A Force in the Field – Battling Late Blight

October 10, 2022

Perhaps no foliar disease is more notorious among commercial potato producers than late blight. Caused by Phytophthora infestans, a fungus-like water mold, this pathogen has the potential to not only reduce yields by causing premature plant death, but it also can infect the tubers themselves, leaving them susceptible to bacterial soft rot after harvest.

Relevant: 75-Day IPM Potato Program

“Late blight is the disease that caused the Irish Potato Famine,” says Jeff Miller, president and CEO of Miller Research based in Rupert, Idaho. “It caused the death of about a million Irish people due to starvation. If it’s left untreated, you can have a complete crop loss.”

While the disease can be found throughout the world, its presence in the Pacific Northwest is a relatively recent occurrence, says Kelly Luff, principal scientist for Bayer Crop Science working across the Snake River Plain in southern Idaho.

“We never saw late blight here until 1995,” he says. “I remember seeing entire fields go down in a matter of days. Once you’ve seen that, it’s so devastating that you’ll never forget it.”

Late blight development is favored by a cool, humid environment. One of three sources of inoculum can lead to initial infection, Luff explains. Infected seed can harbor the disease, the pathogen can be present in cull piles from the previous season, or it can appear in season on volunteer potato plants in rotational crop fields.

“Managing for late blight is a season-long challenge. You want to eliminate those inoculum sources,” Luff says. “Start out by planting certified seed that’s not infected. Destroy cull piles early in the season and control volunteers in your rotational crops.”

Should late blight develop, winds can carry the fungal spores significant distances, potentially into a potato field.

“Thunderstorms are really good drivers for late blight,” Miller says. “With a thunderstorm, we have changes in humidity and pressure. The spores separate from the fungus and get up in the air … they can easily travel 50 to 60 miles.”

When a potato plant is first infected with late blight, Luff says the lesions are inconspicuous ­– small light green to dark green dots. As the lesions grow, they become brown to purplish black with a halo of light green to yellow tissue.

“The lesions move across veins in the leaf,” Luff explains. “They’ll continue to expand and coalesce until they essentially take over the entire leaflet, the entire petiole. They also can expand to the stem and take down the whole plant.”

Growers must be vigilant in their efforts to monitor for late blight. Fields should be regularly scouted, and Miller suggests extra attention should be given to areas that tend to stay wet longer.

“Whether it’s the center tower of a pivot or along a main line where you might have overlapping irrigation, that’s where you want to watch for it,” he says.

Both Luff and Miller recommend that growers subscribe to a preventive fungicide program. Previcur® Flex, a Bayer Crop Science fungicide containing propamocarb, a Group 23 fungicide, is highly effective as a preventive fungicide in a late blight management program.

Luff says fungicide applications should be made in regular seven-to-10-day intervals to ensure protection. Applications should continue through vine kill to ensure that tubers don’t get infected.

“If your neighbor has late blight, you start spraying,” Luff says. “You don’t wait until you see it in your field. The economic impact to growers depends on how quickly they get on their control measures. The sooner, the better.”

For more information about late blight control options as well as other potato pests, refer to resources in the 75-Day IPM Program for potatoes so you can create an agronomic force field.

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