An integrated weed management program is essential for carrot production, because registered herbicides do not control many problem weeds in carrots; mechanical cultivation and hand-hoeing are not feasible because of the high crop plant populations; and, carrots are not competitive against weeds. A weed management program that combines good cultural practices with the use of herbicides will control many weed pests of carrots. However, the limited number of herbicides available and the diversity of weeds that grow in carrot fields make it difficult to maintain adequate control throughout the growing season. Generally, more than one herbicide is necessary to effectively control the different weed species. Usually a pre-plant or pre-emergence herbicide is applied, followed by one or more post-emergence treatments.

The choice of herbicide depends upon the weed species that are expected to occur. Plant-back restrictions need to be considered when selecting herbicides, because soil residues of some products can limit the growth of sensitive rotational crops. Herbicide labels are the best source of plant-back restrictions.

Non-chemical options include solarization and flaming. Carrots have a long germination period, and flaming must be performed after the weeds have emerged but before the carrots emerge. Carrots are planted with 4 to 6 seed lines on the top of a 40- to 42-inch bed. Since the bed top is only 22 to 24 inches wide, there is no room to cultivate the planted area. Only the shoulders and furrows are cultivated. Additionally, because of the high-density planting, hand-weeding is not practiced, and can seriously damage the crop by removing carrots with the weeds.

Monitoring
Monitor fields, and keep records of the weed species that occur in each field during the period of the year when the crop will be grown. Records of weeds occurring at planting time are especially important. Not only are these records valuable in planning which fields to use to grow carrots, they also help track the occurrence of hard-to-control weeds. Avoid fields with high populations of certain weeds, such as sowthistle, nightshades, nutsedge, pigweed, and purslane.

Management Before Planting
Avoid planting carrots in fields last planted to cereals or in fields with known infestations of perennial weeds; available herbicides do not effectively control perennial weeds. To prevent the buildup of weed seed in the soil, cultivate weeds before they set seed in rotation crops. After harvest of the rotation crop, clean-cultivate the field or plant a green manure crop to prevent weed infestations. In a typical farm mix of crops, carrots should be planted in the most weed-free fields.

Soil solarization can be used to control most weeds in carrots. It will also control or suppress some other important pests, such as nematodes. Soil solarization requires a summer fallow season for treatment; it fits in best with a fall-planted crop.

Metam sodium is used in carrot fields principally for control of soil-borne fungal diseases and nematodes. It will also kill emerged weeds, ungerminated weed seed that has become softened by an irrigation, and nutsedge shoots. Application is made about 2 weeks after a pre-irrigation, which should be made before planting the crop. The typical application method is through solid-set sprinklers or into a flood irrigation. The minimum time allowed between application and planting the crop is 14 days, and can be up to 60 days, depending upon environmental conditions, so planning ahead is important.

To control nutsedges, EPTC may be used during the fallow season, if allowed. Apply it in summer before a fall carrot crop to reduce nutsedge tuber populations. Be sure to use it according to label directions; the herbicide must be applied 90 days before carrot sowing, and the field must be irrigated 30 days before planting.

About 2 weeks before planting carrots, pre-irrigate the field, to germinate weed seedlings, and cultivate to destroy them. Carry out this operation as close to planting time as possible to assure that soil temperature and climatic conditions are similar to those that will occur during the crop germination period, thus maximizing the number of weeds controlled. Cultivate as shallowly as possible to avoid bringing up dormant weed seed from deeper soil layers.

Herbicides that are available to use pre-plant in carrots include paraquat (Gramoxone) and glyphosate (Roundup). These products can be used to control emerged weeds just before planting or before the crop emerges. Be sure the crop has not emerged, however, because emerged plants will be killed if contacted by these herbicides. Glyphosate has been particularly helpful in controlling perennial weeds when used as a pre-plant treatment.

Trifluralin is a commonly-used pre-plant incorporated herbicide in carrots. It will control many annual weeds when used for the entire carrot production season.

Management After Planting
Weeds must be controlled in a carrot field throughout the growing season, and require some post-plant control measures. Close cultivation and hand-hoeing are not practiced, because the injury to the crop is generally greater than the benefit received. An herbicide is generally applied before or after the crop emerges, depending on the weed species to be controlled.

Pre-emergence. Trifluralin can also be used as a pre-emergence herbicide in carrots. Sprinkler irrigation is required for incorporation and activation of the material, if it is applied after planting. This method will increase the activity on shallowly emerging weeds, such as common purslane, but will limit soil longevity and effectiveness on weeds germinating from deeper in the soil.

