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• Resistance Management Guidelines for Strobilurin Use on Vegetables
• Why All the Fuss Over GMOs Now?
• Emergence of 'Genesis' Triploid Watermelon Following Mechanical Scarification
• Disease Identification: Botrytis Leaf Blight
• Yield Response of Watermelon to Planting Density, Planting Pattern, and Polyethylene Mulch

Resistance Management Guidelines for Strobilurin Use on Vegetables

This article by Alan MacNab appeared in "The Vegetable and Small Fruit Gazette" Vol. 4, No. 4 (April 2000), published by Pennsylvania State University, Department of Horticulture.

These guidelines are important because some fungi can develop resistance to the strobilurins; in fact, this has already occurred, but only outside North America.

The reasons for this update are (1) to remind farmers that there is a potential for resistance to develop where these fungicides are used, and (2) to encourage growers to follow the guidelines outlined below. By alternating strobilurin materials with other effective materials, and by limiting the number of strobilurin applications per season, the chance for development of resistance to the materials will be reduced.

Information in the guidelines will be included on all new labels. Even though there will be a transition time before all labels have the new wording, all growers will benefit from following the new guidelines now. Guidelines pertaining to vegetables and potatoes are summarized below. Similar guidelines are available for other crops listed on strobilurin labels.

Cucurbits
(1) No more than one application before alternation to an unrelated fungicide that is effective in controlling the causal fungus.
(2) No more than four total applications per season.

Potatoes
(1) No more than one application before alternation to an unrelated fungicide that is effective in controlling the causal fungus.
(2) No more than six total applications per season.

Tomatoes
(1) No more than one application before alternation to an unrelated fungicide that is effective in controlling the late blight fungus.
(2) For all other diseases, no more than three sequential applications before alternation to an unrelated fungicide that is effective in controlling the causal fungus.
(3) No more than five total applications per seasons.

Note: These sequences and totals reflect total strobilurin applications, i.e., at this time, both Quadris and Flint for vegetables. Therefore, if Flint is used for two applications on cucurbits, only two additional sprays can be made, regardless of whether they be Quadris or Flint. Likewise, if Quadris is used for two applications on cucurbits, on two additional sprays can be made, regardless of whether they are Flint or Quadris.

Why All the Fuss Over GMOs Now?

This article appeared in the National Agricultural Biotechnology Council Report "World Food Security and Sustainability: The Impacts of Biotechnology and Industry Consolidation," 1999.

Genetic improvement started with selection of organisms with superior traits, followed by breeding for additional genetic improvement. The power of genetic modification, from the 10th millennium B.C. to the present, progressed from selection to hybridization, Mendelian genetics, quantitative genetics, induced mutation, fusion, somaclonal variation, and molecular genetics.

Molecular methods are the basis of modern biotechnology, providing new tools not only for more rapid but also more precise genetic improvement of organisms; these organisms are referred to variously as genetically engineered organisms (GEOs), genetically modified organisms (GMOs), or transgenic organisms.

Highly domesticated organisms – bacterial, plant, and animal – are genetically modified for improved end-use as food, feed, or fiber crops, as a microbe for fermentative production of a processed food (for example, beer, wine, bread, or miso), as an industrial product (for example, fuel ethanol), as an improved dairy animal, or as an egg or meat producer. These genetically modified organisms are more fit for our domesticated use and usually less fit for existence in the unprotected world.

Our quality of life today is already highly dependent on genetically modified microbes, plants, and animals. Genetic modification is an increasingly major contributor to our capability to provide food for twenty times as many humans in 2000 A.D. as in 1000 A.D. Our knowledge, data, and tools are enabling us to achieve more rational and directed genetic changes at the molecular level by transfer of genes and control of their expression. The new molecular approach enables genes (at most, a few in any one case) to be moved within and across species with greater facility than occurs naturally. Genomic sequencing is revealing much commonality in genes of bacteria, plants, and animals.

What do we know about environmental and human health risks from genetically modified organisms? The most important conclusion is that risk from a product is inherent to that product, not to the process by which it is made. If identical products are produced by either molecular or organismal genetic modification, then they pose identical risks.

