February 2010
Late Blight

New strains of late blight are creating problems for Florida potato growers, leading scientists to find out why the disease is changing before it becomes an epidemic.

Late blight isn’t new to Florida, it has been affecting potato crops since they’ve been grown in the state, and remains a constant threat, impacting crops almost yearly.

It’s been around a long, long time, as long as there’s been agriculture in Florida, there’s been late blight in the state,” said Tim Schubert, administrator for the plant pathology division of Plant Industries, Florida Department of Agriculture & Consumer Services.

“It’s been here long enough that we’d consider it an endemic disease.”

Climate is part of the problem with the disease’s prevalence. Potatoes are a winter crop for the state, and the cooler, humid winters are perfect growing conditions for the disease. Coupled with a lack of heavy frosts, and it leads to a disease problem that won’t go away.

Fungicide applications can manage the disease, but it has been mutating, rendering many fungicides ineffective.
“The A2 strain of late blight came out about 12 to 15 years ago and it is much more aggressive strain than we had previously encountered,” said Danny Johns, manager of Blue Sky Farms, near Hastings. Johns has managed the farm for 25 years, and has had to deal with the disease his entire time at the farm.

“In the past ridomil would stop the disease in it’s tracks, but not with this new type. Prevention has always been key to controlling blight but with these new strains it is even more imperative,” he said.

To combat fungicide resistance, Johns uses a management plan that includes mixing fungicides in the tank, as well as fungicide rotations, with applications almost weekly.

Climate is only one problem facing Florida growers in battling late blight. Many solutions in other potato-growing regions don’t apply to Florida, so to find some answers, Pamela Roberts, an associate professor of plant pathology at the University of Florida’s Southwest Florida Research & Education Center, Immokalee, is studying the disease on potatoes and tomatoes, hoping to find out why it is so different in Florida.

“The situation is changing, and we’re getting a lot of new strains of late blight, and we don’t have a lot of information on how they will act in the field,” she said.
Why the pathogen is changing in Florida is one of the biggest questions left unanswered. There are new strains, and researchers don’t even know if they are from the state.

Late blight’s populations are characterized by genotype, which can be determined by a series of tests. Asexual reproduction creates a genotype similar to the parent strain, while sexual recombination can produce new strains possibly resistant to fungicides. But it’s not known if it occurs in the state.

“One of the things we don’t understand is where the pathogen is coming from. We’re seeing a lot of genotypes, and we don’t know if it’s being produced in Florida or if it’s from an outside source,” Roberts said.

Researchers are also struggling to understand the disease’s host plants. Many solanaceous plants, the family that includes potatoes and tomatoes, are susceptible to the disease, but researchers don’t know much about the host plants in the state. Most growing regions know which plants serve as hosts for the disease and can manage them by removing the plants prior to planting, but many of those same plants may not be hosts in Florida, compounding the problem of where the disease is coming from

“It’s a different situation in other countries, like Mexico and China, where they have documented weed hosts. We don’t have a good handle on that in Florida, and we don’t have a good handle on alternative hosts, like nightshade,” Roberts said.

Another problem is that potatoes and tomatoes can be impacted at the same time, and late blight can easily move between the crops, affecting both in the same season.

The good news is that the disease is manageable in Florida, and experts say one of the best prevention methods is through planting certified disease-free seed potatoes to prevent outbreaks.

“Last season was a mess across the eastern United States because of potatoes and tomatoes in home gardens. Some of the retail planting material was diseased at the point-of-sale. These plants were not as well-protected in the garden, and disease spilled over into commercial fields. By the time it was noticed, it was everywhere,” Schubert said.

Fungicide management plans help control the disease, but they have their limitations. They can be rendered ineffective in the face of mutating strains, and may need as much as weekly applications.

“The new strains are more aggressive, and they can be hard to control with fungicide. We need to stay on top of the disease so we don’t have any future breakouts of it,” Roberts said.

“It’s management, scouting, and fungicide application, and if it gets out of control, (growers) will lose their crop.”

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