the infection contained in the debris or soil is transported as well. Thus, a source of inoculum is introduced into another wise clean field. Sanitizing equipment and containers between each use and movement from field to field may require some time but it can pay good dividends in the long run. Not only will it serve as one of the important practices in an avoidance component of a good disease management strategy but can also serve to prolong the life of these items.

Field location / site selection-

Fields located next to brush areas, pastures and other crops prone to harbor insects that vector viruses and other diseases are more likely to become infected with these diseases than those located some distance away. Whenever possible, do not locate organic field down wind from these locations. Locating fields downwind from conventionally produced crops can also expose the crop to spray drift from prohibited pesticides as well. The same is true when prevailing winds are present in an area and neglect growers are located upwind. Organic fields can become inoculated with spores being blown in from thesis fields.

Field location becomes an important consideration when developing a long term crop rotation plan for the whole farm. Consideration of what fields a given crop will be planted over a 3 – 5 year period to make certain it will not be planted following a closely related crop during this time period.

Selecting field locations that are poorly drained or have low spots can serve to create disease problems and should be avoided. Fields having excessively light sandy soils should also be avoided because these can cause non-pathogenic disease such as blossom end rot, scorching, fruit distortion, sand blasting and fruit drop under conditions of drought and high winds. If such fields must be utilized, it is advisable that windbreaks and irrigation be used.

Field location or orientation can play a role in reducing the incidence of certain diseases. Fields that are laid out in such a way as to have the rows running in the direction of the prevailing winds tend dry faster and have less relative humidity in the plant canopy than those running perpendicular to the prevailing winds. This can serve to remove or lessen the favorable climatic conditions for a disease to develop.

Barriers-

Physical barriers such as row covers, plastic mulch, cage bags and trellises have been effective avoidance tools for disease control. These products prevent direct contact of a plant to inoculum or plant pathogen. Cage bags and row covers are the most effective as barriers against viruses that are transmitted by insect vectors. As expected, these products prevent insects from coming in contact with plants. Plastic mulch are the most value as an avoidance mechanism against soil borne pathogens. Research has shown that a 30% reduction in cantaloupe fruit rot was achievable with the use of plastic mulched bed as compared to bare soil beds (8). Some research has shown that reflective mulches tend to confuse certain insect vectors and prevent them from attacking plants (18) as well prevent spores from splashing onto plants.

Trellising-

Trellises, cages and stakes can also be used to form spatial barriers between soil containing pathogens and fruits of crops such as tomato and cucumber (18). These structures can be used to support vining plants in a vertical position and thereby preventing direct contact with soil containing pathogenic organisms. Some growers have found that coming barrier products such as row covers and bag with cages or trellises can improve the reduction in loss of fruit to rots by preventing both soil borne diseases and insect vectored viruses. They also have achieved the added advantage of enhanced earliness of harvest. Combinations of barriers such as plastic mulch and vertical culture afforded with trellising also aid in reducing favorable microclimates within the plant canopy. These items enhance bed surface and leaf drying and thereby reduce humidity needed by certain pathogens to germinate their spores. Close attention to costs must be made when combining these avoidance techniques.

Irrigation management-

Adequate moisture supplies are not solely needed by crops for optimum growth and yield but by disease inciting organisms as well. Therefore, organic growers will benefit by properly managing irrigation in order to prevent creating favorable wetness conditions for diseases within the plant canopy or by water logging the soil (22). When successfully accomplished fewer disease problems will result. Water volume, application frequency, and method of application can influence disease infection, severity and spread. Too large a volume applied too often can cause certain soil types to become water logged and increase the incidence of seedling damping off or root and fruits. Therefore scheduling duration and volumes to be applied become important considerations. Certain application applications can increase the incidence and spread of some diseases. Phytophora of pepper is spread rapidly down the row with the use of furrow irrigation; black rot of cabbage is spread across a field by sprinkler applied water splashing bacteria from one plant to the next; and leaf wetness periods can be increased with sprinkler applied water thereby causing proliferation of certain diseases such as white rust of spinach. If furrow systems are to be used, apply light frequent applications. If sprinklers are to be used, apply water during the late evening or at night when dew are normally forming. Perhaps the most effective means of applying water from a disease management standpoint is with drip or trickle systems. These systems apply low volumes of water directly to the root system and do not wet the leaf surfaces. Sprinkler systems can be modified to apply water only in the furrow between the plant rows. This will prevent leaf wetting. Such systems are known as LEPA (Low Energy Precision Application) systems.

Planting date modification-

Planting 1 – 2 weeks earlier or later than usual often can mature a crop before or after the peak period of certain diseases. Similarly, altered planting dates may enable the crop to mature before or after the normal invasion dates for an insect vector. A knowledge and understanding of disease and insect life cycles is helpful in determining when a crop is to be planted for this purpose. This will also entail good recording keeping on historic incidences of pests and the weather conditions, which influence their occurrences. The major drawback to altering planting dates is the potential to miss a valuable market window. With the increasing competition resulting from globalization, market windows are shrinking in many regions of the US. Missing a widow may result in the loss of significant revenue.

