April 2017
Battle Plans for Disease Management By David Fairbourn, managing editor

Finding a strategy to fight rhizoctonia and fusarium seed piece decay.

What is the best approach to disease management at planting — seed treatments or in-furrow applications? Or is this really a question about what crop protectant is being applied to manage certain reoccurring, problematic disease like rhizoctonia or fusarium dry rot? Or is keeping diseases at bay more to do with the variety, quality and health of the seed, the location of the field, the water, climate and soil conditions or the length of rotation?

All good questions, but with the uncertainty growers face at planting each year, there’s likely a hundred more. That’s why companies like Miller Research in Rupert, Idaho, are hunting down the answers to all the early season potato production questions and many more.

Earlier this year, Miller Research President and CEO Jeff Miller presented his company’s recent findings at an annual potato pest management seminar near Rupert, Idaho. Over the years, with funding from the Idaho Potato Commission and fungicide manufacturers, they’ve investigated the efficacy of different products in controlling rhizoctonia and fusarium seed piece decay on farms near Minidoka and Acequia, Idaho. Based on the previous years’ results, here’s what is recommended.

“Rhizoc” talks

Rhizoctonia is a disease that goes from bad to worse as the season goes on, and the disease pressure changes from one year to the next. Because it is unpredictable, Miller advises against becoming complacent in managing it.

Rhizoctonia solani attacks the underground stems, stolon, roots and tubers on potatoes. Stems and stolons can be cut off by severe girdling lesions. This can lead to a reduction in tuber number and a decrease in tuber quality.

“There is danger in comparing one year to the next, especially with soilborne diseases like rhizoctonia,” he said. “Why does this disease pressure change from year to year? We think that soil moisture and temperature are big drivers. It may also have to do with crop history and cleanliness of the seed growers plant.”

Not all rhizoctonia is the same. Some grows off sugarbeets, some grows from grain and potatoes. It looks the same but different groups of the disease have been identified that specialize and grow on different host crops. These groups are called anastomosis groups. Rhizoctonia from anastomosis Group 3 is the disease that is primarily responsible for most of the problems in potatoes, Miller said.

But recent research discovered that sometimes this Group 3 rhizoctonia is also found on sugarbeets.

“We used to think the only thing that hit sugarbeets was Group 2, but we’re finding it’s not as cut and dried as we thought,” Miller said. “In fact, in a study, they’re finding some Group 2 on potatoes. So, if sugarbeets are in the rotation, to some degree, it’s keeping up a population that could cause disease on potatoes.”

When growers find rhizoctonia, what’s the best way to treat it? If you know you have it on your seed, then seed treatments are best. Or if you think it’s in the soil, then it should be controlled with in-furrow applications, Miller said.
“If you are on a three- to four-year rotation, you probably have enough rhizoctonia in your soil that would mean an in-furrow spray would be valuable,” Miller said. “If you are on a five-year or longer rotation, then it may be that a seed treatment is the best way to go.

“In essence, whenever we put both in-furrow and seed treatments together, we get better control. When we compare them side by side, it’s somewhat variable, but generally the in-furrow application does better with the three-year rotation on our farm. Now when I was at Aberdeen (University of Idaho), on about a five-year rotation, the seed treatment almost always did better. I think everybody should look at their own rotation and then decide.”

Miller put together a list of crop protectants that would be effective against rhizoctonia. Recommendations for seed treatments are: Tops MZ, Maxim MZ, Maxim 4FS, Moncoat MZ, CruiserMaxx Potato, CruiserMaxx Potato Extreme and Emesto Silver. In-furrow fungicide recommendations are: Quadris, Moncut, Priaxor, Vertisan, Blocker and Elatus.

When rhizoctonia infects tubers, it causes black scurf – the dark brown or black masses resembling dirt that won’t wash off. It’s the most noticeable sign of rhizoctonia, and infected tubers will spread the disease and stunt potato plant growth if they are used as seed.

“When disease severity is lower, more products tend to look good, and things tend to work better,” Miller said. “We tend to see products that appear to work good at the early part of the season. As it gets later in the growing season, we will get more degrees of separation between the products that actually work versus the ones that seemed to work earlier in the year. This is almost the acid test; if you have something that is working really good in August, it’s probably a really good rhizoctonia product.”

The temptation for some people may be to lower their rates on their applications to save money. For instance: deciding to apply Maxim MZ with Mancozeb at a 50:50 rate, which Miller calls a risky proposition.

“It may work in the short run, but I’m not so sure the rhizoctonia is as big an issue as the dry rot,” he said. “Diluting the rates when it comes to dry rot is a recipe for disaster.

“Eventually, we’re going to select for resistance, and while Mancozeb is very good on dry rot, it has no activity on rhizoctonia. Basically, you’re just diluting your protection. On a light disease year, you might get away with it, but under a high-pressure year, it will probably fail. I know it’s done for cost, but I just think that it’s a bad idea. The other thing is, Maxim MZ is effective on silver scurf. If we’re using low rates, we might lose this effectiveness.”

Dry rot thoughts

The dry rot disease cycle begins with the seed. Fusarium coeruleum grows fast and will totally wipe out seed infected with this fungus. Spores from the infected seed will infest daughter tubers and get released in the soil. At harvest, potatoes that weren’t infected can become infected when they are wounded. Dry rot will then develop quickly when these potatoes are stored at 60-70° F, and then the cycle repeats.

Dry rot is caused by several fusarium fungal species. On seed potatoes, it can reduce crop establishment by killing developing seed sprouts, and crop losses can be up to 25 percent. The pathogen enters potato seed pieces, developing at injury sites like cuts or bruises, and the necrotic tissue is usually dry and often rots out from the center. Rotted cavities often have different colors, from yellow to white to pink.

“If you want to get a good sense of your fusarium risk, then look at your seed,” Miller said. “A take home message is this: In-furrow applications do not control dry rot. It’s what you put on the seed that matters. When using a higher rate of product, you’re getting a higher rate of protection.”

If your seed treatment is a dust with Mancozeb or another active ingredient that’s effective against dry rot, it will work very well, but simply applying bark alone will not be effective in managing dry rot. Miller references one study of the effect of seed treatments on fusarium seed piece decay. Comparing Maxim MZ against alder bark, the Maxim MZ seed treatment had nearly zero decay, and seed pieces treated with only alder bark had about 70 percent decay from fusarium.

“You may use bark to help seed pieces suberize better, and that’s fine, but don’t count on that giving you any dry rot protection,” Miller said. “You have to use something that has a fungicide product in it.

“Mancozeb is probably one of the best options we have against dry rot. It’s a broad-spectrum product, and we don’t worry about it developing resistance. By using Mancozeb, it helps preserve the longevity of some of these other products that have more narrow modes of action.”





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