Phytophthora capsici is responsible for multiple disease syndromes of Capsicum annuum but the resistance mechanism is still unknown. Evaluating gene expression during foliar blight can be used to identify expression patterns associated with resistance in Capsicum species. This study reports a direct comparison of gene expression changes during the foliar blight syndrome using two different races of P. capsici on C. annuum host plants with resistant and susceptible phenotypes to those races. Four genes were evaluated for differential expression following leaf inoculation with P. capsici. RNA isolated from leaves at three time points was used to quantify gene expression by quantitative real-time polymerase chain reaction (qRT-PCR). Of four genes tested, two had differential expression in response to P. capsici at 72 hours postinoculation, a xyloglucan-specific endo-β-1,4-glucanase inhibitor protein (XEGIP2) in susceptible cultivar New Mexico Heritage 6-4 (NMH6-4), and a C. annuum cell wall protein (CWP) in resistant Criollo de Morelos 334 (CM334). Both genes had a 5-fold increase in transcription in leaves over the control. These results suggest that both genes are playing a role in disease resistance to foliar blight.
Thrips are the major insect pest of onions grown in South Texas. Four cultivars, `IPA-3', `TG1015Y', `1664' (glossy control), and `1900B' (waxy control), were grown in a split-plot design with insecticide sprayed or nonsprayed treatments as the main plots and cultivar as the subplots. The experiment was conducted at the Texas Agricultural Experiment Station, Weslaco, Texas, in the 1995-96 season. The objectives of the study were to compare `IPA-3' and `TG1015Y' for thrips resistance and evaluate possible resistance mechanisms that may be present in `IPA-3'. The average number of thrips per plant and leaf damage rating were significantly higher for `TG1015Y', indicating that some resistance is present in `IPA-3'. However, there were no significant differences in yield between the two cultivars. A comparison of leaf wax characteristics indicated no significant difference between `TG1015Y' and `IPA-3' using gravimetric or gas chromatography techniques. However, scanning electron micrographs of `TG1015Y' leaves appeared more similar to `1900B' and `IPA-3' appeared more similar to `1664'. The insecticide spray treatment had significantly fewer thrips, less damage, and higher yield than the nonsprayed treatment.
Onion pungency is a major quality attribute with many consumers demanding less pungent onions. In recent years, some growers and retailers have attempted to measure pungency of onions produced in different regions to guarantee a desired level of pungency. However, there are few data on the variability among laboratories using standardized protocols to estimate relative levels of pungencies. Onion cultivars were grown in replicated trials at three locations. Random samples of bulbs from each experimental unit were harvested and shipped to at least three cooperating laboratories, each of which measured soluble solids content (SSC) and pungencies using the same techniques. As expected, cultivars and environments showed significant (P < 0.001) differences. For all three trials, laboratories were a highly significant source of variation (P < 0.024 to 0.001) for measurements of SSC and pungency. Therefore, one cannot make recommendations on relative pungencies of the same lots of onions measured by different labs. The onion research community must identify specific procedures to reduce variation among laboratories to develop a more repeatable standardized assay for the measurement of onion pungency.
The oomycete, Phytophthora infestans, is a devastating pathogen of potato worldwide. Several genotypes of P. infestans are able to infect other cultivated and weed species of the family Solanaceae and cause symptoms similar to late blight. Changes in P. infestans populations have stimulated investigations to determine if potato strains from new immigrant populations infect nonpotato hosts more often than those from the older population. Expansion of the effective host range may be one of the mechanisms involved in pathogenic changes in natural populations of P. infestans and to determine its significance, it is necessary to establish if the pathogen strains on nonpotato hosts represent distinct genotypes/populations or are freely exchanging with those on potato. This article reports characterization of P. infestans isolates from four solanaceous hosts (black nightshade, hairy nightshade, petunia, and tomato) growing within and around fields of blighted potatoes in four U.S. locations and one U.K. location and their comparison with isolates collected from adjacent infected potatoes. Isolates were characterized for mitochondrial DNA haplotype, mating type, metalaxyl resistance, allozymes of glucose-6-phosphate isomerase and peptidase, and DNA fingerprint with the RG57 probe. Analysis showed close similarity of the petunia, hairy and black nightshade isolates to potato isolates. However, tomatoes from New Jersey and Pennsylvania, respectively, were infected by two distinct and previously unreported pathogen genotoypes, which had quite different fingerprints from P. infestans isolates recovered from nearby infected potatoes. Potato growers should be aware that both weed and cultivated solanaceous species can be infected with P. infestans and may serve as clandestine reservoirs of inoculum. Because some of these plants do not show conspicuous symptoms, they may escape detection and fail to be either removed or treated and so may play a major role in the introduction and spread of pathogens to new locations.