COMPONENT III- Pesticides (fungicidal products):

Of the three major components of a sound disease management strategy, the pesticide component is the least emphasized in organic production systems. Although disease control effectiveness is greatly improved by this component, the guide lines set forth for organic production by Texas Department of Agriculture stresses the fact that they cannot be the primary method of control. Therefore, this component must be used in combination with either component I or II. Fortunately for organic producers the number of potentially new fungicidal products to be available in the near future is increasing. Most of these products are derivatives of plants and fungi and are considered to be safe with regard to human health and the environment. A few of the more commonly used fungicidal compounds currently in use are sulfur, copper, Bordeaux mixture (copper sulfate + calcium hydroxide) and fungicidal soaps are shown in Table 18. Table 8 of the appendix contains a complete list of USDA approved pesticidal products.

Table 18. Commonly used fungicidal materials approved for controlling selected vegetable diseases.

Product Target pest Active ingredient Manufacturer/Distributor
Bonide Plant Fungicide
Microthiol Special
THAT
Powdery mildew
Elemental Sulfur
Bonide Products, Inc.
Elf Atochem
Stoller Enterprise
Champion WP
Fungal and bacterial diseases
Copper hydroxide
Agtrol International
Kaligreen
Powdery mildew
Carbonic acid, monopotassium salt
Nichimen America
Kocide 101, 2000 DF
Powdery mildew
Copper hydroxide
Griffin
Mycostop Biofungicide
Root diseases, Botrytis
Streptomyces griseoviridis
AgBio Development
Oxidate
Zerotol (for greenhouse use)
Fungal diseases
Hydrogen peroxide

BioSafe Systems BioSafe

RootShield drench
T-22 Planter Box
Pythium, Rhizoctonia Fusarium
Tricoderma harzianum
BioWorks

Source: http://www.nysaes.cornell

 

A few of the newer generation spreader/sticker such as Niad have been found to offer a degree of control for such diseases. Spray adjutants approved for organic use are Basic H (Shaklee), Natur’l Oil (Stoller Enterprises, Inc.), Nu-Film 17 and Nu-Film-P (Miller Chemical and Fertilizer Corp.), Vaporgard and Wiltpruf.

It should be remembered that just because a product is approved for use in organic production it does not mean that it cannot cause harm to humans and the environment. Products such as copper and sulfur often cause plant injury under certain conditions on certain species. As with all pesticides, always read and follow label use instructions and apply only to crops listed on the label. Products not intended for use as a pesticide should not be used for this purpose because they do not have a label.

The disease control strategy for organic production is farther complicated by disease problems that occur below the soil surface. These problems are difficult to control in any type production system because the control agents, products or plant residues used to control them must be incorporated to considerable depths in order to obtain adequate control. The idea of controlling and manipulating the soil microflora through the use of inoculants, organic amendments and cultural and management that creates a more favorable microbiological environment has become the first line of defense in obtaining optimum crop production (19). Most of these products contain their own indigenous populations of microorganisms and often have the capability to function as biological agents by controlling or suppressing soil borne pathogens. The mode of action of these organic materials generally results through their competitive and antagonistic activities. However, in commercial agriculture, the results obtained with these products have been inconsistent and unpredictable under various environmental conditions. In addition the role of specific microorganisms have not been well-defined (18). The possible mechanisms suggested that can shift the soil microbiological equilibrium following the addition of microbial inoculants and organic amendments are as follows:

Antibiosis- The production of antibiotics by non-pathogenic microorganisms that can induce biostasis and biocidal effects on others.
Competition- The struggle by microorganisms for substrate, space and growth.
Parasitism- The direct parasitic attack on soil-borne pathogens by non pathogenic organisms.
Detoxification-The metabolism of toxic substances by specific microorganisms.
Inhibition- The production of compounds by microorganisms that can inhibit specific metabolic pathways in other species.

For the above processes to be effective as alternative agricultural disease control practices, their vitality is dependent upon the proper and regular addition of organic wastes, residues and other inoculants (18). One of the real positive attributes with the use of these products is that they can perform multiple functions such as providing good nutrition that most other soil borne pathogen treatments do not.

As stated previously, once symptoms are evident it is often too late to avoid crop injury due to disease organisms. Therefore, routine field scouting to ensure early detection is a must in a disease management strategy for organic production to minimize crop loss. When scouting for disease, look for symptoms to occur on a few plants in several locations through out a given field. Always include a regular check of obvious trouble spots such as low spots, poorly drained areas, those next to abandoned fields, brush and pastures etc. Proper diagnosis and recognition of potential pest problems is critical to the success of a good field scouting program. Each disease has its own threshold level, below which no significant crop injury or loss will occur or the level at which it can be easily controlled. Experience, Extension personnel, and professional consultants are the best teachers for determining disease thresholds. Good weather data and recording keeping will greatly improve the benefits for field scouting in a disease management program. Weather data can provide a grower with insight into the potential for disease occurrence. It can also be useful as a means of triggering the implementation of disease control techniques.

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