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.
Embryos of American ginseng (Panax quinquifolius L.) seeds stratified 570 days at 0° or 5°C did not increase in size, whereas embryos of seeds stratified 570 days at 20° grew to a length of 2.5 mm. Embryos of seeds stratified at 5° for 120 days, treated with 1000 ppm gibberellic acid (GA), then transferred to 20° grew to the same length as those held at a constant 20°. None of the GA treated seeds germinated. However, seeds stratified outdoors and those provided 5° for 120 days, 20° for 300 days, and then held at 5°C germinated after 540 days. The embryos of these latter treatments had similar growth patterns. Growth of the cotyledons paralleled the growth of the entire embryo. Cool-warm-cool stratification patterns are necessary for efficacious germination of American ginseng seeds.
Phaseolus vulgaris L. (common bean) ‘Black Turtle’ and ‘Pinto’ differed in 2 major cotyledonary electrophoretic bands. ‘Black Turtle’ contained a 35,000 dalton band whereas ‘Pinto’ contained 28,000 dalton band. Both polypeptides were present in the F1 and segregated in the F2 with a 3 present: 1 absent frequency. No segregation differences were observed between reciprocal crosses.
“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.
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.
Concentrations from 0.00 to 7.00 mg/ml of 2-tridecanone were applied directly to the bottom of petri dishes or to filter paper in dishes and assayed with tobacco hornworm larvae (Manduca sexta L.). The rate and extent of hornworm mortality were greater in the assay without filter paper. The LC50 for the assay with filter paper was 10.6 µg/cm2 and 4.1 µg/cm2 without filter paper. These results indicate that the test without filter paper is a more sensitive bioassay for antibiotic effects on tobacco hornworm larvae.
Clones of 10 F2 mite-susceptible Lycopersicon esculentum Mill. × mite-resistant L. hirsutum Humb. & Bonpl. were raised in summer and winter in the greenhouse. Leaflets from clones grown in both seasons were assayed for two-spotted spider mite survival (Tetranychus urticae Koch); Type IV, Type V, and Type VI trichome densities; and zingiberene and γ elemene concentrations. Clone, season, and their interaction were significant sources of variance of all six variables. Between season, among clones, and among clones between seasons, mite survival was inversely correlated with Type IV trichome density. No evidence for involvement of sesquiterpene presence or concentration or Type V or Type VI densities with mite survival was observed. Covariance analysis of mite survival using Type IV trichome density as the covariable removed the significance of the clone × season interaction, reduced the significance level of clonal variation, and reduced the mean square of seasonal variation. Because Type IV trichome density and mite survival were related across all sources of variation, Type IV trichome density is probably mechanistically and causally related with host-plant resistance of Lycopersicon hirsutum to mites.
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.
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.
Spider mite behavior was characterized on leaflets, varying in extent of expansion, from mite-resistant Lycopersicon hirsutum and mite-susceptible L. esculentum plants. Survival, mortality, avoidance, and fecundity of the mites were related to Type VI trichome density and/or other covariants of leaflet development. At equivalent Type VI densities L. hirsutum leaflets were more resistant than those of L. esculentum. Thus, differential density of the Type VI trichome did not account fully for the resistance of the L. hirsutum plants. Mite mortality was associated with the presence of the L. hirsutum Type VI trichome tips and mite avoidance was associated with other factors that covaried with L. hirsutum leaflet development and leaflet surface.