Alexandria, Virginia, USA
October 7, 2009

Source: American Seed Trade Association (ASTA) Newsletter Vol 9 Issue 20

In September, ASTA was awarded a grant under USDA's Technical Assistance for Specialty Crops (TASC) to improve diagnostics for Stewart's wilt (Pantoea stewartii) on corn. Current diagnostic capabilities have technical challenges that can either produce false positives or be untimely in their detection of the disease, a bacterium spread primarily through a corn flee beetle which can stunt and damage the growth of a corn stalk which either then dies or fails to produce a marketable ear.

ASTA was made aware that the ELISA test for Stewart's wilt, currently recognized by most countries, has been providing what is believed to be unusually high numbers of false positives. Many countries require that corn seed be tested for Stewart's wilt as a condition of entry. If the test is providing false positives, many shipments of corn seed could potentially be rejected based on inaccurate test results. Scientists from ASTA's member companies and Iowa State University (ISU) suspect that there are previously unrecognized species of microorganisms closely related to the Stewart's wilt pathogen that may be responsible for these questionable positive results. For example, recent reports of a maize disease caused by P. agglomerans in Mexico have come to light. Occurrences of other Pantoea strains on maize and sorghum from tropical areas have also been recently reported. P. stewartii has been considered distinct from all other maize bacterial pathogens, so it is possible that these new strains and isolates could also be reacting to the reagents in the current ELISA test.

In addition to the ELISA test for Stewart's wilt, there is a PCR-based diagnostic protocol developed in the European Union. If confidence in the accuracy and dependability of the ELISA test erodes internationally, it is feared that other countries will begin mandating the use of this PCR-based method. The European and Mediterranean Plant Protection Organization (EPPO) bulletin outlining this protocol contains an extensive flow chart that, when followed, often requires weeks to pass a single seed lot for export. The U.S. seed industry has already incurred problems with the lengthy and potentially non-specific EPPO assay where delay of planting and destruction of seed lots have been threatened, but later retracted due to erroneous results obtained from the EPPO protocol. This protocol requires confirmation of suspect positives by PCR, and false positives have been documented.

ASTA submitted a grant proposal prepared by the ISU Seed Science Center to the TASC program to address and hopefully resolve this problem. The proposal was favorably received, and an award of $82,750 was granted. The research will be performed over a two year period by ISU scientists through a contract with ASTA. This project will address three goals: review and re-optimize all components of the Stewart's wilt ELISA kit; conduct an evaluation of the EPPO PCR-based protocol; and characterize isolates of other species in the genus Pantoea that may be responsible for causing the ELISA test to produce false positives.

TASC is a program within USDA's Foreign Agricultural Service (FAS) that provides Commodity Credit Corporation (CCC) funds to eligible organizations, on a grant basis, to implement activities that are intended to address a sanitary, phytosanitary or related technical barrier that prohibits or threatens the export of U.S. specialty crops currently available on a commercial basis. The TASC program is intended to benefit the represented industry rather than a specific company or brand. Examples of expenses that CCC may agree to cover under the TASC program include, but are not limited to: initial pre-clearance programs, export protocol and work plan support, seminars and workshops, study tours, field surveys, development of pest lists, pest and disease research, database development, reasonable logistical and administrative support, and travel and per diem expenses. Eligible projects may take place in the United States or abroad.

"We are very excited that the USDA supported this project," notes Ric Dunkle, ASTA's senior director for seed health and trade. "We continue to be confronted with problems associated with discrepancies over seed testing methodologies and results for phytosanitary pests. It is unfortunately quite common that inaccurate and inconsistent test results disrupt seed movements, thereby causing unnecessary economic impacts to our industry. "

ASTA is currently exploring additional opportunities that may qualify under the TASC program which will facilitate seed trade for its members.

Website: http://www.amseed.org

Published: October 7, 2009