We have substantial experience with organisms modified at the organismal level. In general, such products have been of low risk, but there are a few examples of problems, such as the introduction of kudzu, an exotic pasture legume that became an aggressive weed, and widespread use of corn with cytoplasmic male sterility that was subsequently found to be susceptible to southern corn blight.

We have less experience (about ten years) with molecularly modified organisms; however, no substantiated examples of significant risk to the environment or human health, relative to the products being replaced, has been documented by rigorous and replicated scientific evaluation. Of course, we must continue to be watchful for negative effects in order to assure improved product safety. Our major focus should be on ‘what is’ rather than on the never-ending and often untestable ‘what if’.

There are some process characteristics that help guide risk assessment, the most important element of which is asking the right questions. The involvement of only a few genes of known structure and function in molecular genetic modification helps focus risk-assessment in contrast to organismal processes that involve, for example, the estimated 30,000 or more different genes of a higher plant. This genetic roulette is much less predictable in organismal than molecular genetic improvement. However, genetically improved products should be evaluated for safety on a case-by-case basis, utilizing all of the available information, including experience, to guide the assessment.

The tools of molecular genetic modification continue to improve, and should reduce further concern over food and environmental safety.

Early use of kanamycin antibiotic resistance, expressed by a marker gene to indicate successful transfer of an accompanying target gene, has been criticized because of a theoretical concern that pathogenic microbes might become resistant to this antibiotic. The FDA reported in 1994 its extensive examination of this risk. Kanamycin is almost never used in human medicine due to its high toxicity and evidence of widespread resistance. Transfer, in the human gut, of the resistance gene to a pathogenic microorganism from a plant cell is extremely unlikely relative to the transfer of resistance from the much more abundant antibiotic-resistant microorganisms.

Products in the research pipelines, for the most part, do not have an antibiotic-resistance marker genes. Improved tools, such as genomic site-specific introduction and tissue and development stage-specific expressions, will make molecular genetic improvement even safer.

Other approaches, such as genetic sterility or organelle location of inserted genes, could diminish greatly the concern over gene escape in those areas where there are weedy relatives. Weedy relatives, of course, are not a concern with most domesticated crops in the U.S., such as corn and soybeans.

Editor’s note: In the next issue, Vegetable, Production and Marketing News will highlight a statement from the NABC on GMO regulations.

Emergence of ‘Genesis’ Triploid Watermelon Following Mechanical Scarification

This article by John R. Duval and D. Scott NeSmith, University of Georgia Experiment Station, Griffin, appeared in "J. Amer. Soc. Hort. Sci." 124(4):430-432. 1999.

Mechanical scarification was evaluated as a means to overcome inconsistent emergence and seed-coat adherence. Seeds of ‘Genesis’ triploid watermelon were placed in a cylinder with 100 g of very coarse sand (1.0 to 2.0 mm diameter) and rotated at 60 rpm for 0, 6, 12, 24, and 48 hours in a series of experiments. Number of emerged seed was recorded daily to obtain emergence dynamics.

No significant differences were observed in seed-coat adherence among treatments. The long duration of scarification, however, enhanced emergence, as compared to the control, in three of four experiments. These data support earlier suggestions that a thick or hard seed coat is a factor contributing to poor germination and emergence of triploid watermelons.

Disease Identification: Botrytis Leaf Blight

This article appeared in "Onion World," September/October 1999.

Distribution: North America and Europe

Symptoms: The fungus primarily attacks the leaves. The first symptoms appear as small white spots surrounded by a greenish halo. Center of spots often are tan, making it difficult to distinguish between leaf blight and damage from insect feeding, mechanical damage, or herbicide injury. Lesions expand with age and, when numerous, may cause leaf tips to die back. Eventually, leaf death results, and severely affected onion fields develop a blighted appearance. Bulbs from infected plants may be small because growth is reduced by leaf loss.

