California Researchers Look into the Causes of Vine Decline
By Lisa Lieberman
The Tomato Magazine
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
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
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
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