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.
1. Threat of potato psyllid overwintering on weeds in the Pacific Northwest assessed. Zebra chip, a new and important disease of potato, is caused by the bacterium Liberibacter, transmitted to potato by the potato psyllid. The psyllid host plants are not limited to crop plants, but also include many wild nightshade weeds, which could potentially impact overwintering and breeding of the psyllids and serve as reservoirs for Liberibacter. The potato psyllid was recently reported to overwinter on bittersweet nightshade in the Pacific Northwest. Researchers at USDA-ARS Wapato, and Prosser in Washington and Oregon State University assessed genetic variants of the potato psyllid populations found on bittersweet nightshade in Washington and Oregon and whether these insects were infected with Liberibacter. It was determined that most of the psyllids overwintering on the nightshade were predominantly of the Northwest genetic type and were not carrying the bacterium. This information suggests that these psyllid populations do not pose an imminent threat of zebra chip pathogen transmission to newly emerging potatoes and other related crops in the region, unless a source of the bacterium becomes available.
2. Genetic types of potato psyllid populations in Mexico and Central America identified. The potato psyllid is a serious insect pest of potato and other related crops in North and Central America, where it transmits the bacterium Liberibacter that causes zebra chip disease of potato. Recent studies identified three genetically distinct types of the potato psyllid in U.S., correlating to geographical regions: Central, Northwestern, and Western. Researchers at USDA-ARS Wapato and Prosser in Washington, University of Texas, and Zamorano University in Honduras assessed presence of the different genetic types of the potato psyllid throughout Mexico and Central America. It was determined that potato psyllids from Mexico, El Salvador, Honduras, and Nicaragua, were identified as belonging to the Central variant, which occurs in the central states of U.S. This information suggests that potato psyllids invading potato crops in the Pacific Northwest are not originating from Mexico or Central America and control of this insect pest should consider their movements within the western U.S.
3. A new method for rapid screening of food-based baits to determine attractiveness to wireworms. Development of an optimal bait for monitoring wireworms is hampered by the need to screen a large array of competing food- or grain-based formulations, each of which may show some level of attractiveness in the field. An ARS scientist at the Yakima Agricultural Research Laboratory developed an assay method in which 48-hour weight change of feeding wireworms is used as a proxy for consumption rate. The method allows scientists to quickly screen competing formulations for attractiveness, providing the means to rapidly eliminate less-than-fully attractive formulations without the need for labor-intensive field assays. Advances made in these studies could eventually lead to an optimally attractive toxicant-laced bait for use in an attract-and-kill control program for managing wireworms in potatoes, or to a bait useful for monitoring these pests.
4. Liberibacter reported for the first time on important solanaceous crops in Central America. Liberibacter is an economically important plant pathogen that severely damages several crops including potato, tomato, pepper, and other related crops. This bacterium is transmitted to these crops by the potato psyllid, a serious insect pest in U.S, Mexico, and Central America. Researchers at USDA-ARS Wapato, Washington, and Albany, California, University of Texas, and Zamorano University in Honduras discovered and reported for the first time that this plant pathogen was infecting tomato, pepper, eggplant, and tobacco crops in Honduras, El Salvador, and Nicaragua, causing significant losses to the vegetable and tobacco industry. These crops are economically important to Central America; therefore, it is imperative that both Liberibacter and its insect vectors be effectively monitored and managed to minimize their threat to the region food security.
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.