New tomato yellow leaf curl guidelines combine cultural practices with insecticide resistance management

By Marni Katz

Florida tomato experts have beefed up recommendations for managing tomato yellow leaf curl virus and the whitefly that spreads it to emphasize an integrated, regional approach.

“If you use just cultural practices or just chemical practices, it won’t be sufficient—you have to use both,” says Jane Polston, a University of Florida plant pathologist in Gainesville. “It’s a combination of things you use simultaneously. Growers are really good biologists, and they have to be in order to integrate all of these things into an effective program.”

The Resistance Management Working Group, a panel of researchers, crop consultants and chemical industry representatives, met earlier this year and developed a more detailed set of management recommendations for the virus and the silverleaf whitefly.

Among the panel’s recommendations for controlling whiteflies are: rotating insecticides, host-free crop periods, crop sanitation and reflective mulches.

The group’s goal is to prevent full-blown resistance to neonicotinoids, a widely used insecticide class.

Since the first neonicotinoid was registered for U.S. use in 1995, whiteflies have become increasingly tolerant of the products, says David Schuster, a group leader and UF entomologist with the Gulf Coast Research and Education Center in Wimauma.

The virus defined
Tomato yellow leaf curl virus first appeared in southern Florida in the mid-1990s and has since spread rapidly throughout most Southeastern tomato-production regions.

Southeastern tomato growers battle other insect-vectored viruses as well, including aphid-born cucumber mosaic virus, tomato etch virus and potato virus Y and whitefly-transmitted tomato mottle virus. But none seems to have the combination of virulence and ease of transmission that make the tomato yellow leaf curl virus such a threat.

“Whitefly-borne viruses are our biggest problem in Florida tomatoes, and TYLCV is the most significant of those,” Polston says. “It is now present in every state in the Southeast, including North Carolina, South Carolina, Georgia and even Mississippi, Alabama and Louisiana.”

Infected whiteflies can travel long distances and transmit the virus from plant to plant during 15 to 30 minutes of feeding. Infected plants are marked by smaller, cupped leaves with chlorotic margins or mottling.

Commercial cultivars in Florida are equally susceptible, but often present slightly different symptoms, Polston says. Infected plants often are stunted, have smaller leaves and drop flowers at rates as high as 90 percent.

As with all insect-vectored viruses, the stronger emphasis is on controlling the vector rather than the virus itself.

“The vector is able to transmit the virus to the plant fairly quickly, so the interval between when an adult lands on a tomato plant and the opportunity to kill the vector is very short,” says Charles Mellinger, director of technical services for Glad Crop Care Inc., a Jupiter, Fla.-based crop consulting company.

“With our current insecticides, it is impossible to kill all the vectors before they transmit the virus. About 50 percent of the vectors will die before they can transmit the virus, and another 50 percent will transmit the virus before a neonicotinoid in the plant will kill them.”

As a result, the Resistance Management Working Group recommends keeping insect populations low and preventing them from entering susceptible fields.

Topping the group’s recommendations is maintaining a tomato-free period, which should allow whitefly populations to die down between crop cycles.

“The most important of those recommendations is the observance of a minimum two-month tomato-free period during the summer, preferably from at least mid-June to mid-August,” Schuster says.

While the host-free period may not be practical in all regions because of indeterminate varieties, growers at the very least should work to terminate their crops by mid-June and delay planting the fall crop as long as possible, he says.

Mellinger says growers should minimize the number of whiteflies that can enter a young field by reducing nearby disease reservoirs, such as cull piles, weeds and volunteer plants.

Polston recommends growers rogue symptomatic plants early in the season before second tie to keep resident whiteflies from transmitting infections to neighboring plants.

“I’ve seen examples where the virus in one infected plant can quickly spread to 20 more plants around it,” she says. “So the best thing is to get virus-infected plants out of the field, especially early in the season.”

Quick destruction of crop residue after harvest also is important to keep whiteflies from persisting and moving into neighboring fields or the new crop.

The panel recommends spraying plants within five days after harvest with a burndown herbicide mixed with heavy oil and a nonionic adjuvant.

But just killing plants with an herbicide will not prevent whitefly nymphs from maturing and moving into an adjacent field or host, Mellinger says. Adding oil to the herbicide will keep nymphs on the crop residue from maturing.

Schuster recommends destroying the crop block by block rather than waiting until the entire crop is harvested and allowing whitefly populations to grow.

Florida is considering legislation that would require tomato growers to destroy harvested tomato plants within a specific timeframe, Schuster says.

At the same time, he says growers should be careful about where and when they plant. They should avoid planting near older, potentially infested crops.

Schuster has seen reflective mulches significantly reduce tomato yellow leaf curl virus compared with traditional black and even white mulches.

“I have seen almost 70 to 100 percent of plants infected with the virus on black mulch versus 30 to 40 percent of plants growing on aluminum mulch, with white being somewhere in between,” he says.

The primary problem with reflective mulches is that fungicides will film over the surface and make them ineffective, Mellinger says. The Florida tomato industry is conducting research to find ways to make the reflective or metallic mulches more practical, he says.

Another challenge is cost. The reflective mulches are significantly more expensive than the standard mulches used on raised beds.

But Polston says growers can get around the cost issue by using 4- to 5-inch reflective strips embedded in their traditional plastic mulch. The strips give them the same benefit and they cost much less than full-bed reflective mulch.

Neonicotinoids should be used only during the first six weeks, leaving a neonicotinoid-free period at the end of the crop cycle, Schuster says. In the interim, growers should use alternative chemistry classes.

A typical program may start with a neonicotinoid soil drench at transplanting using Admire, Platinum or the newly registered Venom. Growers should not apply neonicotinoids through the drip system or make split applications.

Based on scouting recommendations, growers also can make foliar applications of neonicotinoids, such as Actara, Provado or Venom, instead of or in conjunction with soil drenches. But these should only occur during the first six weeks after transplanting, Schuster says.

After the six-week period, growers should switch to insecticides labeled for whiteflies—typically pyrethroids alone or in combination with organophosphates—and rotate them with oil or Endosulfan. Schuster recommends applying knockdown insecticides to kill adult whiteflies before cultural practices, such as pruning or tying.

When possible, Mellinger says, growers should focus on selective chemistries for later-season whitefly control, such as insecticidal soap or the insect growth regulators Courier, Knack or Oberon.