Conditions for Disease Development: The fungus may overwinter in infected plant material, or may survive in the soil as small, dark-brown sclerotia. During moist periods with moderate temperatures, fungal spores that arise from sclerotia or infected leaves and debris are dispersed. These spores land on susceptible tissue, and infection occurs. This disease can spread rapidly when environmental conditions are favorable for development.

Control: A good preventive fungicide spray program (Rovral, Bravo, or Mancozeb) is important. Disease-forecasting systems have been developed for some areas, and these are very useful for determining the optimum timing for sprays. Destroying onion- or debris-cull piles will help reduce sources of inoculum. Orienting plant rows and spacing to maximize air movement helps reduce the time that leaves are wet, and results in less disease incidence and severity. Cultural practices, such as deep plowing and crop rotation, will help reduce numbers of sclerotia in the soil.

Yield Response of Watermelon to Planting Density, Planting Pattern, and Polyethylene Mulch

This article by Douglas C. Sanders, Jennifer D. Cure, and Jonathan R. Schultheis, Department of Horticultural Science, North Carolina State University, Raleigh, appeared in "HortScience" 34(7):1221-1223. 1999.

In general, experimental data fit models in which crop yields increase with planting density to a maximum, and then plateau or decrease at some threshold density. This threshold density should produce higher yields as environmental conditions are optimized, as when polyethylene mulch and drip fertigation are used. Optimum plant density may also shift as growing conditions improve. Without mulch, linear increases in watermelon yields were obtained as area-per-plant decreased. Watermelon yield-per-hectare also increased as area-per-plant decreased, and mulch increased yields at all spacings when irrigation was adequate.

In Georgia, the recommended population density for watermelons has been 1,012 to 1,219 plants/A; however, in North Carolina, it was 870 to 1,742 plants/A. Thus, there is a need to evaluate the response of watermelon to high planting density when polyethylene mulch and drip fertigation are used.

Another technique for enhancing earliness is the use of transplants. Like the use of polyethylene mulch, this is more expensive, but the extra cost can usually be more than repaid with premium prices in the early market. One possibility for lowering the cost of transplanting would be the use of two plants/hill. For instance, if the in-row spacing were doubled to two transplant/hill (thus maintaining average area-per-plant), the number of transplant operations per hectare would be halved. This work was undertaken to evaluate the effects of in-row spacing and of planting pattern (one vs. two plants/hill) on yield variables of watermelon, using black polyethylene mulch and drip fertigation.

Watermelon hybrids ‘Prince Charles’ (Charleston Grey type) and ‘Royal Jubilee’ (Jubilee type) were used in experiments in four environments in 1988, 1989, and 1990. The effects of in-row spacing and planting pattern on yield variables were evaluated, with and without polyethylene mulch. In-row plant spacings of 1.5, 2, 3, 4, and 5 feet on 5-foot-wide (center-to-center) beds were used to evaluate the effect of 1 versus 2 plants/hill. All beds were mulched with black polyethylene, and plant water needs were supplied by drip irrigation.

In all locations and years, ‘Royal Jubilee’ yielded significantly (P < 0.05) more than ‘Prince Charles’. “Royal Jubilee’ had higher numbers of marketable fruit per hectare (significant in all comparisons), and tended to have higher individual weights (statistically significant in only two of the four comparisons). There was no varietal difference in percentage of culls except in the 1990 study at one location.

Polyethylene mulch increased marketable yield at nearly all in-row spacings of either one or two plants/hill. Also, the number of marketable fruits increased 30 to 60 percent with polyethylene mulch at close spacings. Weight-per-melon was increased by polyethylene mulch at nearly all in-row spacings and both planting patterns; this increase effectively decreased the percentage of culls from 40 to 25 percent in the very high density (close spacing) treatments, where many melons were borderline in size. The mulch significantly increased the number of larger melons at all plant spacings, but had little effect on the numbers of medium-sized melons, except for an increase at the closest spacing.

These data provide a basis for new, closer spacing recommendations for watermelons (9.7 to 10.7 ft.2/plant) as long as water and nutrients are not limiting. They also support the option of planting double-seeded transplants at half the sites in a row, at a significant savings in labor and transplant cost per hectare.