The bacterial disease Goss’s wilt has become more prevalent in Manitoba corn fields, but its severity is depends on local weather conditions
Goss’s wilt is a bacterial disease which causes leaf and vascular blight. It can result in severely wilted plants and significant yield loss. Plants damaged by hail, wind or sand-blasting are susceptible to infection by the bacteria, which overwinters in corn stubble. Wet weather and high humidity favour development of the disease.
“The severity varies from year to year and is closely related to wet, humid conditions, so the areas that were more severe this year were the areas that received some timely rains,” says Holly Derksen, a plant pathologist with Manitoba Agriculture, Food and Rural Initiatives. “For example the Altona area, in south-central Manitoba, had some significant moisture during the growing season so they’ve seen more symptoms of Goss’s wilt as a result.”
The disease will quickly develop and spread under such wet, humid conditions. “It’s not like it gets to a point where you think, ‘okay, this is as bad as its going to get,’” says Derksen. “As long as there is green tissue, and weather conditions are right, it can continue to spread. Once the crop is ripened it doesn’t spread as quickly or do as much damage.”
Finding Goss’s wilt
Farmers are likely to notice Goss’s wilt first infecting patches along the edge of the field where residue from infected fields can blow in and land. The pathogen also enters leaf tissue which has been wounded by severe weather such as hail, wind or sand blasting, which is often worse around the outside edges of the crop. Although Goss’s wilt can be seed borne, this has not been seen in Manitoba.
Goss’s wilt causes a foliar blight that destroys leaf tissue and causes the plant to wilt. Other symptoms include lesions on the leaf that are parallel to the veins and can give a water soaked or freckled appearance, similar to bacterial blight, during the early stages of disease development. In the later stages, the disease produces an exudate (fluid that moves from the circulatory system into lesions or areas of inflammation) that makes the leaf appear shiny, with a peppering of black spots.
It’s important for corn growers to correctly identify and diagnose Goss’s wilt. That will probably become easier the more familiar they become with the disease.
“Your crop could also be wilting for other reasons as well, like environmental stress,” says Derksen. “But the symptoms of Goss’s wilt are quite different to other diseases because it does give a shiny look to the leaves. So once producers become familiar with it they are probably less likely to misidentify it in the future, but initially, if they are unfamiliar with the disease, it may be hard to identify. If they’re not sure what is going on they should call their agronomist or send samples in for lab testing.”
Spread of Goss’s wilt
Goss’s wilt is a relatively new disease in Western Canada. It was first confirmed in corn fields near Roland, Manitoba in 2009. Since then, DuPont Pioneer has conducted extensive surveys of corn fields throughout Manitoba over the last three years and has found Goss’s wilt throughout all of the traditional corn growing areas. In 2011 they surveyed 270 corn fields and found the disease in 83 per cent of them, but at very low levels which resulted in virtually no yield loss. The data is still being evaluated for 2012, but Derksen doesn’t think it’s going to cause major problems in most fields.
“I think most farmers will only see a patch of Goss’s wilt here and there, and they are not going to see much yield loss,” says Derksen. “But I have heard of fields this year that are going to have a bigger yield loss, maybe up to 30 bushels per acre, where they had warm weather with adequate moisture.”
Because Goss’s wilt is a bacterial disease, fungicides are not effective. The best lines of defense are proactive.
Tillage that buries infected residue and encourages decomposition can be effective to help reduce the rate of new infections. Controlling grassy weeds, such as foxtail or barnyard grass, reduces other plant sources that can act as a host for the pathogen. Crop rotations that include soybean, dry beans, small grains or alfalfa will also help to reduce the primary inoculum sources, but will not completely eliminate the bacteria, which can persist in the soil for some time, making it a recurring problem in subsequent corn crops even if other management measures are taken.
The primary defence against Goss’s wilt is genetic resistance. Although there are many resistant corn varieties being grown successfully in the U.S., where Goss’s wilt has been much more prevalent, there are not yet any resistant varieties registered in Canada.
That is soon likely to change, as many seed companies are working on corn varieties that provide resistance to Goss’s wilt. Dupont Pioneer is one of them and has hybrids with moderate resistance available to Canadian growers.
“Our organization has a long history of breeding genetics with resistance to Goss’s wilt,” says Wilt Billing, DuPont Pioneer’s area agronomist for Manitoba. “We have used that knowledge gained over the last 50 years to begin breeding resistant hybrids or hybrids with increased tolerance to Goss’s wilt in our early maturing corn varieties for Canadian farmers.”
In addition to new resistant varieties, says Billing, the company has conducted three years of nursery trials at Carman, Manitoba. These trials have helped them identify which of the hybrids already available in their current line have performed better when Goss’s wilt is present. Those particular varieties have been tested extensively through plots across Manitoba to compare their performance against competitive products and a susceptible check variety. “Through that effort we have been able to screen our current products in real world scenarios and we are fairly confident that there are products in our current line up that can handle this disease very well,” says Billing.
Billings is confident that farmers will soon be able to access some existing and new corn varieties from DuPont Pioneer which will carry a strong rating for resistance to Goss’s wilt. He feels that those varieties may be quickly embraced by Western Canadian corn growers, not because Goss’s wilt has been a huge problem to date but because it could be in the future. “I think growers will choose to grow varieties resistant to Goss’s wilt for the same reason they grow Bt corn,” says Billing. “They don’t know whether the pests are going to show up but use it as an insurance policy.” †