Liquid seed-piece treatment BMPs
By Carolyn King
Seed potato pieces are at risk of invasion by pathogens that can cause serious emergence and stand problems. Liquid seed-piece treatments are a fairly new option for dealing with some of these pathogens. Dr. Gary Secor, a plant pathologist at North Dakota State University, offers potato growers a set of best management practices (BMPs) for these liquid treatments.
By Carolyn King
Fungicidal seed-piece treatment products come in dust or liquid formulations. Secor looks at two examples of liquid products now available on the Canadian market: Emesto Silver (prothioconazole and penflufen), available with an insecticide in Titan Emesto; and Maxim D (difenoconazole and fludioxonil), available with an insecticide in Cruiser Maxx Potato Extreme.
Secor explains that these two liquids are effective in controlling disease, plus they have some important advantages over the powder products. “The two liquid seed treatments are broad spectrum; they control several seed-borne fungal diseases including rhizoctonia, fusarium dry rot and silver scurf. Although rhizoctonia and fusarium dry rot can also be soil-borne, seed treatments help manage the seed-borne aspects of those diseases.
“The liquid treatments also offer ease of handling compared to the dusts. Dusts are difficult to calibrate and to get the same amount on every seed piece. The liquid seed treatments go on at ultra-low volumes, and you can control the spray to get the same amount on every seed piece.
“But the most important advantage, and the driving factor behind [using liquids], is improved worker safety. Workers are not exposed to all the dust that is produced with a dust seed treatment,” Secor says.
In addition, the two liquid products have some active ingredients that are different than those found in the powder seed treatments and in-furrow fungicides, providing more choices for rotating fungicides to slow the development of resistance.
What about bacterial soft rot?
“Probably the most frequent cause of seed decay is bacterial soft rot. Bacterial soft rot is not controlled by seed treatments directly because it’s a bacterial disease and the seed treatments are fungicides, which control fungal diseases,” Secor notes.
He explains that the bacterial soft rot pathogen requires a combination of three conditions to cause the disease. “It needs an entry point for infection. It also needs wet conditions. And it needs warm temperatures, and the warmer it is the faster the decay goes.”
The bacteria can enter potato pieces through the lenticels (the tuber’s breathing pores) or wounds from cutting the tubers or other injuries, such as the wounds caused by fusarium dry rot.
“So, managing fusarium by using a seed treatment indirectly controls bacterial soft rot,” Secor says.
Since wet conditions favour bacterial soft rot, growers may be worried that applying a liquid seed treatment might increase the risk of the disease. Secor has several recommendations to help prevent problems with soft rot decay when using a liquid: “One is to be sure you put on that ultra-low volume of the liquid seed treatment as the label says – don’t exceed the amount of water recommended on the label.
“Then, if possible . . . allow the seed pieces to dry before the seed goes into the truck or before it’s planted. That will minimize any wet conditions.
“And then, of course, do not plant into wet soil, because planting into wet soil, regardless of whether you have a seed treatment or not, will result in bacterial seed decay,” Secor says.
If you are temporarily storing treated seed pieces before planting, check the product label for any specific requirements. Secor provides some general guidelines for storing seed pieces: “Store the pieces in piles that are no higher than head height, about 1.8 metres high. Provide plenty of air circulating through those cut potatoes; it helps wound healing and it helps get rid of that excess moisture that can result in seed decay. Store the pieces at about 10 C. And, of course, keep the pieces out of the sun and rain.”
Secor has some advice to help growers decide whether or not to apply a seed-piece treatment. “With rhizoctonia, if you have five per cent or greater on the seed, you should probably use a seed treatment. If you have more than one per cent fusarium dry rot in a seed lot, you should use a seed treatment.
“And if you want to control silver scurf because you’re a table stock grower and appearance of the tubers is everything, then one of the best strategies is a seed treatment,” he says, “because as far as we know the inoculum is all seed-borne, not soil-borne.”
However, Secor notes, “The seed treatments do not seem to give control of black dot blemish of potatoes, which is especially important for table potatoes.”
One of the things Secor likes about the liquids is the addition of colorants to some products. “If you’re using a liquid seed treatment that has a colorant in it, you can actually see whether you’re getting good coverage on the seed pieces, and of course, coverage is everything.”
The ability to check that you’re getting complete coverage is very helpful, according to Secor, especially when you’re adjusting, modifying or fabricating applicator equipment. “In my opinion, one of the limiting factors of using a liquid is the application technology. Farmers are really good at inventing and fabricating machinery to accomplish a specific job, and there is some good equipment out there for ultra-low volume coverage of seed pieces,” Secor says.
“But because liquid seed treatments are so new, I think there is room for some improvements to ensure the equipment can absolutely apply the right ultra-low volume with 100 per cent coverage,” he says.
“Certainly, farmers can modify some of the existing applicators to make them even better because farmers are working so closely with the equipment.”
Secor outlines several other practices that help minimize disease and decay early in the growing season.
Of course, one practice is to buy good-quality seed. “There are many factors to consider for selecting good seed, and certainly you want freedom from disease – low amounts of potato virus Y and potato mosaic virus, no ring rot, no late blight, minimal rhizoctonia, minimal fusarium, minimal silver scurf – a minimal amount of the diseases you can see,” he explains.
“It’s also good to have physiologically younger seed because it generally performs better,” Secor says. “And, of course, you don’t want seed that has been injured, frozen, sunburned or mechanically damaged.”
Secor advises inspecting the seed before you buy it and then handling it carefully during the transportation from the seed grower’s farm to your farm and out to your field. “Most of the pathogens will enter through a wound, so handling the seed gently during the whole process is probably the number one factor for making that seed perform well,” he says.
Secor recommends doing everything you can in your operation to prevent injuries to the seed, before and after cutting. That includes things like reducing or avoiding drops, adding padding in places where the seed might be damaged, and using sharp cutter blades so the wounds heal quickly.
Frequent disinfection of cutting equipment is always a good practice to reduce the spread of seed-borne diseases. And it’s especially crucial if you have seed with bacterial ring rot because the liquid seed-piece treatments do not control it. Spread of seed-borne late blight during cutting can also be a serious issue, but growers recently gained a liquid treatment option with the registration of Bayer CropScience Canada’s Reason (fenamidone) for use as a seed-piece treatment to protect against seed-borne late blight.
“My recommendation is to always disinfect the cutting equipment between seed lots. If you have an infected seed lot, the infection may spread within that seed lot, but you don’t want to infect a new seed lot. So the best thing to do is to completely sanitize your cutter and cutting equipment between seed lots at a minimum,” he says.
Secor also recommends doing everything possible to get the potato plants up and growing on their own root system, independent of the seed pieces, as quickly as possible. He suggests three practices that will help.
“Warm the potatoes prior to planting so they just begin to sprout and then plant them into warm soil. The optimum we recommend is that the seed and the soil be at the same temperature and that temperature should be 10 C – that’s an absolutely perfect way to plant potatoes,” Secor says. Often growers are able to warm the potatoes from the storage temperature of 4 C up to 10 C by simply using fans to move outside air, because outside air is typically around 10 C as planting time approaches. A pulp thermometer can be used to monitor the pulp temperature of the seed.
Secor continues, “Also, plant potatoes into medium soil moisture; the soil should not be too wet or too dry.
“And don’t plant them too deep. Sometimes it is better to plant them shallow to get them up on their own roots,” he says. “Then you can go through the hilling process at the same time as you’re doing another field operation like applying a herbicide or doing a cultivation.”
If you’re dealing with seed-borne fungal diseases, a liquid seed-piece treatment can be a valuable tool in an integrated approach to getting your potato crop off to a great beginning.