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Research

California Researchers Look into the Causes of Vine Decline

By Lisa Lieberman

The Tomato Magazine
June 2006

The most frustrating thing about vine decline is that a tomato grower can have a beautiful crop, and then just a few weeks before harvest, his plants start losing their canopy cover for no apparent reason. This results in his vines becoming weaker and his fruit becoming overexposed to the sun.

Most tomato growers are seeing some form of vine decline in their fields, but up until now, no one has been quite sure about what causes it or how to treat for it. Scientists still aren’t sure what the root cause of vine decline is, but this year, Gene Miyao and Mike Davis, two extension specialists with the University of California at Davis, are embarking on the second year of a study on vine decline that hopefully will help them come up with some answers for growers in the near future.

“It’s possible that vine decline could be bacterial, but it’s more likely caused from a fungal pathogen,” Miyao says. “We’re also looking at plant nutrients and water stress as possible causes. We speculate that the problem may not be related to a single factor.”

It’s normal for growers to see vines becoming less green and more yellowed toward the end of the season, but vine decline has additional tell tale signs, Miyao notes.

“When we’re dealing with determinate growth plants, the plants should not remain vegetative forever,” he points out. “But the tomatoes have got to be 90 percent ripe before the foliage begins to slow down. With this problem, the decline is happening 30 days away from harvest. At that particular growth stage, most of the fruit is green and you just have a few fully ripe tomatoes on the plants. When this happens, not all of the green fruit matures.”

Part of the lost tonnage caused by vine decline comes from rot caused by sunburn or black mold. Even if the plants fail to contract these diseases, Miyao believes vine decline causes yield losses.

“We’re probably losing fruit yield because the fruit isn’t able to size up as well as on a healthy plant,” Miyao observes.

In really bad fields, growers could lose 50 percent of their crop, although losses are more likely to be in the 10 to 15 percent range, the researcher says. But if you have a loss of 10 to 15 percent on a 40-ton crop, that’s at least four tons and would be considered substantial.

Aside from reduced tonnage, there’s another problem that develops in fields with vine decline. When plants start losing foliage, they also lose their ability to photosynthesize and produce sugars.

Miyao is especially worried about problems developing with vine decline this year since most California tomato growers got a very late start due to unseasonably late spring rains.

“We’ll probably have a very light harvest in the month of July and maybe into the middle of August,” he says. “Once we reach the middle of August, we’ll start getting in large volumes of fruit and the plants will be exposed to very high temperatures with an increased chance of sunburn.”

In response to vine decline, some growers try applying more nitrogen to their fields only to find the approach mostly ineffective since the plants are not nitrogen deficient, Miyao says. Some also try to maintain plant health by irrigating closer to harvest.

“ But with furrow or overhead irrigation systems, this creates additional problems with fruit rot as well as contributes to lower soluble solids,” Miyao observes.

Results from last year’s experiments show that applying more water to the plants near harvest doesn’t do much good anyway, since soil moisture in the trials was not a problem, Miyao adds. Another technique growers have tried to use with vine decline is to cover exposed fruit with a white clay-based material to deflect the sun.

The goal is to slightly lower fruit temperatures.

“It’s clear that these sun reflectants are effective in lowering temperatures, but, for the most part, are kind of a band aid and are not sufficient enough to reduce sunburn. Hence, we’re trying to work on the problem from a different angle,” Miyao explains.

In years past, researchers noticed that when they sprayed for late blight or for bacterial speck or spot in tomato fields, there was a corresponding reduction in vine decline, even though vine decline is not something growers normally spray for, Miyao says.

“We’re thinking that there could be a minor pathogen that no one would ever treat for coupled with other factors such as a low-grade soil or foliar pathogen. It’s possible that these three things are weakening the plant and are part of a more complex problem,” Miyao explains.

Ron Timothy, a tomato farmer in Dickson, Calif., grows 1,500 acres of tomatoes. He feels that, economically, vine decline is his biggest problem in the field.

“Usually this problem is associated with our later plantings, stuff that’s harvested in the last part of September,” he says. “The vines will grow spindly. They’ll set a lot of fruit, but you’ll have really small stems that won’t have any strength before the fruit sizes. The vines tend to collapse, and the tomatoes size up one-quarter to one-third less than their normal sizes.”

Timothy says he’s not sure what causes vine decline, however, he feels that he may have stumbled on something that can help prevent it.

“We’ve had some fields before that finished real strong?ones where we went in with a full insecticide program because we had leaf miner. We’ve also sprayed fungicides for powdery mildew in the past, and on those fields, the vine decline didn’t seem to show up,” Timothy says.

In his own trials to date, Miyao says that neither foliar fungicidal sprays nor soil fungicide treatments have helped lower vine decline incidence. The next step he hopes to test is to treat an infected piece of soil with methyl bromide.

“If we apply methyl bromide, and it’s highly effective, and we compare it to the nontreated controls, we could dig up the roots of the plant and see what the difference is to try to isolate the pathogen and make a comparison,” he theorizes. “Of course, methyl bromide is not going to be available in a few years. However, at least we will have identified a pathogen and be closer to finding a solution.”


© 2006 Columbia Publishing

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