Verticillium wilt, an old San Joaquin Valley cotton nemesis virtually forgotten for decades, reappeared in 2004 in a wake up call to growers and researchers alike.
"Just because you may have not seen vert for several years does not mean you can let your guard down if you have fields with a history of verticillium," said University of California cotton specialist Bob Hutmacher.
The wake up call was very evident in a pair of San Joaquin Valley Cotton Board variety trials where ideal weather conducive to late-season verticillium wilt was the worst Hutmacher has seen in more than two decades of working in SJV cotton.
"The average infection in random cut stem sections of susceptible varieties was well over 60 percent. In some varieties, 80 percent to 90 percent of plants cut show evidence of verticillium," said Hutmacher, adding in many cases the vascular symptoms of the disease were detected in the stems of green plants.
Verticillium has never been a problem throughout the entire San Joaquin Valley. However, it became so severe in certain areas three decades ago the only cure was the development of resistant varieties. It seemed to be spreading before resistant varieties were developed. Those varieties saved the day.
Apparently, though, it has not been a major focus of some breeders because the verticillium separation between varieties was striking in at least two SJVCB trials.
Striped like zebra
In photographs Hutmacher took of the trials, the trials were striped like a zebra. "The stripes are right to the row of the different varieties planted at the site, and for the most part, the symptoms go all the way through the plots," said Hutmacher.
Once infected, the pathogen remains in a field and attacks the plants under the right soil and weather conditions like those this season.
"I do not believe there should be a huge alarm sounded for widespread verticillium, but 2004 was a reminder than when conditions are just right the disease can be a significant problem," said Hutmacher. One of those conditions was late season irrigations in an attempt to set a top crop.
Hutmacher said some of the new varieties coming through the SJVCB screening trials, including some of the new Roundup Ready Flex entries, are "highly susceptible to verticillium damage in a year like this one.
"In looking at a lot of our Acala variety trials, it has been fairly consistent that Phytogen 78 was generally the worst affected by verticillium out of our approved Acalas. Phytogen 72 is intermediate, but certainly both seem more affected than most CPCSD (California Planting Cotton Seed Distributors) entries," said Hutmacher.
The verticillium appearance in the two trials only served to bolster Hutmacher admonition that growers should take care to match varieties with conditions on their farm, using results from SJVCB and UC variety trials.
‘Don’t ignore vert’
"Making the right choices for varieties in your area grows in importance when you see the significant varietal differences in verticillium resistance we saw this year," he said. "Don’t ignore vert if there is a history of it in your area. If you do not make those right choices, you are setting yourself up for potential problems."
The situation with verticillium susceptibility, added Hutmacher, is also worth a "heads up" for researchers and breeders. Dollars have grown short for university research and there has been talk of cutting back on variety screening for verticillium.
This year may reverse that.
"We may need to re-think how much effort is needed to both screen varieties for verticillium and put some education efforts together to remind growers the importance of watching for developing problems, what to look for, and the importance of using a range of varieties on farm until you have a good handle on susceptibility to verticillium and other problems," said Hutmacher.