Breeding for resistance to insects and other arthropod pests in vegetables has been a difficult endeavor. Greater public awareness of health and environmental issues requires that we as horticultural scientists reexamine why breeding for resistance has been difficult. The literature clearly suggests the potential for a genetic solution, and the literature also reveals some reasons why achievement of genetic resistance to arthropod pests has not been as successful as the achievement of resistance to pathogens. The thesis of my presentation is that the complexity of plant-arthropod interactions often prevents simple genetic approaches to breeding for resistance. Data using Lycopersicon hirsutum and its interaction with spider mites will provide examples of the these complex interactions. L. hirsutum is a wild relative of L. esculentum, the common tomato, and is nearly immune to insect attack. However, there are few or no clear examples of this taxa contributing to the insect resistance of tomato. The complexity of the interaction between mites and trichomes on L. hirsutum will be highlighted as it pertains to environment and genetics of the plant, and the development of the arthropod.
Brent Rowell and John C. Snyder
“We are a tobacco state” is frequently heard among farmers and agricultural leaders in Kentucky; the state's farm economy has always revolved around burley tobacco production. Tobacco, grown in Kentucky for nearly two centuries, remains the most valuable crop earning approximately $694 million in 1995. Even our unusual terminology of “alternative,” “supplemental,” or “opportunity” crops denotes the prime position of tobacco and attitudes toward vegetable crop production. This long tradition and attitudes associated with it contribute to a serious lack of confidence and low expectations when it comes to diversification with vegetable crops. These low expectations and the consequent circular pattern of experience with vegetable production were revealed in a multidisciplinary, 5-year research project designed to determine opportunities for and constraints to vegetable production in the state. The study showed that nearly half of Kentucky's commercial vegetable growers also were tobacco growers and that there were no fundamental incompatibilities in tobacco–vegetable cropping systems. Although farmers considered lack of markets a major constraint, economic research revealed that growers were often unwilling to use and take the risks associated with existing market structures and channels. As a result of these findings, a major on-farm demonstration program was implemented to raise expectations and break the “circular syndrome”. More recently, new partnerships and collaborative relationships have been established between university horticulture and marketing specialists and the Burley Tobacco Growers Cooperative Association for the promotion of “supplemental crops” among Kentucky's tobacco growers.
Zhenhua Guo and John C. Snyder
Choice and non-choice bioassays were used to examine deterrence in vitro and in vivo of Tetranychus urticae Koch. In vivo deterrence of leaflets from 11 Lycopersicon hirsutum accessions as well as the tomato cultivar `Ace 55' was measured as was in vitro deterrence of their leaf hexane extracts. Leaf surface chemistry was examined by gas chromatography. All 6 accessions of L. hirsutum f. hirsutum contained sesquiterpene hydrocarbons. Each of these extracts also contained one or a few late eluting components. All were deterrent in vitro and 5 out of the 6 were deterrent in vivo. The one lacking in vivo deterrence had low density of type IV trichomes. All 5 accessions of L. hirsutum f. glabratum contained methyl ketones. These accessions were less deterrent in vitro and 4 out of the 5, less deterrent in vivo. The one accession having high in vivo deterrence also had high density of type IV trichomes. `Ace 55', having few hexane extractable compounds was neither deterrent in vitro nor in vivo. Within an accession, secretions from different types of trichomes shared similar chemical profiles and were similar to leaf profiles.
John C. Snyder, George Antonious, and Richard Thacker
Many accessions of Lycopersicon hirsutum are highly resistant to insects. Trichomes and their secretions have been extensively indicated as factors of resistance. One mechanism of resistance mediated by secretions is repellency, a mechanism that is consistent with the observation that few insects visit plants of L. hirsutum. Trichome secretions from certain accessions of L. hirsutum f. typicum are repellent to spider mites. However, the composition of secretions from different accessions of f. typicum are chemically diverse. Sesquiterpene hydrocarbons are prevalent in secretions, but are structurally diverse. How structure may relate to repellency is of interest but difficult to address because isolation of pure sesquiterpene hydrocarbons from hydrocarbon mixtures is difficult. To begin examining relationships between structure and activity we determined how chain length of n-alkanes related to repellency of spider mites. n-Alkanes having chain lengths from 8 to 22 carbon atoms were assayed for repellency. The C16-C18 alkanes were most repellent. Smaller and larger hydrocarbons were less repellent. The EC50 for n-hexadecane was equal to that of the most repellent natural products we have isolated from trichome secretions of L. hirsutum.
Brent Rowell, William Nesmith, and John C. Snyder
Virus and fungal disease pressures limit fall production of summer squash (Cucurbita pepo L.) in Kentucky. Twenty-five summer squash cultivars (nine zucchini, eight yellow straightneck, and eight yellow crookneck entries) were evaluated for marketable yield, appearance, and disease resistance in a late summer planting. Genetically engineered virus-resistant materials and new conventionally bred resistant or tolerant cultivars were compared with popular susceptible hybrids. Virus incidence was determined visually before and after final harvest and was also determined by enzyme-linked immunosorbent assay (ELISA). Watermelon mosaic virus (WMV) was most frequently detected and appeared to have caused most of the observed symptoms. Conventionally bred cultivars containing the precocious yellow gene and two transgenic lines were in the highest yielding group of yellow straightneck squash despite high virus incidence in precocious yellow cultivars. Among yellow crooknecks, transgenic cultivars were clearly superior for disease resistance and yields. Conventionally bred cultivars with virus tolerance were among the highest yielding zucchini types. Most transgenics were superior to their nontransformed equivalent cultivars for virus resistance and yield. Cultivars and breeding lines varied considerably in color, shape, and overall appearance. ELISA results revealed that some (but not all) transgenic cultivars tested positive for the coat protein corresponding to the virus resistance present in that cultivar. Also, mild virus-like symptoms were observed in transgenic squash plants after the conclusion of harvest.
