Aphis gossypii Glover causes direct and indirect damage to Cucumis melo L. crops. To decrease the harmful effects of this pest, one of the most economically and environmentally acceptable options is to use genetically resistant melon varieties. To date, several sources of resistance carrying the Vat gene are used in melon breeding programmes that aim to prevent A. gossypii colonization and the subsequent aphid virus transmission. The results suggest that the resistance conferred by this gene is associated with a microscopic hypersensitive response specific against A. gossypii. Soon after aphid infestation, phenol synthesis, deposits of callose and lignin in the cell walls, damage to the plasmalemma, and a micro-oxidative burst were detected in genotypes carrying the Vat gene. According to electrical penetration graph experiments, this response seems to occur after aphid stylets puncture the plant cells and not during intercellular stylet penetration. This type of plant tissue reaction was not detected in melon plants infested with Bemisia tabaci Gennadius nor Myzus persicae Sulzer.
Xylem colonization by Xylella fastidiosa promotes physiological, biochemical, and morphological alterations in citrus plants causing citrus variegated chlorosis (CVC) disease, which might infiuence the feeding behavior of vectors of this bacterial pathogen and its spread in citrus groves. By using the electrical penetration graph technique, we compared the numbers and durations of stylet penetration activities by adults ofthe sharpshooter vector Bucephalogonia xanthophis (Berg) (Hemiptera: Cicadellidae) on healthy and X. fastidiosa-infected sweet orange seedlings {Citrus sinensis (L. ) Osbeck, cv. Pera). Infected plants were either symptomatic, exhibiting the typical CVC, symptoms or totally asymptomatic. The mean time needed to contact xylem and start xylem sap ingestion after the onset ofthe first probe was similar among treatments. However, the average time elapsed between the onset ofthe first probe and the beginning of sustained xylem ingestion (>5 min) was longer on plants with CVC symptoms than on infected asymptomatic or healthy plants. In addition, the length of time spent in ingestion activities was much shorter on symptomatic plants. Our results showed that CVC symptomatic citrus plants were a less acceptable host than uninfected or asymptomatic X. /astidiosa-infected plants. Furthermore, our results support the hypothesis that symptomless infected citrus trees may be more important as sources for CVC spread than severely diseased ones.
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