Relationship Between Late-Season Severity of Stewart's Bacterial Wilt and Seed Infection in Maize

Abstract

The relationship between the amount of foliar disease on maize plants and seed contamination by the causal bacterium, Pantoea stewartii, was studied by comparing disease severity on adult plants with results from laboratory seed tests. Seventy-seven naturally infected maize lines (25 in 1990 and 52 in 1992) 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 to 2%, 6 to 14%, 25 to 34%, 35 to 49%, 50 to 74%, and 75 to 100%. Ears were harvested from 10 to 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 ≥25% disease severity; however, 45 of 63 such seed lots had no infection. Three seed lots had >5% infected seeds, all from plants with >50% disease severity. The results suggest a possible threshold level between 15 and 25% disease severity for Stewart's wilt on leaves before bacteria are detected in seed. This study describes a relatively simple method for estimating the incidence of infected seeds in a seed lot and contributes additional evidence to indicate that the chance of spreading P. stewartii from U.S.-produced maize seeds is low.

Geographical and Temporal Dynamics of Chaetocnema Pulicaria and Their Role in Stewart's Disease of Corn in Iowa

“…Alternatively, a seed corn company could attempt to prove that the seed was not infected by paying for costly growout or ELISA tests to prove that a seedlot was free of Stewart's disease, thereby, allowing for its exportation (Block et al, 1999).…”

“…This disease has tremendous economical implications considering that the prevalence of Stewart's disease in Iowa was 25% of seed production fields in 1998 and 58% in both putter et al, 1998Esker and Nutter, 2000). This high prevalence would require either costly grow-out tests or enzyme-linked immunosorbent assays (ELISA) (on seed) be performed by companies to verify that the seed was indeed fiee ofPantoea stewartii (Block et al, 1999).…”

Quantifying the Feeding Periods Required by Corn Flea Beetles to Acquire and Transmit Pantoea stewartii

“…Michener et al (18) reported that seed-toseedling transmission was 0.038% after testing 58,300 kernels obtained from infected plants. Studies by Block et al (2)(3)(4) revealed only 0.14% seed-to-seedling transmission of P. stewartii from infected kernels harvested from artificially inoculated plants (29 of 19,735 seed), and 0.022% (1 of 4,563) seed-to-seedling transmission from P. stewartii-infected kernels harvested from naturally infected plants (3).…”

“…Early reports suggested that seed transmission played an important role in the occurrence of Stewart's disease (24,30). However, recent studies by Michener et al (18), Block et al (2)(3)(4), and Khan et al (12) all have shown that the risk of seed-to-seedling transmission of P. stewartii is extremely low. In their study, Khan et al (12) grew 75,000 plants from seed that was harvested from P. stewartiiinfected plants and did not find any seedto-seedling transmission of P. stewartii in the test plants; however, they did not determine the actual percentage of P. stewartii-infected (or infested) seed.…”

“…In their study, Khan et al (12) grew 75,000 plants from seed that was harvested from P. stewartiiinfected plants and did not find any seedto-seedling transmission of P. stewartii in the test plants; however, they did not determine the actual percentage of P. stewartii-infected (or infested) seed. Both Michener et al (18) and Block et al (2)(3)(4), however, adjusted their estimates of seedto-seedling transmission to account for the percentage of infected seed in a seedlot. Michener et al (18) reported that seed-toseedling transmission was 0.038% after testing 58,300 kernels obtained from infected plants.…”