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Pythium update: a new battle begins

Pythium has been lurking under the radar lately on the crop breeding scene, and that is not a good thing. “Companies nowadays are developing new soybean varieties very quickly,” says Ohio State University professor Dr. Anne Dorrance, “and how these varieties stand up to what are considered minor diseases like Pythium has been somewhat ignored. That may come back to haunt us in a significant way.”


March 16, 2010
By Treena Hein

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pythium
According to OMAFRA’s Albert Tenuta, growers are getting better at distinguishing Pythium from Phytophthora, pictured here.

Photo courtesy of Albert Tenuta, OMAFRA.

Pythium has been lurking under the radar lately on the crop breeding scene, and that is not a good thing. “Companies nowadays are developing new soybean varieties very quickly,” says Ohio State University professor Dr. Anne Dorrance, “and how these varieties stand up to what are considered minor diseases like Pythium has been somewhat ignored. That may come back to haunt us in a significant way.”

Most species of Pythium inhabit the soil and cause a variety of diseases, including seed rots and damping-off; root, stem and fruit rots; foliar blights; and postharvest decay. This genus of plant fungal-like pathogens, closely related to other oomycete plant pathogens such as Phytophthora species, has a large geographic distribution and host range. There are approximately 120 recognized species. “Pythium has been a historically important issue in Ontario and is getting more important,” observes Ontario Ministry of Agriculture, Food and Rural Affairs field crop plant pathologist Albert Tenuta. “There are species that affect only corn or wheat or soybean, and others that can affect two or all three crops.”

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Tenuta believes that identification and awareness of the disease are improving among farmers. “People are learning to distinguish it from Phytophthora and other early season root rot diseases,” he says. “Its foothold depends a lot on environmental conditions. It likes to establish in cool wet conditions, 15 degrees C or less, and then it decreases as it warms up, and Phytophthora takes over.” Surveys in Ontario during the last few years have shown that there are about 14 species in eastern Ontario that affect soybean, corn and wheat, with some affecting only one or two of the three. The species affecting corn seem to be increasing more than others.

Perhaps Pythium’s most worrying aspect is that it is a major cause of seed rot prior to germination, especially in poorly drained soils. “Almost all germinating seeds and developing roots are exposed to infection by Pythium,” says Tenuta. Hence the importance of fungicide seed treatments will be important until such time as effective resistance genes are identified.

Work is now underway
It is not easy to beat down a pathogen that has such a wide range of deleterious effects, even if it is well understood at the molecular, genetic or genomic level. However, the good news is that with the recent sequencing and annotation of the pathogen’s genome, researchers are now better prepared to do battle.

Dr. André Lévesque, a mycologist with Agriculture and Agri-food Canada, with project leader Dr. Robin Buell of Michigan State University and collaborator Dr. Ned Tisserat of Colorado State University, initiated the genetics project in 2006 after obtaining funding from the US Department of Agriculture. Almost 50 scientists from around the world are currently finalizing the genome annotation, which will be released in late 2010. “Having the genome completely sequenced and annotated is going to make a huge difference,” Lévesque says. “We know that cultivars with root rot resistance to Phytophthora are not resistant to Pythium. We also know that the two pathogens have different ways of attacking the plant; Phytophthora has a sophisticated “signalling system” that helps it enter the root, and Pythium has more of an opportunity-based system. Now, with the genome mapped, we’ll be able to understand better how Pythium attacks, and pinpoint the genes that relate to its different life stages.”

He adds, “Knowing more about the genes that are responsible for the way the pathogen works at root surface will also help breed resistant plants.”

Dorrance is equally excited about the avenues that having the genome sequenced will open up. “Being able to compare the genome of different species, which attack different parts of the plant,” she notes, “will go a long way to also helping us develop effective seed coat treatments.”

Dorrance recently isolated a new species of Pythium that attacks in warmer temperatures and on older plants, as opposed to the usual infestation of younger plants during cool, wet springs. Despite the fact that this particular species is not very aggressive, it is causing growers to conduct costly replanting in Ohio. “We’ve discovered it in a few locations, but it’s hard at this point to say what its range is,” she says.  

If the climate models are correct and warmer temperatures become more common in the future, Tenuta says that Pythium species that require colder temperatures might begin to suffer, but those that can tolerate warmer temperatures might increase. “Pythium is very flexible,” he says. “There are species that can function well in both types of scenarios.”

The fact that there is not yet much information about things like the virulence of various Pythium species or the degree of resistance to them that may exist in the new rapidly developed soybean varieties means that growers stand on dangerous ground, in Dorrance’s opinion. “The last three summers have been dry and so haven’t been conducive to proper screening for Pythium,” she says. “We need expedited screening methods to get the information we need.”


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