STEPHENVILLE -- Mark Allison, Comanche County farmer, first noticed a few yellow vines in his watermelon crop in the late 1990s but didn't think much of it.
There are a number of diseases that cause melon vines to yellow, but this one thumbed its nose at conventional treatments. The next year, it struck again, turning half his crop into a mass of yellow, wilted heap of vines, leaves and unmarketable fruit.
At war with diseases, often besieged by unpredictable weather, melon farming is a high-investment, high-risk crop. With tens of thousands of dollars in production costs at stake, Allison called his first line of defense, Bob Whitney, Texas Cooperative Extension agent for Comanche County.
Whitney brought in Dr. Forrest Mitchell, a Texas Agricultural Experiment Station entomologist. Mitchell, who is based at the Texas A&M University System Agricultural Research and Extension Center at Stephenville, got right to work. Mitchell had been involved in the hunt for the cause of yellow vine disease since the mid 1990s.
In 1991, yellow vine disease made its premier performance in Texas and Oklahoma, striking melon and cantaloupe fields with a vengeance. Some fields were completely wiped out. By the mid-1990s, it was clear the melon industry throughout the region was at risk.
In response to the threat, Mitchell joined a multi-disciplinary task force that included Texas A&M System scientists, U.S. Department of Agriculture scientists at Lane, Okla., and Oklahoma State University researchers. Work proceeded on various fronts, as some theories about the disease were proven and others discarded.
For example, in 1997, Mitchell and his colleagues at the Stephenville center were the first to positively identify a rod-shaped cell – bacterium-like organism – that was at work in yellow vine disease.
The scientists knew an insect had to be spreading the disease, but which one remained a mystery for some time. At first the chief suspect was a little known species of leafhopper. But as more data was accumulated, it became obvious that another insect, the squash bug, was the culprit.
The proof involved "Koch's postulates," a series of rules for finding the cause and vectors of disease. In use for more than 100 years. Koch's postulates have been used to identify causal agents behind everything from AIDS to simple bacterium-related diseases.
In the case of yellow vine disease, following Koch's postulates meant:
Identifying the bacterium in tissue samples of plants during all stages of infection; Isolating the bacterium by growing it in culture dishes; Reinfecting a healthy plant with the isolated bacterium to see if it would result in infection; and Letting squash bugs feed on infected plants, and then seeing if they would in turn infect a healthy plant.
"It was a surprise to many," Mitchell said. "The squash bug is a plant feeder, but it had not been known to transmit any other disease agents."
Further work proved the damage done by the squash bug was two-fold. First, it infected the plant with the bacterium. The bacterium eventually clogged up the plant's phloem, the principal food-conducting system. But the damage didn't stop there. The squash bug's feeding lacerated the xylem, the water-transporting system.
For the melon plant, it was as if it suddenly and simultaneously developed severely clogged arteries and a digestive blockage.
Worse, the disease would often not become evident until the melons were nearly mature. A week or so before harvest, the grower would look out upon his fields and see a healthy, profitable crop. Practically overnight, vines would begin to turn yellow. And two days later, half or more of his melon crop – and his chances for profit – would be gone.
"When the foliage died, the melon would immediately develop sunburn. But it didn't matter. As the vines died, the melons would stop maturing. The losses were 100 percent in some cases," Mitchell said.
With the insect vector identified, the entomologists could develop a control strategy. As with any disease, it's best not to contract it in the first place, but this was particularly true with yellow vine.
"Usually, once it started, it was unstoppable." Mitchell said. One strategy was to plant a trap crop, something the squash bugs would find more attractive than a melon. And what do squash bugs like best? You guessed it: squash.
The idea was to plant a squash crop early in a strip bordering the field. Better yet, have the squash up and growing good-sized fruit while the melon crop was just getting started. Incoming squash bug would be drawn to the squash borders where they could be sprayed with insecticide before they had a chance to infect the main crop.
That was the theory. It had even been tried on a small scale and without scientific controls.
But theory is one thing and reality out in the field – particularly in commercial agriculture – can be something quite different, Mitchell noted.
So when the Extension agent contacted him, Mitchell couldn't have been happier. He now had the opportunity to test the theory in large scale commercial fields.
"Without Whitney, we would have been at a loss. He knew all the growers personally. He vouched for us and smoothed the road," Mitchell said.
With $50,000 or more in production costs at risk, Allison was glad to cooperate. He planted a bordering squash crop to Mitchell's specifications.
"Actually, he did better. We were recommending a squash trap crop only on the north side of the field because that's the direction the bugs fly in from. But he planted squash on all four sides. He established a perimeter," Mitchell said.
Following Mitchell and Whitney's suggestions, Allison sprayed the squash crop and the melons early in the season.
The results were good that year and the next. Allison has been spared any serious infestations since using the trap crops. Other growers in the area have seen similar results. By all regards, the technique is a success and has kept melon growing a profitable venture in Comanche County and the region, Whitney said.
Comanche County growers alone plant 800 acres of melons annually, with a potential value at the farm gate of $5,000 per acre, Whitney said. And the industry has a huge economic effect, not only on the agricultural community but on the small and mid-size towns throughout the area.
"Starting now (mid-June) our growers will ship a million pounds of watermelon a day," Whitney said.
The project has been a success from a research perspective too, Mitchell said. It has brought to bear a diverse group of professionals to address a real-world problem directly affecting people's livelihoods.
"It was truly a team effort," he said.
Source: Eurekalert & othersLast reviewed: By John M. Grohol, Psy.D. on 21 Feb 2009
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