Some years are worse than others when it comes to late blight in Florida tomato crops, says Pamela Roberts, professor of plant pathology at the University of Florida’s Southwest Florida Research and Education Center in Immokalee.
Symptoms of late blight include brown lesions surrounded by a “white sporangial mass” that show up early in the morning on the leaves of the tomato plants, she says.
The condition also can cause “big brown, greasy lesions” on the fruit.
“If there is no control, or if the weather is conducive, it can destroy your entire planting within weeks or days,” she says.
Sometimes, regular fungicides, conventional farming methods and early detection are enough to control late blight, but other years, growers must take more aggressive action.
Often the fungicide mefenoxam, developed in the 1980s and marketed under the brand Ridpmil Gold as well as several other names, is effective in controlling late blight. But there’s no guarantee.
“You need to know your population every year as to whether it’s sensitive to Mefenoxam,” Roberts says.
If the late blight pathogen does show resistance to mefenoxam, some newer alternatives are available to which the fungi have not yet demonstrated resistance, she says.
Presidio, with the active ingredient fluopicolide, is one. Revus, with the active ingredient mandipropamid, is another. And there are even more on the market.
“Use a late-blight specific compound when there has been a specific outbreak and you want to protect your field,” Roberts advises.
The website http://www.usablight.org tells highlights the latest outbreaks.
“Late blight usually is manageable,” she says, if you use a protective fungicide, monitor your field, pay attention to late-blight alerts and use a late-blight specific fungicide as needed.
No longer a rarity
Tomato late blight used to be a fairly rare occurrence at Lipman, an Immokalee-based tomato grower, says Gerry Odell, chief farming officer. But he says in recent years, it’s been turning up pretty much every season.
“The university will tell us when late blight first shows up,” he says.
The university also will alert him about whether the strain is sensitive to products that contain mefenoxan, Odell says. Then he’ll decide which to apply.
“We try to use as many different chemistries as we can that have activity against late blight,” he says.
Odell typically uses Bravo, but adds products like Ranman and Curzate when late blight shows up.
“We’ve got four or five different materials that we use in rotation to control late blight,” he says. “It’s been a fairly successful program for us. We haven’t had any wipeouts.”
Typically, you won’t find brown rot in peaches in a well-maintained orchard, says Guido Schnabel, professor of plant pathology at Clemson University. But if you find greater than 5 percent to 10 percent, he says, “You can probably guess that there might be resistance going on.”
Brown rot likely is present if fruit is completely rotten and mummified or perhaps half rotten with brown lesions and a “fuzzy brown fungus on top of the lesions,” he says.
Together, Georgia and South Carolina produce more peaches than any other states except California, he says.
“We have seen resistance in both states in most of the chemical classes.”
Growers have commonly used the pesticide Orbit but now seem to favor a similar product called Tilt to combat brown rot, he says. Propiconazole, a DMI fungicide and Tilt’s active ingredient, also is available under other brand names.
Growers are seeing resistance to Orbit and Tilt-like products and also to Topsin-M, which has the active ingredient thiophanate methyl—a different chemical class, Schnabel says.
There also is a combination product of two chemical classes—QOI fungicides and SDHI fungicides—called Merivon.
“It’s a new product that just came out last year, Schnabel says. “It’s really good for brown rot control and for strains that are resistant to Topsin M and Orbit.”
If brown rot is resistant to propiconazole, you still can control the resistant strain with a high rate of a similar-type product, such as Indar, he says.
You can’t increase the rate of propiconazole because of residue issues, but other members of the same chemical class can be increased.
Fenbuconazole is the active ingredient in Indar.
Mike DuBose, partner in Cotton Hope Farms in Monetta, S.C., has been having an increasingly difficult time finding a chemical to control rots.
“We are beginning to see resistance to everything that is available now,” he says. “We’re having to rotate different products to combat resistance.”
DuBose, whose family has been growing peaches for more than 100 years, says that in the past, new products would come along to replace those to which rot had developed a resistance.
That’s not the case today, he says.
“We’re just rotating all of the chemical classes that we have, so that we can take care of the ones that are resistant to one chemical class,” he says.
The good news is some new products are being developed, he says, and some existing products soon may be registered for use on peaches.
Gummy stem blight
Many fungicides are used to combat gummy stem blight in watermelons because there are no resistant cultivars and cultural practices don’t offer a lot of protection, says David Langston, professor and Extension vegetable pathologist for the University of Georgia.
Symptoms of gummy stem blight, the most widespread watermelon disease, are large, irregular-shaped brown lesions on leaves and water- soaked brown lesions on stems.
“Severe infestations could lead to complete defoliation of a crop,” Langston says.
The broad-spectrum protectant Bravo, with the active ingredient chlorothalonil, is the most common fungicide used against gummy stem blight in watermelons, he says. But Bravo does not work as well as some others under high disease pressure.
Bravo also can’t be used safely within 21 days of harvest because it has a history of burning watermelon rinds, Langston says.
Other fungicides are single-site modes of action and present a higher risk of developing resistance, he says.
“Strobilurin fungicides used to work, but we’ve got widespread resistance to those,” Langston says. “Those just don’t work anymore.”
Nor do the benzimidazoles, such as Topsin M and the now-discontinued Benlate, he says.
Many growers use DMI fungicides, such as tebuconazole and some others, because they are off patent.
Inspire Super is a different type of DMI that is premixed with fungicide from a different class and “works really well,” Langston says.
Another group, SDHI fungicides--such as Pristine and Fontelis, used to be very effective, but growers have seen a lot of resistance to them in recent years, he says.
Luna is an SDHI that works well and has no resistance, Langston says.
“That’s probably the most effective fungicide used against gummy stem blight today,” he says. “We don’t see cross resistance between Luna and the other SDHIs.”
All of the fungicides are sprayed preventively, not in response to what is seen in a field, Langston says.
Grower Greg Leger, owner and president of Leger & Son in Cordele, Ga., is grateful for the work researchers like Langston have done.
“Every time it rained, [gummy stem blight] would eat us up,” he says.
But that has changed since researchers have recommended effective mixtures and combinations of sprays.
“Now when we spray, we feel confident that we can keep it in check,” he said. “Gummy stem blight is not one of our major issues right now.”
Leger asks those who grow under his label to use Bravo before the fruit matures. Inspire Super and a combination of Topsin and Penncozeb are some of the other fungicides he uses.
“You have to stay on a spray program,” he says, “especially in rainy weather.”