February 2021

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Spudman • February 2021 21 "common garden experiment," where they grow potatoes in small plots using soil that has never been exposed to potatoes and soil that has been exposed to potatoes. They plan to characterize the soil by quantifying all of the microbes present in both soil types. "We expect to see the potatoes grown in the virgin soil perform better," said Wheeler, who is the principal investigator on the project. He is working in collaboration with soil scientists and pathologists. Using DNA sequence data, the aim is to identify fungi, bacteria and nematodes present in the soil. "We hope that the yields are higher and disease intensity is lower," he said. "If it does work, if we do see that effect, then we'll try to reproduce it at scale by maybe finding some of the microbes that are very associated with the virgin soil and perhaps trying to inoculate them into soils that are not virgin, for example." But the process is very complicated and there are many factors that impact the disease. Wheeler is also part of a project that is working to identify the verticillium genes that are associated with disease in potatoes, as well as infection in asymptomatic crops like mustard. "The reason why that might matter is that if we can find genes that are associated with genes in potato, we can give those data to breeders and they can help develop resistance," said Wheeler. In the future, the hope is that geneticists are able to convey resistance via RNA interference using gene- editing technology. "The advantage of that would be that we wouldn't have to apply fungicides if it worked," said Wheeler. "That would be awesome. But we have to convince consumers that it's safe, and that's a very big hurdle." In the meantime, Steven Johnson, Extension Crops Specialist, University of Maine, said the best course of action is still prevention. Avoid introducing the pathogen to uninfected fields, including via dirty equipment, and use clean, high quality seed, he said. For those farmers whose fields already have verticillium wilt, rotation is the best long-term solution. The success of this management strategy was particularly evident this past growing season when most of Maine's potato farmers faced severe drought. "When there's a drought, verticillium wilt comes on faster, it's more exacerbated and a plant that could probably handle a low level of verticillium and not show a significant symptom shows it now because the vascular system is so stressed it doesn't take much plugging with the pathogen," said Johnson. "The people who had longer rotations… it was very noticeable," he added. A three-year rotation is best, advised Johnson. And while that wouldn't be long enough in other states, Maine experiences long, cold winters, which helps reduce pathogen load come spring. "But the real bottom line here is that extended rotation did show a lot of value," Johnson concluded.

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