Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/1/1999
Publication Date: N/A
Interpretive Summary: Stewart's bacterial wilt of corn is a plant disease that can act as a major trade barrier for U.S. seed corn producers who want to export seed to international markets. More than 100 countries have quarantine restrictions that prevent the importation of corn seed unless the seed is certified as Stewart's wilt-free. Seed is often certified by a field inspection of the growing plants, but the current regulations (set by the importing country) have no tolerance for Stewart's wilt. Finding even a single plant with a few leaf spots will cause the field to fail inspection. The producer's only options are then to wait until harvest and have the seed tested for bacteria or not to export the seed. Our results showed that the simple presence of Stewart's wilt in a field was a poor indicator of the likelihood for seed infection. Instead, there was an excellent correlation between a corn variety's disease resistance and seed infection. Resistant plants are able to limit the growth and damage caused by the bacteria to relatively small areas on the leaves. Seed infection did not occur in any of the resistant or moderately resistant varieties that we tested. We found infected seed only in varieties where bacteria were able to spread from the leaves to the stems, and eventually to the ear, by traveling in the water- conducting vessels of the plants. These findings provide valuable data for the seed corn industry and the American Seed Trade Association in their efforts to negotiate international quarantine regulations for this important disease that can be justified on a scientific basis.
Technical Abstract: This study examined the relationship between Stewart's bacterial wilt severity on leaves of adult corn plants and the resulting incidence of infected seed. Leaf disease severity was rated late in the growing season (Sep 1 to Sep 10) and seed harvested from the plants was tested for Pantoea stewartii (syn. Erwinia stewartii) infection. Seventy-seven naturally-infected maize lines were selected and assigned to one of six disease severity classes, based on the percentage of ear leaf tissue killed by Stewart's wilt: trace-2, 6-14, 25-34, 35-49, 50-74, and 75-100 percent. Ears were harvested from 10- 20 plants representative of the disease class for each maize line and seeds were tested by enzyme-linked immunosorbent assay (ELISA) of bulk-seed samples. Seed infection percentages were estimated from the bulk-test results by using statistical equations for group testing. The accuracy of the bulk-seed method for estimating seed infection was validated by comparison with 300-kernel single-seed tests. Infected seed was detected only from seed of plants with greater than or equal to 25 percent disease severity, however 45 of 63 such seed lots still had no infection. Three seed lots had >5 percent infected seeds, all from plants with >50 disease severity. The results suggest a possible threshold level between 15 and 25 percent disease severity for Stewart's wilt on leaves before bacteria are detected in seed. This study describes a relatively simple bulk-seed testing method for estimating the incidence of infected seeds in a seed lot and contributes additional evidence to indicate that the chances of spreading P. stewartii from US-produced maize seeds are low.