Linuron (Lorox) can be used for pre-emergence application. Crop safety is marginal; be careful not to exceed label recommendations on rate. This herbicide will control annual broadleaf weeds better than trifluralin, particularly nightshade, mustards, and sowthistle.

Post-emergence. Following crop emergence, fluazifop-P-butyl (Fusilade) can be applied for grass control, and linuron (Lorox) for annual broadleaf weeds and yellow nutsedge. Fluazifop-P-butyl is effective in controlling small seedling annual grasses and some perennial grasses. Effectiveness is reduced when grasses are under moisture stress. Later growth stages of annual grasses are more difficult to control. Follow label instructions regarding the use of adjuvants with fluazifop-P-butyl.

Linuron can also be used as a post-emergence herbicide in carrots. It is applied over the top of the crop when the carrots are 3 inches tall. It controls emerged weeds, and also has soil residual activity against later emerging weeds. Linuron will control (suppress) yellow nutsedge, but has little to no effect on purple nutsedge. A repeat application is allowed, but a total of 3 lb a.i./A is the limit per season. Some carry-over can occur under certain conditions, creating a plant-back problem. Consult the herbicide label before application.

Increasing plant populations from 1,211 to 3,628 plants per acre resulted in significantly greater fruit number and yield at both locations and for both varieties. Average fruit size declined at the highest populations.

Increasing row width from 6 feet to 12 feet resulted in a slight, but significant, decrease in number of fruit per hectare, with no effect on other yield parameters. At one location, the effect of row width on yield and number of fruit per hectare depended on the population. At low populations, row width did not influence yield or fruit number; at high populations, wide rows produced lower yield and fewer fruit than narrow rows.

The results demonstrate that growers may increase pumpkin yield by increasing plant populations, but they should use narrower row widths and wider in-row spacing. Growers who choose higher populations should ensure that all inputs are optimized to reduce potential plant-to-plant competition, and use regionally adapted cultivars.

The year 1999 was definitely an interesting time for the produce industry. The industry has seen a business landscape that has fewer and fewer retail chains, but the ones that remain have nearly doubled in size. Industry participants are also facing technological changes that will affect the way transactions are made. Strides to increase the ease of trading globally have also been made. In the last year of the millennium, the produce industry has undergone monumental changes that will affect the way industry participants will do business in the years to come.

During 1999, grower/shippers witnessed the dawn of supply-side economics. Consolidation ran rampant among large retail chains. A study has shown that the nation’s eight largest supermarkets now control 50 percent of U. S. sales. This figure is up from 1975, when the eight largest retailers controlled only 25 percent. One effect of this consolidation has been that retailers, especially larger ones, are demanding more consistent year-round supplies. The number of suppliers forging their own alliances began to rise as well. These supplier alliances took various forms: mergers, joint marketing arrangements, and collective branding, to name a few. Marketing programs grew larger in volume during 1999, spreading to more production areas across the globe. This larger global presence should aid in providing year-round availability. As a result of supply-side consolidation, distributors increased warehouse and processing capacity to keep up with the larger volume. Another result has been an increase in demand for experienced managers to run these larger and sometimes more sophisticated operations.

As 1999 came to an end and the new millennium began, a new era in produce distribution beckoned. Fax machines and telephones, which have been an essential element in connecting buyers and sellers, are now facing competition from the computer. Most companies have resolved any Y2K issues, and their attention has now turned to connecting with customers and suppliers through e-commerce. E-commerce applications are diverse, ranging from business-to-business transactions (like a retailer buying produce from a wholesaler) to business-to-business sales (like HomeGrocer.com selling fresh produce to Internet customers), as well as load-monitoring, which lets produce buyers monitor the progress of products en-route. E-commerce has been used for more than twenty years in the form of electronic data interchange (EDI), but recently, the Internet has added a different twist. Internet-based e-commerce companies allow any retailer to deal with any supplier directly, without the need for specific EDI systems. Though proponents of the various e-commerce options differ on whether Internet-based solutions will supplant traditional EDI, they do agree that business-to-business e-commerce is the future of produce trading.

