Start Scouting for Stewart's Wilt

Over the past week Stewart's wilt symptoms have been observed in some seed corn in southwestern Ontario. Although most field corn varieties are resistant to or tolerant to the bacteria there are differences in seed corn inbred lines and sweet corn hybrids susceptibility to Stewart's wilt. As a result Stewart's wilt can be devastating in seed corn and sweet corn production. Although the disease can be detected every year, Ontario growers have not experienced significant losses since 2002.

An annual seed corn and commercial corn disease survey conducted by Agriculture and Agri-Food Canada (AAFC) Ottawa (Zhu, Reid, Woldemariam) in conjunction with OMAFRA Ridgetown (Tenuta, Van Herk) support these observations (Figure 1). From 1999 to 2002, the percentage of Ontario fields surveyed that had Stewart's wilt ranged from 30 to 55 %. The following three years were very low and not until 2006 did we see an increase (20%).

Figure 1. Stewart's Wilt Survey results indicate the disease could be on the rise again
after a brief reprieve

Figure 2. Corn Flea Beetle populations in most years mirror those of Stewart's wilt. In other words, the more corn flea beetles present the more disease we see.

The corn flea beetle is the predominant vector of the disease in Ontario and as shown in Figure 2, the disease is highest in years when corn flea beetle populations are also high. Mild winters allow more corn flea beetles to survive the winter. In contrast, our Canadian cold winters kill most of the flea beetles. The beetles move from grassy areas into adjacent corn plants, therefore, the incidence of Stewart's disease is usually higher along grassy areas of fields or in areas with grass weed pressure.

What had the insect models forecasted for 2007?

Since the early 40's the Steven-Boewe's model has been used to alert growers and company representatives of the potential risks of Stewart's Wilt. This bacterial disease, when infected early, causes severe wilting and death of corn seedlings. However when infection occurred later in the season, which is more the case in Ontario, the foliage shows long wavy streaks which later turn yellow, then brown giving a leaf blight appearance. Not only are there losses in corn yields but the most serious impact may be on export of seed from fields with confirmed diagnosis of Stewart's Wilt. Several countries include the Stewart's Wilt bacterium among their quarantined pests. In those countries, quarantine regulations require seed from fields with confirmed Stewart's Wilt disease must be tested by a certified laboratory prior to export and the seed be certified as pathogen-free. Stewart's Wilt can be confirmed using a commercially available antibody-based detection kit.

The insect vector of this bacterial disease is the corn flea beetle (Figs. 3 and 4). All of the known predictive models attempt to determine the rate of insect survival. The greater the winter mortality of this pest, the less risk of Stewart's Wilt.

Figure 4. Stewart's Wilt Symptoms

The predictive insect model developed by Steven & Boewe, summed the average daily air temperature for the winter months of December, January and February. The developers set a criteria that ranked the survival risk from Moderate-Severe to Trace.

More recently researchers at Iowa State developed their own model. They averaged the air temperatures during these same winter months of December, January and February, then counted the number of those months where the average temperatures were equal or greater than -4.4C. The warmer the months, the greater risk of corn flea beetle survival, thus increasing the risk of more insects spreading the Stewart's Wilt bacteria.

Even more recently researchers are using average monthly soil temperatures for these same winter months.

What did each of these models predict?

It would be helpful if each model predicted the same risk levels for each of the locations studied. However, even though there are variations in the predictions, there appears to be a trend. Fields in southwestern Ontario experienced winter weather conditions that appeared to favour the survival of the corn flea beetle across all three models.

It was indeed another strange winter we experienced where the first two months were warmer than the months of February, and also March. March is not included in any of these models and it may have played the significant role in killing these insects. Since no one included March in their predictive models we will learn this summer how significant March weather conditions were for the survival of the corn flea beetle.

The ability to accurately predict Stewart's Wilt of corn can allow for better management of the disease and the corn flea beetle. Field evaluations of the various disease predicting models is one component of a larger "Best Management Practices for Ontario Seed Corn Project" that is under way. This project is supported by the Seed Corn Growers of Ontario which obtained funding through contributions by Canada and the Province of Ontario under the Canada-Ontario Research and Development (CORD) Program, an initative of the federal-provincial-territorial Agricultural Plicy Framework designed to postion Canada's agri-food sector as a world leader. The Agricultural Adaptation Council administers the CORD program on behalf of the province.

1. Steven-Boewe - sum of the monthly average air temperatures

2. Iowa State Model - number of months >= -4.4C

3. Insect survival prediction based on average soil temperature ranges