Late blight may have captured headlines last summer because of the devastation the fungal disease caused to tomatoes and potatoes on the East Coast.
But the leaf spot complex—a group of four early season fungal diseases—can be just as insidious and cause annual headaches for potato growers, say plant pathologists.
Compounding the problem is the difficulty in identifying the diseases within the complex and the limited number of products that control some of them.
The leaf spot complex comprises four fungal diseases—early blight, brown spot, black dot, and Botrytis cinerea or gray mold—in generally descending order of importance. The actual level of importance varies among regions.
“To the untrained eye, it can definitely be difficult,” says Neil Gudmestad, a plant pathology professor with North Dakota State University in Fargo. “I think they’re a real problem for potato growers—how much of what they have is early blight and how much of what they have is brown spot?”
To help growers and consultants, Gudmestad and his research assistant, Julie Pasche, have put together a two-page color guide that provides identification photos and tips for Midwestern potato diseases. It also includes an efficacy chart ranking all of the registered products available for potato disease control.
Companies can purchase the copyrighted publication, which includes free updates and customization with the company’s name.
Choosing a fungicide
Knowing the disease or diseases you have in your field is key to determining what crop protection materials to choose, Gudmestad says.
Early in the season when disease pressure is low, protectants such as mancozeb and chlorothalonil can be used.
As the season goes on and disease pressure increases, Gudmestad says growers will probably need one to four additional applications of other materials.
The strobilurins control early blight, but they’re ineffective against brown spot. In addition, early blight has become resistant to strobilurins in many areas, so growers should tankmix them with another mode of action.
The early blight organism— Alternaria solani —has mutated, and many populations carry the F129L resistance gene.
Those populations are 10 to 15 times more resistant to strobilurins than sensitive ones.
That doesn’t mean you can’t use a strobilurin against early blight, Gudmestad says. It just means the level of control if used without a tankmix partner is reduced to that of chlorothalonil.
In unpublished research, Gudmestad says he found the same F129L resistance gene in A. alternata, the brown spot organism. But that may be moot since strobilurins have never been particularly effective against that disease.
“In the Pacific Northwest, they’ve seen a shift from solani to alternata, and there’s a lot of interest in getting away from the strobis or having products that also contain other than a strobi with it,” says David Laird, fungicide brand manager for Syngenta Crop Protection in Greensboro, N.C.
Quadris Top, which will available in very limited quantities this year, is a combination of difenoconazole and azoxystrobin, a strobilurin, Laird says. In addition to early blight, Quadris Top controls brown spot and black dot.
“The only thing I would caution is these products do have some curative activity on alternata, but that doesn’t mean they have rescue activity,” Laird says. “If you have black dot in the mix that requires an early application probably prior to row closure, then I’d recommend Quadris Top very early because it will pick up black dot, early blight and brown spot.”
Revus Top, a premix of mandipropamid and difenoconazole from Syngenta, and Scala (pyrimethanil) from Bayer control both early blight and brown spot. Revus Top also is active against late blight.
In addition, Scala is effective against Botrytis but doesn’t control late blight, white mold or black dot, Gudmestad says.
He adds that Bayer hopes by the end of the year to have registration of Luna Tranquility, a premix of the active ingredients in Scala and fluopyram, which is in the same family as boscalid.
Field trials show Luna Tranquility is effective against early light, brown spot and Botrytis.
An integrated approach
Randy Cherney, an agronomist with Plover River Farms in Stevens Point, Wis., says he presumes most of the leaf spot diseases in the fields he oversees are early blight, although he hasn’t had them tested.
He says he takes an integrated approach to managing the disease, and “fungicide is just one part of the equation.”
Maintaining proper plant nutrition, keeping insect pressures low and properly irrigating to reduce plant stress are equally important.
“As long as your plant is healthy, it’s better at fighting off early blight,” Cherney says.
He typically applies his first fungicide—a protectant such as the chlorothalonil product Echo Zn—at about 300 P-days or just before row closure.
Then it’s on to a seven-day schedule, rotating an application of chlorothalonil with a tankmix of chlorothalonil and another fungicide, such as Scala, Quadris or Endura.
Chlorothalonil has a multi-site mode of action, so the chances of the early blight organism becoming resistant to it are much less than a single-site product, such as Scala or Endura, Cherney says.
“You have to use all of these different products that have different ai’s or [belong to] different classes. We wouldn’t use all Endura,” he says.
Depending on the variety and the length of growing season, Cherney says he may make eight, nine or possibly 10 fungicide applications for early blight. He continues spraying until vine kill.
“If you can keep the early blight low in your fields in August, your plants will stay green and you’ll get really good bulking, especially in the late-season varieties,” Cherney says.
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Consider several factors for identification
Identifying whether you have early blight or brown spot or both can be a challenge. Neil Gudmestad, a plant pathology professor with North Dakota State University in Fargo, recommends examining several factors, including visual symptoms, disease behavior on the plant and in the field, climatic conditions and variety.
Early blight, caused by Alternaria solani, produces larger lesions with concentric circles. Brown spot, caused by A. alternata, involves more, but smaller, lesions that are scattered all over the leaf in a shotgun pattern. The lesions may eventually coalesce or merge together.
But looks can be deceiving, says Koen van den Eynde, Raleigh, N.C.-based North American vegetables and sugar crop manager for Bayer CropScience. Bayer and several researchers conducted identification tests using PCR, which is similar to the DNA genetic fingerprinting used in crime labs.
Plant pathologists were asked to send in samples of what they thought was early blight and brown spot.
“There was no correlation whatsoever with what they saw on the leaves with the PCR results,” van den Eynde says. “For example, they thought a leaf with many little spots was brown spot, but this wasn’t necessarily the case. The bottom line—even the experts had a hard time distinguishing between one and the other.”
That is one reason why Gudmestad says growers and consultants should use several factors when making an identification.
Early blight tends to develop from the bottom of the plant up, he says. A. solani is aggressive on senescing tissue, starting on the yellowing leaves and working its way up.
Brown spot, on the other hand, tends to start on the top of the plant and works down. It will be more prominent on the top of the canopy and on new foliage.
Brown spot also tends to develop in pockets in a field whereas early blight will be more evenly distributed throughout a field.
Varietal susceptibility also should be factored in, Gudmestad says. Atlantic, Pike, Ranger Russet and Yukon Gold tend to be very susceptible to brown spot. Russet Norkotah, on the other hand, is susceptible to early blight but not so much to brown spot.
Climate also plays a roll in what disease or diseases typically are found in a field. The chronic morning dews of the Midwest are conducive to early blight epidemics and also contribute to brown spot development.
“With Midwestern conditions, we can’t get season-long disease control of this complex with mancozeb and chlorothalonil,” Gudmestad says of the protectant products. “We need one to four [applications] of specialty products to control those diseases.”
The specialty products he was referring to include Scala, Luna Tranquility, Revus Top, Quadris Top and Endura.