Easier international trade has long been a goal of many in the produce industry in North America. The North American Free Trade Agreement (NAFTA) has helped propel this dream into a reality. For some time, many in the produce industry have felt that a tri-national dispute-settlement mechanism that could ensure greater trade volume would be necessary to fully develop NAFTA. The year 1999 saw the development of just such a committee. The Dispute Resolution Corporation (DRC) was developed during the course of three meetings, and is intended to act as a private, member-driven entity which will provide efficient and skillful dispute resolution services for the produce industry. The organization opens for business as of February 1, 2000.

Slotting fees, although nothing new to the produce industry, were put in the spotlight in September when the U. S. Senate conducted hearings on retail buying practices. Two supplier witnesses testified before the Senate Small Business Committee with their faces hidden, due to fear of retaliation from large retailers. A retail spokesman defended slotting fees as a way to allocate scarce shelf space and short out the thousands of new products that come onto the market each year. The retail spokesman also claimed that slotting fees allow retailers as well as producers to share the risk of incurring the cost of failed products. A Notre Dame University research and anti-trust lawyer indicated that these fees cannot be legitimately justified, and that they often limit competition and consumer choices, raise prices, and stifle innovation. There were also allegations that the slotting fees are higher than the actual cost of putting the product on the shelves – meaning that these fees are simply a way to increase profit for the retailer. The U. S. Department of Agriculture’s Economic Research Service is currently working on a one-million-dollar study of structural changes and trade practices in the produce industry. The purpose of the study is to analyze the impact of slotting fees and other industry practices.

The last year of the twentieth century proved to be an exciting and climactic one. Several new changes took place, and they are sure to change the way the produce industry conducts itself for many years to come. The turning of the calendar to a new millennium has not shut down businesses or the economy; we can now put to rest our concerns about Y2K, and turn our attention to what the future holds.

Natural fungi may help rein in important insect pests of vegetable crops like broccoli. Agricultural Research Service entomologist John D. Vandenberg has been focusing on whether two parasitic fungi – Beauveria bassiana and Paecilomyces fumosoroseus – could become biological controls for diamondback moths.

The diamondback moth is a worldwide pest of cabbage, broccoli, and other crucifers. Each year, growers world-wide spend more than one billion dollars to control it – primarily with chemical insecticides. But in many areas, the moth has become resistant to conventional insecticides as well as natural bacterial controls such as Bacillus thuringiensis.

Vandenberg has conducted laboratory and field tests that show the moth succumbs to both fungi. But only Beauveria had a consistent effect in the field. Vandenberg works at the U. S. Plant, Soil, and Nutrition Laboratory operated in Ithaca, New York by the Agricultural Research Service, the chief research agency of the U. S. Department of Agriculture. Vandenberg and colleagues were the first to field-test Mycotrol, a commercial formulation of B. bassiana, against the diamondback moth. Weekly or twice-weekly applications significantly reduced insect populations and damage to seedlings, compared to chemical controls. Mycotrol was first developed to combat silverleaf whiteflies through a cooperative research and development agreement between ARS and Mycotech Corporation of Butte, Montana.

Research Proves Successful
At least ten other insects, including Indian meal moths, have developed some resistance to Bt, especially where growers have used it extensively. But fungi are promising alternatives, according to Vandenberg.

“With P. fumosoroseus,” Vandenberg says, “field studies on application rates and residual effects on subsequent populations are needed to select appropriate strains. We also need to better understand and manage the fungal infection process.” In laboratory studies, Vandenberg and Cornell University graduate student Jennifer Altre compared eight strains of P. fumosoroseus for the ability to infect and kill the diamondback moth. “We found dramatic differences among these strains in their relative virulence, spore size, germination speed, and ability of spores to attach to the surface of the insect,” Vandenberg says. “The highly virulent strains have larger spores that attach easily to the insect’s cuticle and germinate quickly. A strain with smaller spores doesn’t attach well, germinates slowly, and isn’t able to infect the moth.”

In laboratory experiments with B. bassiana, all larval stages of the moth were susceptible to infection. But larvae exposed to the spores shortly before molting avoided infection, because they quickly shed their cuticle. “Larvae died more quickly at moderate temperatures and when exposed to higher doses of B. bassiana spores,” he says. “These studies will help us to predict the success of timely applications of field-applied fungi.”

In related research, Vandenberg and Anthony M. Shelton, a Cornell University entomologist, investigated B. bassiana for diamondback moth control on greenhouse-grown cabbage seedlings. Growers who grow cabbage rely on these seedlings, which they transplant and grow in their fields. “Commercial crucifer seedlings can be contaminated with insecticide-resistant diamondback moths,” Vandenberg says. “By the time these seedlings become established in the field, diamondback moth larval populations can be high enough to require control.

