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COUNTRY LIFE IN BC • NOVEMBER 2019
Bacterial leaf streak lacks chemical controls Genetic research could help end a losing streak for corn
Bacterial leaf streak (BLS) is a foliage disease in corn
and cereal crops. It is found around the world and in recent years it became an emerging problem in the US, especially in the western corn belt where corn is irrigated.
Research by MARGARET EVANS
Then, in 2013, it showed up in southern Alberta. In humid conditions after rainfall or irrigation, the
destructive bacteria, Xanthomonas vasicola pv. vasculorum, squirm through the open stomata on the leaf surface and infect the veins to form the distinctive long stripe-like greyish or brownish streaks as the corn grows. While no one is totally sure what is going on inside the plant, the lesions disrupt the photosynthetic ability of the leaf, which become necrotic and die. This can impact yield. The severity of BLS can be driven by weather conditions and how susceptible the plant host is.
Symptoms of the disease can vary from brown to tan
to yellow narrow streaks or stripes between leaf veins. If the leaf is backlit by the sun the yellow colour can be very noticeable. The streaking can be shorter than two centimetres to several centimetres in length. Lesions often develop on the lower, older plant leaves then spread upwards. But the type and severity of the symptoms may vary between corn varieties and hybrids. The pathogen can overwinter in infected corn crop debris after harvest so it could be capable of re-emerging year after year. Crop rotation could help to give the bacteria time to break down. Around the world, the bacteria also cause gumming disease in sugar cane. “The disease was first identified in 1949 in South Africa,” says Tiffany Jamann, an assistant professor with the Department of Crop Sciences, College of Agricultural, Consumer and Environmental Sciences, University of Illinois at Urbana-Champaign. “It seems that there were some genetic changes and the pathogen is more virulent now than it was when it was first discovered. It was also discovered in Argentina around the same time as in the US and more recently in Brazil. It is unclear whether it independently evolved or was imported.” To make things more challenging, the bacterial disease looks like the fungal
disease, grey leaf spot. But the telling difference between the two is that grey leaf spot has rectangular shapes with smooth, linear margins whereas the bacterial disease has wavy margins on the streaks. However, it can be
CROPWATCH - UNIVERSITY OF NEBRASKA-LINCOLN PHOTO
confusing for farmers who may spray the crop with a fungicide, thinking they are dealing with grey leaf spot. This is not only ineffective, since the disease in question is bacterial in origin, it’s an unnecessary cost. But the real problem is that there is no chemical control for bacterial leaf streak and, so far, no fully effective corn hybrid exists. The pathogen is spread by the wind or moved from plant to plant by insects. However, Jamann’s research team has made some
promising progress by discovering that some corn lines have moderate resistance, and this could lead to the development of genetically resistant corn lines in the future. “We don’t have hard numbers yet, but this pathogen in other systems can cause up to 20% yield losses,” said Jamann. “There's no effective chemical control. Fungicides don't work, as it's a bacterial disease. That's why host resistance is going to be critical, just as it is for controlling other bacterial diseases of corn, such as Goss's wilt and blight.” The research team inoculated 26 diverse lines of corn with the bacteria and they found corn varieties that had moderate resistance as well as genetic regions that were associated with some levels of bacterial resistance. “Although we didn’t identify any large-effect
resistance genes, the study is the first report of host resistance to bacterial leaf streak that has ever been published,” says Jamann.
The team also looked for some relationship between any resistance to the disease and resistance to other corn foliage diseases for which host resistance is better understood. It was hoped that the same genes that protected corn against other infections would also resist the bacterial disease, but it was not quite as straight forward as they had hoped. “We find that the genetic architecture of resistance is complex, with lots of
regions of the genome contributing to resistance,” says Jamann. While the study was not totally clear cut, the genetic information thus far will be valuable to breeders developing new hybrid lines with an ability to resist bacterial leaf streak. And even though management practices may be able to mitigate against losses to this disease, they may only be partially successful. As Jamann notes, conditions cannot always be controlled to check disease spread. Ultimately, host resistance will be critical. Jamann’s research going forward will continue to identify regions of the corn genome that will provide resistance to bacterial leaf. Margaret Evans is a freelance writer based in Chilliwack specializing in agricultural science.
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