Induction of Volatile Emissions from Peanut Plants in Response to Fungal and Insect Damage

Induction of Volatile Emissions from Peanut Plants in Response to Fungal and Insect Damage

Summary

In the present study we tested the effect of peanut, Arachis hypogaea L. (Leguminoseae), stem infection by the white mold fungus, Sclerotium rolfsii Saccodes (Basidiomycetes), on the feeding preference of beet armyworms, Spodoptera exigua Hübner (Lepidoptera:Noctuidae). We also evaluated and compared the evolution of volatile compounds from S. rolfsii fungal cultures and from peanut plants under individual or simultaneous attack by the white mold fungus and by beet armyworms.

Objectives
1) Analyze, identify, and compare compounds from head space collections from diseased and healthy plants.
Non-inoculated control plants released myrcene, linalool, (E)-4, 8-dimethyl-1,3,7-nonatriene throughout the duration of the experiment, but their amounts were relatively small compared to those released by white mold-infected peanut plants. Volatile profiles of fungi-infected plants consisted of b-pinene, the fungal-produced compound 3-octanone, (Z)-3-hexenyl acetate, myrcene, linalool, (E)-4, 8-dimethyl-1,3,7-nonatriene, methyl salicylate, nerolidol, and (E,E)-4,8,12-trimethyl-1,3,7,11-tridecatetraene.
2) Determine the effect of pathogen defense induction on the production and release of herbivore-induced volatiles by the host plant.

The emission of volatiles from peanuts in response to BAW-damage was not negatively affected by infection of white-mold fungus on the plant. In fact, white mold infected peanuts damaged by BAW released all the volatiles typical of a healthy plant damaged by BAW. Additionally, headspace collections from white mold/BAW-damaged plants also contained volatiles normally produced by plants in response to white mold infection alone. The compounds methyl salicylate and 3-octanone were only present in volatile emissions from white mold-infected plants and not in those of plants damaged by BAW alone.

3) Evaluate the effect of volatiles emitted from healthy, diseased, herbivore-damaged, and the combination of disease and herbivore damage on oviposition site selection by herbivores and on the host-searching behavior of parasitoids.

BAW feeding was not negatively affected by infection of S. rolfsii on peanut plants. To the contrary, larvae showed a significant preference for young and old leaves from fungi-infected plants over those from healthy, non-infected plants. This preference was more prevalent when insects were confined to young versus old leaves. Further experiments will be conducted to evaluate the effect of fungal infection on oviposition preference by adult herbivores and on host searching behavior by their parasitoids.

Conclusions: The feeding preference of BAW for leaves from white mold infected plant may indicate an interference of the fungal infection with the plant’s direct chemical defenses against the herbivores.

Peanut plants release a specific set of chemical signals in response to S. rolfsii infection. This volatile profile differs not only qualitatively, but also quantitatively from signals emitted in response to BAW damage.

Previous infection on the plant by the white mold fungus does not interfere with the emission of volatiles by the diseased plant in response to BAW attack.

The white mold-derived compound 3-octanone and the plant-produced methyl salicylate were only recovered from plants infected with S. rolfsii, thus the presence of these compounds could potentially be used in the future for the detection of infected plants in the field.