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Target spot severity varies among cotton varieties

NORTH CAROLINA STATE Cotton Specialist Keith Edmisten says growers should make some careful evaluations before spraying fungicides for target spot
<p> NORTH CAROLINA STATE Cotton Specialist Keith Edmisten says growers should make some careful evaluations before spraying fungicides for target spot.</p>
&bull; Little is known about the impact of the fungal disease on yield and quality, but recent research indicates there may be significant differences among varieties. &nbsp;

Target spot has created much concern among cotton growers in the Southeast over the past few years and began showing up in cotton fields in the Carolinas and Virginia for the first time last year.

Little is known about the impact of the fungal disease on yield and quality, but recent research indicates there may be significant differences among varieties.

Target spot has been reported in cotton in Georgia, Alabama and Florida since 2010, but prior to last year had not spread into the Upper Southeast.

Now that the disease has been documented in the region, growers are left to wonder whether target spot is something they should plan for in their varietal selection and cultural practices.

Target spot, technically Corynespora cassicola, began showing up in South Carolina cotton fields in late August last year and in quick order was reported in North Carolina, then on Aug. 26, of last year it was first reported in cotton in Virginia.

Clemson University Plant Pathologist John Mueller says there doesn’t seem to be any distinctive pattern as far as which fields are and which aren’t attacked by the diseases.

“We’ve seen it in fields where we had cotton behind cotton, and in fields where cotton followed two years of corn or one year of peanuts and one year of corn — it’s not just showing up in long cotton rotations, the veteran Clemson plant pathologist says.

Weather last year in South Carolina and most of the Upper Southeast created an ideal environment for the disease pathogen to grow. Cloudy, wet weather over extended periods of time seem to be ideal for the fungus to develop.

North Carolina State University Cotton Specialist Keith Edmisten says target spot first occurred in eastern counties in North Carolina last August. How widespread it was across the state isn’t clear.

“We have other leaf spot diseases (Stemphylium, Alternaria, Cercospera) that we commonly see in North Carolina. These diseases are often associated with dry periods and/or potassium deficiency.

First seen at top of plant

“These diseases are normally seen first on the top of the plant, compared to target leaf spot which tends to start on the lower portion of the plant,” Edmisten says.

“Typically, North Carolina growers don’t spray fungicides to manage these diseases, because there hasn’t been any documented proof that they can increase yields,” he adds.

Target spot has been more of a problem in the Lower Southeast, since it first began to show up in yield-damaging levels in 2010 and again in 2011.

Though it’s not a welcome visitor to cotton fields, on some varieties this disease will actually aid in defoliation, whereas fungicide applications may make defoliation more difficult.

As in the Carolinas, target spot’s arrival in Virginia created great concern among cotton growers, but how much real damage it caused to cotton is not clear.

If target spot defoliates mostly bottom limbs of cotton plants, it could have a positive effect by creating better air movement and lowering the incidence of boll rot.

However, if the upper leaves of the cotton plant are defoliated, it seems more likely that a significant yield loss would be possible if the disease reaches the mid-canopy level prior to 10 to 20 percent of plants with open bolls.

If the disease defoliates the cotton prematurely, then reduced micronaire and yields should be expected.

How widespread the disease will be in 2013 is hard to predict.

History from other cotton growing states indicates it won’t likely go away. However, it is highly dependent on extended periods of warm, wet weather to form.

Unlike most common cotton fungal diseases, target spot takes 24-48 hours to infect cotton roots — more than twice as long as other fungal diseases on cotton.

The most likely targets for the disease is cotton that is irrigated. The second most likely is cotton that is in a no-till or minimum-till situation, and in areas with conducive weather patterns.

Spores for the disease are more likely to be moved by seed than by air currents. The spores are too large to move great distances in the air, though tropical storm systems could increase the distance they move to some extent.

Research in the Lower Southeast indicates the disease causing fungi do survive in crop residue. So cotton following cotton or in rotation with grain crops and grown in minimum-tillage production may be at higher risk to the disease.

Since target spot has been more prominent in the Lower Southeast, most of the research on fungal disease has been done there.

Some question about seriousness

Auburn University Plant Pathologist Austin Hagan has worked with the disease pathogen for the past couple of years in cotton and says he still isn’t sure how big a problem it can be.

“I work primarily with peanuts, and when I see 40-50 percent defoliation, I’m expecting big yield losses.

“Even on varieties with some tolerance or resistance to the disease, we saw 50 percent defoliation from target spot on cotton. So, from that standpoint, it is a scary disease,” Hagan says.

“However, when you look at the impact the disease has on cotton yield, it’s a little like a beagle — its bark can be much worse than its bite,” he adds.

Target spot occurrence and severity is definitely different among different cotton varieties, the Auburn researcher adds.

In tests in 2011 and last year, he says PhytoGen varieties, in particular Phytogen 499, were more susceptible to target spot than Deltapine varieties, especially DPL 1050, which appears to be somewhat tolerant or resistant to the disease.

“In untreated plots at two different locations in the southern part of Alabama, we consistently saw 80 percent defoliation in Phytogen varieties, compared to 50 percent on Deltapine varieties.”

Hagan says this increased defoliation did not equate to lower yields in the PhytoGen varieties.

“By applying fungicides, we were able to see a 120 pound per acre increase in lint on PhytoGen 499, versus a 50-60 pound increase for DPL-1050, per application of Headline, and we can only use two applications of any of the strobilurin fungicides,” he says.

“Still, there was no statistical difference in yield between the two varieties in these tests,” he adds.

“Exactly how much target spot affects cotton yields is just not clear. We know heavy pressure and subsequent defoliation on PhytoGen 499 caused a 200 pound per acre loss.

“However, the yield potential of this variety is so high, maybe 7,000 pounds per acre, so that 200 pound loss could be more like 1,200-1,400 pound loss from the true yield potential of the variety, Hagan says.

Likewise, the plant pathologist says the efficacy of strobilurin fungicides for managing target spot on cotton is not clear.

“I’m used to working with peanuts, and when I put out 4-5 fungicide applications, I expect to see a clean field.

“With this disease, we were still getting 50 percent and higher defoliation of cotton plants,” he says.

For growers in the Upper Southeast, the jury is still out on target spot. At the very least growers should keep an eye out for the disease and make cost effective decisions whether to spray fungicides, if it occurs.

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