The diamondback moth is a pest of cole crops. This pest was introduced from Europe before the middle of the nineteenth century, and is now found everywhere its host crops are grown. The moth has trouble over-wintering in cooler regions, but when over-wintering populations are present, they increase significantly. Trade winds disperse the moth throughout the United States during the growing season.

The adult diamondback moth is small, grayish-brown, with fringed hind wings. The adult moth can be identified by the pattern of white diamonds present in a line down the insect’s back when at rest. Both coloration and the manner in which the wings are held create this pattern. The eggs of the diamondback moth are round, white, and smaller than a pencil point. Eggs are found in groups of one, two, or in rare cases three, and will most often be located on the underside of leaves, adjacent to a large vein. The larvae are pale green, with a black head-capsule and sporadic black hairs. In contrast to some of the other cabbage worms, the larvae of the diamondback moth will wiggle furiously when picked up. The larvae are capable of producing silk, and may be observed hanging by a silken thread. The pupa is approximately one-quarter of an inch long, covered with a loose silken cocoon, and will frequently be found stuck to the plant.

Adult diamondback moths will feed in small amounts, and the damage is often so light that it will go unnoticed. It is the larvae of the diamondback moth that can cause significant economic damage.

Larvae have two distinct feeding styles. Shortly after emerging from the egg, larvae are extremely small and will feed by mining into the leaf (feeding on the leaf tissue between the upper and lower epidermis). The larvae themselves are not visible during this time (because they are inside the leaf). The only evidence is a long, tortuous white trail that is the result of underlying dead leaf tissue. As the larvae grow, they become too large to feed as leaf miners. It is at this point that larvae begin to burrow through the leaf tissue, resulting in many small holes in the leaf. This type of damage is most commonly called ‘shotgun damage’ due to the appearance of many small ‘shotgun pellet’ holes.

Economic damage is most severe after heading begins. Larvae will burrow into the developing head, destroying its marketability. The recommended treatment threshold for diamondback moth depends upon the plant’s growth stage. From seeding to cupping, the recommended threshold is when 20 percent of the plants are infested. After heading begins, the recommended threshold decreases to 5 percent infestation.

The diamondback moth over-winters as a adult in the debris of the previous year’s crop. In the spring, when temperatures increase, the moth will mate, and then begin a host search. Oviposition occurs on the underside of leaves, usually immediately adjacent to a vein. Eggs are usually oviposited in groups of singles, doubles, or triples, although more than one group may be oviposited per leaf. Larvae will feed for approximately ten days before beginning pupation. The pupal case of the diamondback moth is made of silk, and is used to adhere the pupa to the plant. An adult moth emerges from the pupal case in five to ten days, depending on the temperature.

The diamondback moth may be able to over-winter in most areas of Texas during mild winters. When populations successfully over-winter, it greatly increases both the amount and intensity of moth generation. Immigrating diamondback moths are the result of prevailing winds from the south.

The cornerstone of good pest management is monitoring. Optimal monitoring for the diamondback moth is the use of pheromone traps to monitor adults. When the adult population rises, begin scouting the plants for larvae and eggs.

When spraying, be sure to use a high-pressure blast sprayer (>200 psi) with three hollow-cone nozzles per row. One nozzle should point straight down into the row, while the other two spray from either side. Additionally, use a spreader-sticker to increase coverage. This procedure sprays the plants from three sides, and reduces the spray shadow that occurs as a result of the change in topography between the leaf and the vein. Keep in mind that it is this region — adjacent to the veins – where the diamondback moth oviposits its eggs. For proper control, it is critical to ensure that optimal spray coverage has been achieved.

In some cases, the intensity of summer infestations of the diamondback moth can be reduced by eliminating the over-wintering habitat. Because the diamondback moth over-winters in the debris of the previous year’s field, all plant materials should be plowed under immediately after final harvest. Additionally, fields should be rotated between years as far as possible. This makes it more difficult for adults who have managed to over-winter to find the current year’s crop.

Jill Stavenhagen, Web Site Design
Produced by Extension Horticulture, HFSB 225
Texas A&M University - College Station, Texas 77843-2134
(979) 845-5341 - fax: (979) 845-8906
http://aggie-horticulture.tamu.edu/