The Potential for Dickeya dianthicola to be Vectored by Two Common Insect Pests of Potatoes

Dickeya dianthicola (Samson) causing blackleg and soft rot was first detected in potatoes grown in Maine in 2014. Previous work has suggested that insects, particularly aphids, may be able to vector bacteria in this genus between plants, but no conclusive work has been done to confirm this theory. In order to determine whether insect-mediated transmission is likely to occur in potato fields, two model potato pests common in Maine were used: the Colorado potato beetle (Leptinotarsa decimlineata Say) and the green peach aphids (Myzus persicae Sulzer). Olfactometry and recruitment experiments evaluated if either insect discriminates between infected and uninfected foliage. To determine whether other insect species may display discriminatory recruitment, pitfall traps and adhesive trap cards were set up beside infected and uninfected plants placed in the field. In the laboratory, beetles and aphids were fed plants infected with Dickeya dianthicola and then transferred onto uninfected plants to determine if bacteria would be transmitted between plants. Both plants and insects were sampled and tested using polymerase chain reaction (PCR) for Dickeya spp. In 2017 and 2018, a single potato field was divided into five rows of four plots with randomly assigned insecticide treatments to selectively suppress aphids and/or Colorado potato beetles. Disease spread was monitored among plants in 2017, and tubers were harvested in both years to test for Dickeya. Neither Colorado potato beetles nor green peach aphids were attracted to infected foliage in either olfactometry or recruitment experiments. To the contrary, the presence of 2,3-butanediol, which is product of Dickeya fermentation, significantly reduced beetle attraction to the odor of potato foliage. Green peach aphids preferred uninfected foliage, but only when conspecifics were present. Flea beetle (Coleoptera: Chrysomelidae: Alticinae) captures by adhesive cards were affected by the infection status of provisioned potato plants, but the effect was inconsistent between the dates of trap deployment. Neither Colorado potato beetles nor green peach aphids acquired and transmitted D. dianthicola through feeding on infected plants in the laboratory. In the field, neither insect’s abundance correlated significantly with the spread of this disease. This study did not find indications that D. dianthicola is vectored by either Colorado potato beetles or green peach aphids. Therefore, controlling these pests is unlikely to prevent blackleg outbreaks in potato fields. Instead, the efforts to limit spread of this disease should focus on sanitation, water management, and seed screening