Yasmin Cardoza, Mentor

Rena Bryan studied what effect various Xanthomonas bacterial concentrations had on the volatile emissions of pepper plants. She also examined the feeding preference of the beet armyworm, Spodoptera exigna, on healthy leaves vs. those infected by Xanthomonas.

Putting internal standard into sample vials for GC analysis.

Inoculating one pepper plant in one of the dilutions of bacteria.

Feeding choice experiment. Healthy pepper plant is on the left; infected on the right. Leaves from each plant extend into the shallow cage at center, where beet armyworms are contained.

A look at the beet armyworm (third instar)

Rena's Abstract:

Chemical interactions between plant, pathogen, and herbivore

        A complete understanding of the interactions between the plant and its pathogenic and herbivorous pests will help to safely protect gardens and farms, securing optimal agricultural yields in the future. Plants possess a number of chemical defense mechanisms that are often induced by herbivore and/or pathogen attack. These chemical defenses can directly influence the development and survival of the pests or act indirectly by recruiting natural enemies of insect pests. Previous experiments have determined that pepper plants, Capsicum annuum, release a specific blend of volatile chemicals in response to attack by Xanthomonas campestris pv. vesicatoria bacterial infection. However, further experiments need to be conducted to understand the dynamics of plant volatile production in response to these organisms, and to determine if such volatiles are a product of the plant's active defense against the attacking pest.

        The objectives of my research were first, to study the effect of three concentrations (108, 104, and 102 cfu/mL) of Xanthomonas bacteria on the volatile emission by pepper plants. I found that plants released more volatiles in response to increasing amounts of bacterial inoculum. Second, I wanted to examine the effect of bacterial infection on the feeding preference of Spodoptera exigua, beet armyworm (BAW). Results from my experiment suggest that, in a dual-choice situation, BAW may prefer the infected pepper leaves to healthy counterparts.

        The identification of pathogen and herbivore induced plant volatiles and knowledge of their effect on the biology and behavior of pathogens and herbivores will greatly contribute to the development and improvement of environmentally sound methods for control of the pests. The data obtained from my study adds to the better understanding of the interactions between pepper plants, the bacterial leaf spot pathogen, and BAW. Additionally, the information obtained from my project provides the basis for the determination of thresholds for disease detection and defense activation. It is hoped that in the future, the data collected from other experiments like this will help in the enhancement of naturally produced chemical defenses in plants against pathogens and other pests.