Late blight is a fast-spreading airborne fungus that can destroy a tomato or potato crop in just days. And it has — the most famous infestation of Phytophthora infestans is the Irish Potato Famine in the 1840s. Fortunately, there are a number of effective fungicides available now, but prevention and diligence are still vital to keeping the disease from affecting your crop. Once symptoms are seen, it is too late to provide adequate control.
Being aware of weather conditions that are favorable to the fungus and applying preventive fungicides on a regular basis are the best ways to stay ahead of late blight, says Shine Taylor, NA IFS Insect and Nematode Mgmt. Technical Expert at DuPont.
“Late blight is a weather-driven disease,” Taylor says. “The ideal temperature range is 50°F to 70°F with high humidity. It’s a real problem in southwest Florida this time of year. When you get an overcast, soggy, high-humidity day along with cool temperatures, it is just perfect conditions for late blight. It can blow up overnight.”
Spotting the Signs
The fungus causes purplish or brown irregularly shaped lesions on the leaves and stems of potato and tomato plants and on the tubers and fruits as well. All stages of growth can be affected. White, cottony growth (sporangiophores) can be seen on the foliage and stems when conditions are wet. Sporangiophores collectively produce millions of tiny sporangia, tiny structures that spread via wind and rain and infect healthy plants wherever they land. It only affects members of the members of the Solanaceae, or nightshade family, which includes potatoes, tomatoes and petunias.
Because the disease spreads so quickly and cannot be cured once symptoms appear, preventative spraying is the main approach to control.
“Know when conditions are going to be favorable and make sure you are spraying every five to seven days with a protectant fungicide,” Taylor says. “There are quite a few out there that are good for late blight. DuPont’s are Tanos® and Curzate®, but there are others, and it’s important to rotate between modes of action. If weather conditions are favorable several days in a row, you may want to tighten up your spray interval. Make sure you have a fungicide on the crop before a rain event.”
Be Aware of Symptoms on Neighboring Fields
Taylor also recommends being aware of what is happening in adjacent fields; if your neighbor has late blight in their field, make sure you are spraying for it in yours. Regular scouting can identify a problem early enough to limit damage. If early symptoms are seen in just one or two spots, infected plants should be destroyed along with healthy-looking plants around them. Systemic fungicide should be applied immediately on remaining plants, and protectant fungicides should applied frequently on a tight schedule.
“The big picture on late blight is it’s such a devastating disease,” Taylor says. “It’s a pathogen people have been dealing with a long time, and it has the ability to move entire populations around the world. Growers have to stay on top of it, especially at this time of year.”