Seed corn is usually more affected than field corn.
By Heather Hager
Stewart’s wilt is caused by the bacterium Pantoea stewartii, which is transmitted from plant to plant by the corn flea beetle. Many field corn hybrids have adequate levels of tolerance against the bacterium, so its effects on yields in these hybrids are not usually economically injurious. Seed corn inbreds and sweet corn, however, can be quite susceptible to the disease, and it can be an issue for both the yield of sweet corn and quality of seed corn.
|In some years, Stewart’s wilt can have serious effects on mature corn plants.|
Xiaoyang Zhu, a biologist with Agriculture and Agri-Food Canada (AAFC) in Ottawa, performs yearly cornfield disease surveys with other biologists from AAFC and the Ontario Ministry of Agriculture, Food, and Rural Affairs (OMAFRA). According to Zhu, Stewart’s wilt occurs mainly in southwestern Ontario and occasionally in eastern Ontario, and has not been found in Quebec since 2000. The disease’s distribution is mainly related to its mode of transmission via the corn flea beetle. Thus, the disease can be tracked by understanding the beetle’s population dynamics.
Timing of infection
Corn plants can be infected early as seedlings or later as mature plants, depending on when they are exposed to the bacterium by the corn flea beetle. Other grassy plants and insect species may harbour the bacterium as symptomless reservoirs, but little is known about the extent of their involvement in the disease process.
The ability of the corn flea beetle to overwinter affects the risk of early infection. “The Stewart’s wilt bacterium overwinters in the gut of the corn flea beetle,” says Tracey Baute, field crops entomologist with OMAFRA. “The beetle can then transmit it to the young crop when the crop is just emerging.” Transmission occurs when the beetle takes a bite of the plant and then regurgitates or defecates into the wound. Early infection takes place in spring and often kills the young corn plants.
Baute explains that the disease is less prevalent in eastern Ontario and Quebec than in southwestern Ontario because the harsher winters in the more northerly areas usually kill many of the corn flea beetles. In southwestern Ontario, the beetle’s abundance in spring is related to the severity of the preceding winter. Colder winters kill off more beetles, reducing the risk of early infections in corn. Seed treatments may also help to reduce early infections, says Baute. “A lot of seed treatments now do control corn flea beetle, with the exception that the beetle still has to take a bite out of the plant and could transmit the bacterium.” Baute says that the beetles do not have natural predators that specialize on them, but wet weather can be detrimental to them in the spring because it benefits fungi that can kill the beetles.
The surviving beetles eat, potentially transmit the Stewart’s wilt bacterium, lay eggs, and die in late spring. The bacterium is not passed to the eggs, says Albert Tenuta, OMAFRA plant pathologist, so the new adults that emerge in June must acquire the bacterium by feeding on infected plants. They spread Stewart’s wilt to mature plants at around the tasseling stage. This is known as the late phase
“If we had a cold winter, we would observe few early season effects of Stewart’s wilt, unless it was an extremely susceptible variety,” says Tenuta. “We may see the disease later on in the season because both the corn flea beetles and the bacterium would have time to build up.” The late phase of infection may also be supplied by the movement of corn flea beetles from the Midwest Corn Belt on wind currents, says Tenuta. “The later they arrive, the less time there is to infect the plants, and therefore, the effect on the crop is less. But over the past number of years, we’ve been seeing Stewart’s wilt relatively early, which would indicate that we have a base level of infected corn flea beetles that has overwintered from year to year.”
OMAFRA has been evaluating several models that predict the risk of corn flea beetle survival over winter. Generally, these models use December to February temperatures, although Baute says that the extent of snow cover can also be a factor by providing the beetles with some insulation from extreme cold. Predictions of high risk in spring should prompt producers to monitor and manage corn flea beetle populations. “By managing the vector, particularly early in the season, you can reduce the potential impact of the flea beetle and limit its feeding so you have less transmission of the bacterium,” says Tenuta. However, he says that genetic resistance is probably the best management tool to reduce the spread of Stewart’s wilt. “We see the effectiveness of genetic resistance in our commercial corn hybrids, where we have good tolerance to Stewart’s wilt. We’re seeing more in the in-breds as well, but we still have in-breds that are very susceptible to the disease.” This means that seed corn producers need to continue to watch out for the disease.
Whereas infection in the young plant can cause it to wilt and die, infection in the mature plant causes leaf lesions, which can contribute to other problems such as stalk rot. In seed corn, mild infections usually remain in the leaf tissue, but severe infections can lead to systemic disease and some infection of the seed. “What we see is that if the plants have more than 75 percent of the leaves infected, some of the seeds may contain the bacterium,” says Zhu.
|Colder winters tend to kill off more corn flea beetles, thus leading to lower infections of Stewart’s wilt.|
This is a concern for seed producers because Stewart’s wilt is a quarantine pest for certain countries that import corn seed. The conditions that must be met to export seed to these countries differ depending on the country’s specific import requirements, says Lois McLean, Ontario network plant protection specialist with the Canadian Food Inspection Agency. “Usually for Stewart’s wilt, phytosanitary certification is based on the condition that the seed is grown in an area free from the pathogen, a field inspection during the growing season, or laboratory testing of the seed,” she explains.
In field inspections, leaf tissue is sampled. The presence of the bacterium in the leaf does not necessarily mean that it is present in the seed, so most exporters prefer to test the seed. Samples are tested using an enzyme-linked immunosorbent assay (ELISA) to detect the Stewart’s wilt bacterium. In the ELISA, a specific antibody that recognizes only the Stewart’s wilt bacterium changes colour if the bacterium is present. “If we find it and it’s being shipped to a country that has it on their regulated list, we can’t issue a phytosanitary certificate,” says McLean. “In that case, the producer could ship it to a country that doesn’t regulate for Stewart’s wilt or sell it domestically.”
Glen Hellerman, plant manager with Syngenta Seeds Canada, says that seed companies rarely treat for the corn flea beetle unless there is a severe infestation because it is not economically advantageous to do so. “Typically, the results of the seed tests do come back favourable, so even though Stewart’s wilt does exist in the seed corn production area, it doesn’t usually show up in the seed itself,” says Hellerman. “It does exist, but it really has not been a hindrance for Canadian seed companies to do business with other geographical areas.”