Green no cotton yield guarantee: Stem cuttings best vert identification

“It's not that easy bein' green; Having to spend each day the color of the leaves. When I think it could be nicer being red, or yellow or gold - or something much more colorful like that.

“But green's the color of Spring. And green can be cool and friendly-like. And green can be big like an ocean, or important like a mountain, or tall like a tree.”

Green is fine for identifying Kermit the Frog because after all he is a frog.

However, green may not be best way to identify a cotton variety's resistance to verticillium wilt, an old nemesis that made an unwelcome return to certain areas of the San Joaquin Valley in 2004.

The soil-borne disease created quite a stir last fall when some varieties in at least two University of California or San Joaquin Valley Cotton Board Acala cotton variety trials exhibited striking, seemingly yield-robbing signs of verticillium wilt compared to other varieties in trials.

“Vert” was a big issue in the valley 30 years ago when it had a major impact of cotton yields, especially on the east side of the valley. New verticillium-resistance variety development solved the problem and since then cotton breeders have made sure there was enough verticillium resistance to withstand major wipeouts. Apparently that worked last year because even though some varieties looked like verticillium had significant hurt yields in the variety trials, the final yield results proved otherwise.

University of California Extension cotton specialist Bob Hutmacher says if verticillium wilt leaf symptoms appear too early and attack a low resistance variety, it can significantly affect yields. Last year, symptoms appeared too late to have a major impact.

Verticillium looked worse that it was last season. Nevertheless, it may have had a negative impact on key varieties. However, there were no wipeouts, emphasized the cotton specialist.

When verticillium symptoms appear too early, the disease can discolor leaves, reduce leaf canopy and reduce photosynthesis, thereby reducing yields.

Cut and inspect

However, Hutmacher said at a UC Cooperative Extension meeting in Visalia, Calif., recently, a better way to detect verticillium early is by cutting stems to see if there is brownish discoloration of stems and leaf veins.

Hutmacher added the intensity or extent of vascular staining is not a good indicator of eventual severity of damage to leaf area or yield. However, it may give an early clue to the potential of the problem, allowing growers and consultants to take mitigating measures.

Last year, Phytogen 78 had the highest level of vascular staining of any other variety in one variety trial. It has vascular symptoms in 40 percent of the plants, he said, but verticillium did not have a dramatic effect on yields.

High-yielding Phytogen 72, one of the most popular varieties in the valley, also exhibited leaf and vascular signs of verticillium yet recorded excellent yields.

However, verticillium may have had an impact on Phytogen 72 yields, according to Hutmacher. In past years, Phytogen 72 has outyielded Maxxa, the valley standard, by an average about 15 percent in side-by-side comparisons. In 2004, this was reduced to about 7 percent overall, said Hutmacher. That change may have been a result of verticillium.

“However, there certainly were no wipeouts due to verticillium in 2004,” he added.

Visual impact

Probably the most visually dramatic visual impact of verticillium wilt was in a San Joaquin Valley Cotton Board first year screening trial where high levels of verticillium were readily evident in an experimental Phytogen Roundup Ready Flex variety. “You could see it from the pickup. It was the worst looking variety in the trial — hands down,” said Hutmacher. “And yet the variety topped the trial in yields.”

In the Merced County trial where stem sampling indicated as many as 40 percent of the Phytogen 78 had vascular staining, the variety finished third in yields. This was the highest yielding of the seven UC approved Acala trials with an average yield of 2,142 pounds across all varieties. Phytogen 78 yielded 2,212 pound per acre. Phytogen 72 finished eighth in the trial at 2,090 pounds. However, only 14 pounds separated positions seven, eight and nine in the trial and only 108 pounds separated fourth through eighth place.

The lowest rated verticillium in this trial was for Ultima EF, which topped the trial at 2,320 pounds per acre and Ultima, which finished fifth at 2,195 pounds per acre.

Verticillium can infect cotton as early as seedling development. The extent of damage to the growth and yield is more impacted by the degree of tolerance in the variety as well as the weather and cultural conditions favorable for development of verticillium.

Weather and cultural conditions that generally favor more severe verticillium symptoms include prolonged periods of cool, wet weather, cool weather with frequent irrigations and excessive fertilization or irrigation. The extent and timing of foliar symptoms is strongly impacted by those factors as well as the verticillium inoculum levels in the soil.

Defies conditions

In some years, Hutmacher said conditions are right for verticillium development, but it does not happen. He acknowledged that the reason why verticillium does not cause problems is not totally clear, but he believes it may be partly because soils levels of the more virulent verticillium inoculum in many cotton fields may be relatively low after many years of growing rotation crops or verticillium-resistant cotton varieties.

Hutmacher said verticillium's appearance after several years of anonymity is a wake-up call.

In the wake of last year's reappearance of verticillium, he suggested growers and consultants “consider any potential cultural controls and keep an eye out for possible varietal differences in disease susceptibility in choosing the mix of cotton varieties planted and crop rotations used.

“However, I to not believe there is great cause for concern. Seed companies are on this and are screening for verticillium as part of their operations,” he said.

Another thing learned about the verticillium red flat that went up in 2004 was “don't pick a variety just because it looks green. It may look green because it does not have a boll load,” he said. Verticillium wilt is a “much more complicated issue that just picking green varieties.”

You can tell a frog by its green color. You cannot identify a high yielding cotton variety by the same criteria.

e-mail: [email protected]

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