Re-evaluation of seed transmission of Clavibacter michiganensis subsp. nebraskensis in Zea mays

Authors: Block, Charles; SHEPHERD, LISA - Iowa State University; MBOFUNG, GLADYS - Iowa State University; SERNETT, JEFF - Monsato Seed Company; ROBERTSON, ALISON - Iowa State University

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/6/2018
Publication Date: 1/10/2019
Citation: Block, C.C., Shepherd, L., Mbofung, G., Sernett, J., Robertson, A. 2019. Re-evaluation of seed transmission of Clavibacter michiganensis subsp. nebraskensis in Zea mays. Plant Disease. 103:110-116. https://doi.org/10.1094/PDIS-02-18-0292-RE.
DOI: https://doi.org/10.1094/PDIS-02-18-0292-RE

Interpretive Summary: The spread of Goss's bacterial wilt and leaf blight of corn, caused by Clavibacter michiganensis subsp. nebraskensis, to a wider geographic area has generated concerns about the possible role of seed transmission in disease spread. The objectives of this research were to: (1) study the correlation between disease severity on seed corn plants and bacterial contamination of the seeds harvested from these plants, and (2) to assess how often an infected seed would grow into an infected plant. Infected seed lots were produced on resistant, susceptible, and moderately resistant hybrids. Plants were scored for disease severity throughout the season. As expected, the highest disease severity occurrred on the susceptible hybrid with 88% leaf area killed. The lowest disease severity occurred on the resistant hybrid with 1.6% leaf area killed. The two moderately resistant hybrids were intermediate, but had significantly more disease than the resistant hybrid. The percentage of infected seed ranged from 0% in some plots to an average of 21.6% for the susceptible hybrid. Seed transmission studies were conducted in field and greenhouse grow-outs, with 12 cases of seed transmission identified. A seed transmission rate was then calculated from the fraction of plants estimated to have grown from infected seeds (potential-positive plants). This approach eliminated dilution of the rate by healthy seeds and yielded an estimated seed transmission rate of 0.040% (12 events from 30,088 potential-positive plants). This means that one of every 2,500 infected seeds might grow into an infected plant, a very low frequency but still possible. This was the first study to closely examine the relationship between disease severity on the parent plants and resulting seed infection and also the first large scale seed transmission study. Based on the low rate of seed transmission observed and previous research on how disease spreads from a single source, it seems very unlikely that seed transmission could create a disease outbreak in one growing season. The risk of seed transmission has more relevance in the phytosanitary area, which involves the introduction of disease to areas where it does not occur. At this time, Goss's wilt has not been detected outside North America, and while the risk of seed transmission is very low, the risk is not zero if infected seeds are present in the seed lot. Fortunately, C. michiganensis subsp. nebraskensis is relatively easy to detect by seed health testing and at-risk seed lots can be readily identified.

Technical Abstract: The spread of Goss's bacterial wilt and leaf blight of corn, caused by Clavibacter michiganensis subsp. nebraskensis, to a wider geographic range has generated concerns about the possible role of seed transmission in disease spread. The objectives of this research were to: (1) assess the relationship between disease severity on parent plants and resulting seed infection, and (2) to conduct a large scale seed transmission study. Infected seed lots were produced on resistant, susceptible, and moderately resistant hybrids by mechanical pinprick leaf inoculation. Eighty plants per plot were rated biweekly for leaf disease severity. On the final assessment date the highest disease severity was observed in inoculated plots of the susceptible hybrid, with 88% average leaf area killed and 21.6% average seed infection. The non-inoculated plots of the susceptible hybrid averaged 27% leaf area killed and 5.1% seed infection. In contrast, the inoculated plots of the resistant hybrid averaged 1.6% leaf area killed with infected seed found in three of four replications, ranging from 1% to 6%. The non-inoculated plots of the same resistant hybrid averaged 1.2% leaf area killed, with seed infection found in two of the four replications, 1% and 2%, respectively. Disease severity for the two moderately resistant hybrids was not significantly different from the resistant hybrid among the non-inoculated plots, but was significantly higher in the corresponding inoculated plots. Seed transmission studies were conducted in field and greenhouse grow-outs in 2013 and 2014, with 12 events of seed transmission identified among 242,000 total plants. A seed transmission rate was calculated from the fraction of plants estimated to have grown from infected seeds (potential-positive plants). This approach eliminated dilution of the rate by healthy seeds and yielded an estimated transmission rate of 0.040% (12 events from 30,088 potential-positive plants), approximately one positive plant per 2,500 infected seeds. A one-sided 95% upper confidence limit was also calculated. The upper confidence limit of 0.065% suggests a maximum seed transmission and approaches the observed rate as more plants are tested. The nearness of the observed rate and the upper confidence limit indicates that a sufficiently large plant population was included to have good confidence in the seed transmission rate. This was the first study to closely examine the relationship between C. michiganensis subsp. nebraskensis infection on parent plants and resulting seed infection and also the first large scale seed transmission study. Based on the low rate of seed transmission observed and previous research on how disease spreads from a single source, it seems very unlikely that seed transmission could introduce enough inoculum to create a disease outbreak in one growing season. The risk of seed transmission has more relevance in the phytosanitary area. To date, Goss's wilt has not been detected outside North America, and while the risk of seed transmission is very low, the risk is not zero if infected seeds are present in the seed lot. Fortunately, C. michiganensis subsp. nebraskensis is relatively easy to detect by seed health testing and at-risk seed lots can be readily identified.