Cotton genotypes vary in foliar elicitation of plant defenses by fungal spores and subsequent effects on aphids

Fungal entomopathogens for biological control are typically applied as foliar sprays and evaluated on their ability to kill insects. If and how plants respond to these foliar treatments is largely unknown. Moreover, spore viability following application can be affected by environmental factors such as UV light exposure, resulting in both live and dead spores as potential elicitors of responses on the plant surface. We tested whether foliar treatments of viable or dead fungal spores affect the defensive responses of cotton (Gossypium hirsutum) against herbivory by the cotton aphid (Aphis gossypii). Live or dead spores of three fungi (Beauveria bassiana, Phialemonium inflatum, Chaetomium globosum) were evaluated for their effects on aphid performance and plant defensive gene expression when applied to two different genotypes of cultivated cotton (DP0912 and PHY367). We found strong effects of both plant genotype and spore viability on both aphid performance and plant defensive gene expression. Aphid population growth was significantly higher on PHY367 plants treated with dead spores of B. bassiana or C. globosum. No other combination of plant genotype and spore status (viable or dead) was different from controls. Treating plants of both genotypes with the plant defense elicitors methyl jasmonate and Actigard 50WG yielded the same effects on aphid performance as the fungal spore treatments indicating that plant defensive responses to dead spores were likely responsible for the observed effects on aphid reproduction. Moreover, foliar application of Chitosan suggested that MAMP recognition might play a key role in the responses of PHY367 plants to dead spores. Defensive gene expression analysis corroborated our hypothesis that fungal spores induce changes in defensive phytohormone signaling and that variation in these responses is strongly affected plant genotype.