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Project description

Biology and overwintering of the corn leafhopper, Dalbulus maidis, and corn stunt spiroplasma, Spiroplasma kunkelii, and epidemiology of corn stunt disease in the San Joaquin valley. (02FE025)
Program UC IPM competitive research grants program
C.G. Summers, Entomology, UC Davis/Kearney Agricultural Center
Host/habitat Corn
Pest Corn Leafhopper Dalbulus maidis; Corn Stunt Spiroplasma Spiroplasma kunkelii; Corn Stunt Disease
Discipline Plant Pathology
Applied Field Ecology
Start year (duration)  2002 (Two Years)
Objectives Determine the overwintering capabilities and overwintering sites of corn leafhopper and corn stunt spiroplasma in the San Joaquin Valley.

Utilize Geographic Information Systems(GIS) technologies in the surveillance and monitoring of corn stunt disease and it leafhopper vector to describe disease epidemiology.

Final report A three-year study was conducted in the southern San Joaquin Valley of California to determine the overwintering survival of the corn leafhopper, Dalbulus maidis (DeLong and Wolcott) and the mollicute, Spiroplasma kunkelii, the corn stunt spiroplasma (CSS), the causal agent of corn stunt disease. Corn leafhopper populations were sampled from November to March using yellow sticky cards, D-vac suction samples and inspection of volunteer corn plants and spring planted corn. S. kunkelii presence was determined by sampling sentinel plants placed in the field during the winter, leafhoppers collected throughout the winter, evaluation of volunteer plants over the winter, and the sampling of spring planted corn. Leafhoppers were collected on yellow sticky cards in every month throughout the winter during all three years. They were also regularly recovered from alfalfa, Medicago sativa L., winter forage (wheat, Triticum aestivuum L. and triticale, Triticale hexaploide Lart.) and riparian areas by D-vac suction sampling. Females constituted the majority of leafhoppers (>80%) recovered on both sticky cards and from D-vac samples. S. kunkelii was recovered from leafhoppers throughout the winter from sentinel plants placed in the field and in spring planted corn found to contain leafhopper adults shortly after seedling emergence. Volunteer plants were determined to be a critical key in leafhopper overwintering and consequently the survival of S. kunkelii. Volunteers extended the season by as much as two months, thus shortening the period of time the leafhoppers were forced to go without.

ArcView 8.3 software, developed by Environmental Systems Research Institute, was used to digitize trap locations to create a "trap layer" over existing reference maps such as Department of Water Resources' land use boundaries and California Spatial Information Library roads, and Township, Range and Section maps. Each week, starting in April 2003, reports of trap counts were provided to the Kearney Agricultural Center (KAC) Geographic Information System (GIS) staff, and the data were added to the trap layer map as attributed information. A new map was created with a standardized legend and provided as a .jpeg image on the KAC GIS Web site.

Studies continued on the overwintering biology and control of corn leafhopper and corn stunt disease. Leafhopper survival was determined using yellow sticky cards, examination of volunteer plants and D-vac samples. Spiroplasma survival was determined using overwintering corn plants, sentinel corn plants, and PCR analysis of leafhoppers. The role of volunteer corn in overwintering survival of corn leafhopper, corn stunt spiroplasma, and the epidemiology of corn stunt was evaluated. Corn leafhopper adults were recovered in D-vac samples from November (the end of harvest season) to March (the beginning of the new season). Leafhoppers were recovered from alfalfa, weeds, and winter forage. Adult leafhoppers were recovered on yellow sticky cards during the same period with the exception of January when temperatures were too low to permit flight. The most important discovery was the presence of leafhoppers, both adults and nymphs, in volunteer corn plants. The winter of 2002-03 was extremely mild. Although the tips of the leaves from volunteer corn plants were frozen, the whorl remained unfrozen and intact. Leafhoppers burrowed deep into the whorls and survived the winter. Twenty volunteer corn plants were examined for the presence of leafhoppers in January, February, and March at the Kearney Research and Extension Center in Fresno County. One adult and 0.3 nymphs per plant were recovered during this time. This is significant since nymphs are not known to overwinter in this area. We also found approximately 50% of the adult population was composed of males. New-season corn was planted adjacent to the volunteers on March 12, it emerged on March 24, and on April 7 & 8, 15 of 132 plants examined contained one adult leafhopper each, five of which were infected with spiroplasma. Based on sticky card traps, leafhoppers densities were higher in 2003 than in 2002. In 2003, densities were higher in Tulare county that in Kings county. Lemoore again was the center of the leafhopper density within the three-county area.

Since its discovery in the San Joaquin Valley it has been assumed that corn leafhopper could not overwinter but was reintroduced annually from Mexico. Recent events, however, suggest that leafhopper does overwinter here. Beginning in October 2001, studies were conducted to determine the success of leafhopper overwintering in the San Joaquin Valley and the location of overwintering sites. Corn leafhopper populations were monitored using yellow sticky cards, taking D-vac suction samples, caging leafhoppers over possible overwintering hosts, sampling volunteer corn, and placing sentinel corn plants in areas suspected of harboring leafhoppers. We evaluated overwintering of the corn stunt spiroplasma within the adult leafhoppers and volunteer corn using PCR and ELISA. Adult leafhoppers were captured on yellow sticky cards and in D-vac suction samples continuously from October through late March although no corn was present. By late March, leafhoppers were collected from newly emerged corn. Adults were found in D-vac samples from October through March. We determined that volunteer corn makes a significant contribution to leafhopper overwintering by extending the period of time food is available. Leafhoppers and sentinel plants were positive for corn stunt spiroplasma indicating that the pathogen also overwintered. Leafhoppers were discovered in all Valley counties from Kern to Sacramento, Yolo and Solano counties. Corn stunt spiroplasma was detected in leafhoppers from Sacramento County. The spread of the leafhopper and corn stunt disease was monitored using GIS. These studies confirmed that leafhoppers and corn stunt disease appear first and with the greatest severity around the Lemoore area, and spread from there through the region. Further analysis is under way to describe the epidemiology of corn stunt disease.

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