1a. Objectives (from AD-416):
Objective 1: Develop monitoring methods for integrated pest management of potato insect pests. Objective 2: Develop bio-intensive methods to manage insect vectors of zebra chip and purple top diseases. Objective 3: Develop and apply baits or attract-and-kill control technology based on semiochemicals and toxicants.
1b. Approach (from AD-416):
Objective 1: Identification of the psyllid pheromone will involve: 1)olfactometer assays to determine male psyllid behavioral responses to females and female volatile chemical samples; 2) analytical chemical procedures to isolate and characterize pheromone compounds; and, 3) olfactometer and field assays to confirm pheromonal activity. Development of psyllid pheromone as a monitoring tool will be done by: 1) determining field attractiveness of pheromone; 2) comparing psyllid captures in traps over a range of pheromone release rates; 3) comparing psyllid captures in a variety of traps baited with pheromone; and 4) comparing the effectiveness of the optimized trap and pheromone lure versus the standard monitoring methods. Objective 2: Factorial experiments will determine interacting roles of insect vector density and potato plant growth stage on disease symptoms. Experiments will evaluate beet leafhoppers as vectors of the purple top disease pathogen and potato psyllid as vectors of the zebra chip disease pathogen. We will determine the time needed for an uninfected psyllid to acquire the Liberibacter pathogen from foliage, and the time needed for an infected psyllid to transmit the pathogen to an uninfected plant. We will determine the relative susceptibility of potato cultivars to zebra chip disease by comparing disease incidence and severity in varieties of potato in field cages, with timed numbers of infective potato psyllids. Objective 3: We will develop a toxicant bait for attracting and killing wireworm larvae when applied to potato fields at spring planting to provide protection of tubers at the end of the season. Baits will be tested in the field to determine efficacy against wireworms.
3. Progress Report:
Locations of the causal organism of zebra chip disease of potato (Candidatus Liberibacter solanacearum) in the potato psyllid were examined using fluorescent in situ hybridization. Results indicated Liberibacter was located throughout the insect body including the gut, bacteriome, hemolymph, and salivary glands. Compared with adults, nymphs were less likely to harbor Liberibacter and less likely to infect host plants. Ongoing screening assays identified six wild potato germplasm lines with putative resistance to the potato psyllid. Potato psyllid was shown to be surprisingly cold-hardy, which forces scientists and growers to seriously consider that the insect may be able to overwinter under local winter conditions. Animal proteins were found to be highly stimulatory to feeding wireworms. Efforts to incorporate animal proteins into baits for use as monitoring tools or in attract-and-kill programs are ongoing. Field studies further confirmed that zebra chip-infected potato tubers do not significantly contribute to the disease spread as these tubers generally do not sprout and if they do, usually produce disease-free and short-lived potato plants. This information will greatly benefit seed certification agencies and promote national and international trade of fresh potatoes. For a second year, it was determined that asymptomatic potato tubers produced by plants infected with liberibacter late in the season could develop zebra chip symptoms in storage. This is particularly important for the Pacific Northwest where the potato psyllid colonizes fields late in the season and where most of the potatoes are stored for several months pending processing. This information will help potato producers make informed decisions on whether to store tubers from fields impacted by zebra chip. In collaboration with several private and public potato breeders, potato clones were screened for zebra chip resistance. Six of the advanced potato breeding lines showed some tolerance to zebra chip. This information enables the development and adoption of better and sustainable management strategies for zebra chip. It was found that applications of Cyazypyr are effective in killing the potato psyllid rapidly. However, although Cyazypyr insecticide effectively suppressed psyllid populations, it did not prevent transmission of liberibacter to potatoes. Information from this research will be incorporated in integrated pest management programs for potato psyllid. In collaboration with ARS and university scientists, the zebra chip bacterium was found on several important crops in Central America (Nicaragua, Honduras, and El Salvador), including tomato, pepper, eggplant, and tobacco. The genetic variants of the potato psyllid in U.S., Mexico, and Central America were identified and their spatial and temporal distributions described.
Nelson, W.R., Sengoda, V.G., Alfaro-Fernandez, A.O., Font, M.I., Crosslin, J., Munyaneza, J.E. 2013. A new haplotype of 'Candidatus Liberibacter solanacearum' identified in the Mediterranean region. European Journal of Plant Pathology. 135:633-639. DOI:10.1007/s10658-012-0121-3.