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Stop resistance to vegetable fungicides

In a bid to protect valuable crops — whether row type, vegetables, or fruit and ornamentals — producers have increasingly turned to fungicides. As more products come on the market, university and Extension researchers have stepped up educational efforts about the proper use of fungicides.

Too often, says David Ingram, growers don’t realize “there are different chemical groups with different modes of action against certain fungal and bacterial pathogens. This has led to a need for fungicide resistance management. If a grower doesn’t have a good grasp of these newer fungicides — and some are excellent products — they won’t know the mode of action is very site specific.”

Based in Raymond, Miss., Ingram has been employed at Mississippi State University for 20 years. For the last decade, he’s been working a split appointment as an Extension and research plant pathologist with major emphasis in vegetable crop production.

“I also have responsibilities in some other minor crops like pecans, the Christmas tree industry and blueberries. Most of my research work has been in greenhouse tomato production. Lately, I’ve been working on a bacterial canker disease that’s a major problem for large tomato operations.

“If the grower isn’t educated on how to correctly use fungicides, there is a possibility that a resistant population of a fungal pathogen can develop in the field.”

That result will appear to the grower as a failure of the product. But in actuality, the product hasn’t been used properly.

Currently, there are no examples of that occurring in Mississippi commercial vegetable production. But Ingram cites reports of a case in the Southeast where a watermelon farmer, attempting to control gummy stem blight, “put out repeated, sequential applications of a new fungicide.” As a result, fungal resistance developed.

“In nature, there is already a small portion of the fungal pathogen population that’s resistant to almost any fungicide that’s developed. That population is minute.

“But if you repeatedly put these site-specific fungicides out — they have one target site that’s active against the fungal pathogen — it’s easier for the fungus to mutate and overcome the fungicide’s chemistry. The product then ‘fails,’ although it isn’t really the product’s fault.”

Instead, the “insensitive strains” of the fungus multiply in the environment to become the prevalent population.

“It’s sort of like johnsongrass developing resistance to Roundup. With massive usage of a particular chemistry — and it may be in a bunch of products using different trade names — the resistant population is inadvertently selected.”

In early December, Ingram will speak on fungicide use at the Deep South Fruit and Vegetable Conference and Trade Show in Mobile, Ala.

“I’ll explain why growers need to rotate different chemistries. A simple analogy is a batter and pitcher. If the pitcher throws the same pitch every time, after a while the batter will find it easier and easier to hit a homerun. You need to add a curve, a slider or a knuckleball to keep the batter off balance.”

Producers can buy five to seven trade names, “all possibly in the same chemical class with the same basic mode of action. The grower may believe he’s rotating chemistries, but he isn’t.”

Growers need to do some homework, says Ingram. Labels will help and should be followed. “One might say, ‘Make only two sequential applications of this product. Then rotate to a different mode of action.’”

Also, don’t forget other disease-management practices. “In vegetable production we use plastic mulch, drip irrigation, proper fertility. Cultural things like that — in addition to fungicides and the integrated pest management approach — really help.”

In the past, “we had products like Bravo, protectant multi-site fungicides with two or more places on the fungus where they had activity. The newer fungicides generally have only one site of action in the pathogen. That makes it easier for the pathogen to overcome the product.

“That’s another reason we’re finding ourselves limiting the number of back-to-back applications of some of these newer fungicides. Some growers are rotating the newer products with the older, protectant ones.”

In vegetables, particularly tomatoes, growers now spray fungicides every week to 10 days on a calendar basis.

“It isn’t like scouting for soybean rust and saying, ‘I found it and it’s reached a certain level and it’s time to spray.’ In vegetable production, many crops are on an exact spray schedule we’ve developed that tells the grower what product to use in week one, week two, all the way through about 16 weeks. That helps keep the fungi from developing resistance.”

Ingram says it’s complicated nowadays with 150-plus chemistries being sold on the market.

“It’s hard for people in my profession to even keep up with the names and suggested rates. And the same active ingredient may be sold as three different products — one for row crop agriculture, one for ornamental agriculture and one for fruit and nut agriculture. It can be confusing.”

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