By Vicky Boyd

University of Florida researchers admit they still don’t know much about a new tomato disorder that first appeared in the state in 2006.

But they say they do know what tomato purple leaf disorder isn’t.

“The only thing we’ve really ruled out definitely is it’s nothing associated with chemical application, and it’s totally independent of nutrition,” says Gary Vallad, an assistant professor of plant pathology based at the Gulf Coast Research and Education Center in Balm, Fla. “And as far as we can tell, there’s no environmental cause—it’s definitely not ozone.”

Regardless of the cause and possible vectors that transmit it, Vallad recommends that consultants and growers keep their eyes peeled for the problem. If they suspect they see it, they should report it to their county Extension agent or contact him at or (813)634-0000.

Charlie Mellinger, a principle in the Jupiter, Fla.-based Glades Crop Care, says he and his team of consultants are doing just that in the Homestead and Immokalee areas, where they scout fields.

“We haven’t seen it in the Homestead area this season,” says Mellinger, a plant pathologist. “We’ve seen a few plants here and there in the past but not nearly the size of the problem that seems to have occurred in the Palmetto and Ruskin areas, where it was originally identified.

“We’re keeping our eyes open for anything new and potentially problematic.

That’s why we need to stay alert—just in case it turns out to be yield grabbing, we don’t want to be surprised.”

Bob Gilbertson, a University of California, Davis, professor of plant pathology, says Western pest control advisers should take a similar approach.

So far, the disorder appears confined to Florida. But Gilbertson says PCAs should be on the lookout for it as well as a new ilarvirus first reported during the 2008 season. The ilarvirus was found in scattered fields from Fresno County, Calif., north to Colusa County.

“If they’re going in to monitor fields for tomato yellow leaf curl virus, this is something they should add to the things they’re looking for,” Gilbertson says of tomato purple leaf disorder. “It’s certainly a reason to minimize the movement of transplants, particularly from places like Florida into California. The jury’s still out on how damaging it is, but it’s clearly something you don’t want to see show up here.”

Purple leaf symptoms
Tomato purple leaf disorder first appeared during the 2006 season in Hillsborough and Manatee counties. It has since spread to Suwannee County and the Miami-Dade area.

In addition, growers who use a wide range of management practices have reported the disorder.

“Last fall, we didn’t see it in too many areas, and this spring, it’s too early to tell,” Vallad says. “The year before, it was pretty widespread, but it’s hard to tell how many acres were involved. Whether it’s causing economic damage is still kind of a big question mark.”

Probably the most striking symptom is the purpling of the leaf, he says. The only other problem the purple leaf disorder could be confused with is mild phosphorus deficiency.

With phosphorus deficiency, Vallad says, the entire leaf turns purple. But with the leaf disorder, only the portion between the veins is purple.

In addition, tomato purple leaf is typically found on the top portion of the leaf and not the underside. Any part of the leaf that is shaded by other leaves or fruit remains green, while the portion in the sun will turn purple.

Affected leaves don’t seem to have decreased chlorophyll production, which suggests the purpling is due to the increased accumulation of anthocyanins—reddish and purple plant pigments, he says.

Symptoms are more pronounced on grape and cherry tomatoes. But Vallad says that could be because the indeterminate growth habits of many of these varieties extends their growing season, allowing more time for symptoms to develop and increased exposure to the causal agent. The disorder also has been reported in round and Roma tomatoes, but symptoms generally are less severe.

During the first season, Vallad says growers reported the symptoms appeared six to 10 weeks after planting. But during the fall of 2008, some growers reported their fields were symptom-free until the last couple weeks before the December freeze.

Growers enhance knowledge base
Much of what Vallad and fellow pathologist Jane Polston, who’s based in Gainesville, Fla., surmise about the disorder has come from anecdotal information from growers.

Many report the disorder is associated with fields with high whitefly populations.

Until researchers can determine the causal agent, Vallad says they have yet to confirm that whiteflies are vectors.
Growers also have reported that one season only a few plants may show symptoms, Vallad says. The following season, it shows up in force throughout the field.

Initially, he says researchers had a hard time determining whether the disorder caused economic losses since it typically appeared with other diseases.

But last year, they were contacted by a couple of growers who had apparently clean fields except for the purple leaf disorder.

“It was the first time you could see this phenomenon with no other diseases interfering with it, and you actually saw the vines decline with this,” Vallad says.

Vallad and Polston continue to try to identify the causative agent and method of transmission. Vallad’s initial experiments using rooted field cuttings suggests that the disorder may be caused by a biological agent, but the method of transmission remains unknown.

He currently is conducting graft-transmission studies to see if the disorder can be moved between plants.

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Researchers hope this season sheds light on new ilarvirus

University of California researchers say they hope to learn more this season about a new ilarvirus tentatively named Tomato necrotic spot virus that appeared in scattered fields throughout much of the state’s tomato central and northern production areas last season.

The ilarvirus family includes two other members that infect tomatoes: Tobacco streak virus and Parietaria mottle virus.

They are spread by thrip-mediated pollen transfer.

Infected pollen from a plant—whether a tomato or another species—lands on a tomato leaf where thrips have fed and caused cell damage. The virus enters the plant through the feeding site.

The new ilarvirus causes light brown necrotic lesions predominately on the leaves but also on some fruit, says Bob Gilbertson, a University of California, Davis, professor of plant pathology.

Unlike tomato spotted wilt virus, where the necrotic lesions are mostly along the leaf vein, spots associated with the new virus are scattered across the leaf.

The necrotic lesions also tend to be lighter in color than the darker brown ones caused by tomato spotted wilt.

Fruit infected with the new virus typically don’t have the darker rings surrounding the lesions and don’t tend to be bumpy or misshapen, as they are with tomato spotted wilt, Gilbertson says.

Researchers say they are interested to see if the virus appears in 2009 and at what level, or whether it was an unusual occurrence favored by some unknown factors during 2008.