Brent Rowell, R. Terry Jones, William Nesmith, and John C. Snyder
Bacterial spot epidemics, caused by Xanthomonas campestris pv. vesicatoria (Doidge) Dye, continue to plague bell pepper (Capsicum annuum L.) growers in a number of southern and midwestern states. A 3-year study designed to compare cultivars and breeding lines under induced bacterial spot epidemic and bacterial spot-free conditions began soon after the first release of cultivars having the Bs2 gene for resistance to races 1 to 3 of the pathogen. Bacterial spot epidemics were created by transplanting `Merlin' plants (inoculated with races 1 to 3) into plots of each test cultivar at an isolated location in eastern Kentucky. Plots of the same trial entries at a second location were kept free of bacterial spot for 2 of the 3 years of trials; however, a moderate natural epidemic occurred at this location in 1996. Bacterial spot resistance had the greatest impact on yields and returns per acre in the inoculated trials. Cultivars with only Bs1 or a combination of Bs1 and Bs3 were highly susceptible in the inoculated trials. There were statistically significant and economically important differences in resistance among cultivars and breeding lines having the Bs2 gene; some were nearly as susceptible as susceptible checks. Although many Bs2-gene cultivars showed satisfactory levels of resistance, only a few were highly resistant, horticulturally acceptable, and comparable in yields to the best susceptible hybrids in a bacterial spot-free environment.
C.R Roberts, Dean E. Knavel, John Snyder, Terry Jones, and Dave Spalding
Internal brown spot (IBS) was found consistently in the `Atlantic' cultivar at Lexington in 1967, 1968 and 1989, and at Owensboro and Quicksand, KY in 1987, Treatments of foliar and soil applied CaSO4 in 1987, soil-applied CaSO4 in 1988, and straw mulching in 1989 did not reduce IBS. Irrigation increased IBS because of larger tubers and increased Ca content of plants as compared with non-irrigated plants. Tubers showing IBS had higher Ca content in affected tissue than in non-affected tissue. Both IBS and Ca content of leaves increased as the plants aged.
George F. Antonious, Matthew E. Byers, John C. Snyder, and Douglas L. Dahlman
The development and deployment of crop varieties that resist or tolerate insect attack is one tactic of pest management that can eliminate one or more spray applications per season, a significant savings to the grower. Seven tomato, Lycopersicon esculentum Mill. cultivars (Marmand, Edkawy, VF-145, GS-27, Pakmore-B, Flordade, and UCX) were evaluated under greenhouse conditions for differences in mortality and feeding behavior (leaf-area ingested) of the 4th instar larvae of Spodoptera littoralis (Boisd) and the Colorado potato beetle, Leptinotarsa decemlineata (Say). The most resistant cultivars to S. littoralis during two summer seasons, 1990 and 1991, were Edkawy and UCX (37% mortality) and VF-145 (33% mortality). Mortality was least (20%) on the F1 hybrid GS-27, indicating that GS-27 was the most favorable cultivar for S. littoralis. L. decemlineata larvae reared on excised tomato leaflets of the same varieties indicated similar trends. Factors responsible for greater resistance of Edkawy and UCX to S. littoralis and L. decemlineata are under investigation.
John C. Snyder, Richard Thacker, Jan St. Pvrek, and Jack P. Goodman
Spider mites (Tetranychus urticae Koch) readily colonize the cultivated tomato. Lycopersicon esculentum L. However, mites have extreme difficulty colonizing the wild relative of tomato, L. hirsutum Humb. and Bonpl. When mites approach leaves of L. hirsutum, they often veer away, suggesting the presence of a deterrent or repellent. Initial experiments indicated that trichome secretions on leaflets of L. hirstum deterred mites. In vitro bioassays indicated that at least four distinct compounds present in these sequiterpenoid secretions of L. hirsutum P.I. 251303 were deterrent. At least two of the compounds were soluble in dilute NaOH. Based on mass spectra and 1H and 13C NMR the structure of two base soluble compounds were established as two related bisabolane derived carboxylic acids.
Timothy Coolong, Derek M. Law, John C. Snyder, Brent Rowell, and Mark A. Williams
Thirty-eight leafy greens, eight kale (Brassica oleracea acephala group), nine mustard (Brassica juncea), six arugula (Eruca sativa), five swiss chard (Beta vulgaris var. cicla), five collards (B. oleracea acephala group), and five turnip (Brassica rapa ssp. rapa) varieties were evaluated during Spring and Fall 2007–08 to determine suitability for organic production with respect to yield and stability. Trials were conducted on certified organic land using organic production practices. For mustard, kale, collards, and arugula, there were significant variety by season by year interactions. Despite these interactions, some varieties consistently performed well throughout the trial. ‘Florida Broadleaf’ was the highest yielding mustard in three of the four seasons evaluated. ‘Siberian’, ‘White Russian’, and ‘Red Russian’ were in the highest yielding group of kale varieties for overall yield. For collards, ‘Georgia/Southern’ and ‘Flash’ were part of the highest yielding group as determined by Duncan’s multiple range test in three of the four seasons examined. Turnip and swiss chard had significant year by variety interactions. Overall yields of ‘Alamo’ and ‘Alltop’, both F1 hybrids, were better than other turnip varieties assessed. Despite the interaction, ‘Fordhook Giant’ had superior yields in both years of the study. Arugula performance was significantly and negatively affected in Spring 2008. Overall, ‘Astro’, ‘Apollo’, and ‘Arugula’ had the greatest yields. This trial was designed to provide recommendations specifically for organic growers marketing directly to consumers.