Study of the effects of an elicitor (Acibenzolar-S-Methyl) on melon resistance to two viruses (CYSDV and CMV) and on the behavior of their respective vectors (Bemisia tabaci and Aphis gossypii)

Abstract

In California, melon crops are strongly impacted by whiteflies and aphids-transmitted viruses. Despite attempts to control their vectors populations, the disease pressure is still high and melon production is impacted. As California is the main production area for melons in the United States, it is an important economic issue. A synthetic elicitor could help to improve natural plant defenses against these pathogens. It was shown that the molecule Acibenzolar-S-Methyl (ASM) is able to induce the systemic acquired resistance (SAR) on plants. In this master thesis, the effects of ASM are analyzed on melon resistance to two viruses: the Cucurbit yellow stunting disorder virus (CYSDV) and the Cucumber mosaic virus (CMV). The ASM effects will be also analyzed to evaluate its ability to disrupt the feeding behavior of a CMV vector: Aphis gossypii, and its effects on the aphid feeding preferences. The results of CYSDV-inoculated plants cannot be presented as the ASM rate applied was wrong and caused strong plant weight reduction. Thereafter, the optimal ASM rate was evaluated to be between 0.5 and 2.5 mg ASM sprayed per plant. Half of the CMV-infected plants sprayed with 0.5 mg of ASM per plant displayed no symptoms at two weeks post inoculation, with a virus titer on ELISA twice lower compared to plants with symptoms. To evaluate the impact of ASM on feeding behavior of aphids, the electrical penetration graph (EPG) was used in another experiment. At 1.5 mg/plant, the aphids spent less time inside to ingest phloem sap and more time inside the intercellular pathway. A choice test was also performed between four leaves, under the ASM treatment or not, and CMV infected or healthy leaves. Almost no reliable results were obtained because of technical problems, but there is a trend for a preference for CMV-infected leaves without ASM after 30 minutes. We can conclude that ASM acts efficiently against CMV infection, but studies to find an optimal ASM rate and on virus transmission must also be conducted.