| dc.description.abstract | Two genotypes of cotton DeltaPine 0912 and Phytogen 367, three species of fungi Beauveria bassiana, Phialemonium inflatum, Chaetomium globosum, and the cotton aphid Aphis gossypii were used in this study to test whether foliar treatments of fungal spores (viable or dead) affect the cotton plant and how the cotton aphid performs on these plants. Three-week-old plants were infested with two aphids and two weeks later the total number of aphids per plant was counted. Aphid population was significantly higher on Phytogen 367 plants treated with dead spores of B. bassiana or C. globosum, no other combination of plant genotype and spore status (viable or dead) were different than controls. Furthermore, plants of these two genotypes treated with the plant defense elicitors methyl jasmonate and Actigard 50WG yielded same results as fungal spore treatments denoting that changes in the plant defense profile were responsible for the observed aphid results. Foliar applications of Chitosan provided clues that chitin recognition might be the reason why Phytogen 367 plants responded to dead spore applications. Gene expression analysis corroborated the hypothesis that fungal spores induce changes in the plant defense hormone profile, however it was found (in consistency with the preceding experiments) that only one plant genotype showed significant differences in gene expression. Moreover, the gene expression from plants treated with viable spores was opposite to the one from dead spores.
As a follow up experiment, microscopic observations of the leaves were done to determine the behavior of spores on the leaf surface following treatment. No spore germination was observed at 1, 3 and 7 days post treatment, suggesting spores undergo a state of fungistasis upon landing on the leaves. However, it was found that enrichment of spore suspensions with molasses combined with high humidity induced the germination of all three species of fungi. Moreover, upon germination, fungal leaf colonization through stomata openings was observed. Additionally, it was found that the fungus B. bassiana was able to reproduce on the surface of cotton leaves through the process of microcycle conidiation. | |
