Target spot pathogens appear to be repeat offenders

Odds are that if you observed symptoms of target spot or Corynespora cassiicola in your cotton this year you’re probably going to see it again if you plant cotton in that field next year.

That’s the advice Heather Kelly, an assistant professor in field crops plant pathology at the University of Tennessee Institute of Agriculture, gave farmers attending the 2016 Cotton Tour at the West Tennessee Research and Education Center in Jackson.

Dr. Kelly, who also serves as Extension plant pathologist in west Tennessee, said target spot historically has been less prevalent in the upper Mid-South than in fields in south Alabama and Georgia. That has changed in recent years.

“We saw some symptoms in 2013,” she said. “It came back in 2014 and 2015, and this year, 2016, is the first year we’ve definitely seen severe infestations in commercial fields. You can probably find a little bit of target spot in every field in Tennessee based on the feedback I’ve received.”

Target spot, which shows up as brown to brick-red lesions that display concentric rings as they get larger, first appeared in south Georgia and south Alabama before 2008. The same fungal pathogen causes target spot on many other hosts including soybeans and tomatoes.

Cross over to different hosts

“Research is being done to try to better understand why it all of a sudden has become problematic on cotton in this area and to understand why the same fungal strains can cross over into different hosts,” says Dr. Kelly.

(Crop consultants in Arkansas reported observing Corynespora leaf spot lesions in a number of soybean fields late season in their area.)

Dr. Kelly said target spot typically won’t appear on the lower leaves of the cotton plant until the canopy closes. “Target spot likes it really humid so it’s down in that lower canopy and only when you start to see defoliation in the lower canopy does it start to climb up the plant into the middle and upper canopy.

“The first thing to do is to make sure it’s identified based on that symptomology rather than the other leaf spot complexes. It can be responsive to fungicides,” she noted, adding scientists are investigating reports target spot has also appeared on the squares and bolls of cotton plants. “Those have not been confirmed.”

University of Tennessee scientists conducted trials in 2014 and 2015 on the efficacy of several fungicide treatments on different varieties of cotton, including two that Dr. Kelly included in her presentation during the Cotton Tour (Deltapine 1321 and Phytogen 499).

More vigorous varieties

“We’re seeing some varietal differences, but we don’t see any varieties that are resistant to target spot,” she said. “We think the response is based more on the growth of different varieties – the more vigorous growing the variety, usually the more target spot you will see on it.

Scientists also saw an increase in the levels of defoliation for both varieties between 2014 and 2015 although the weather patterns were relatively similar for each year.

“I believe it was the buildup in inoculum that’s happened over the course of time and with that we’ve gotten a little earlier disease onset,” she said. “Last year the disease came on in the last week in July, and, in 2016, we’ve also seen it come on in the last week in July. There may be some commercial fields where it did come on in the middle of July, but I don’t know we’ll see it much earlier than that in Tennessee.”

Scientists also looked at application timing, spraying some plots at the first week, third week or fifth week of bloom. Some plots received double applications – the first week and third week or the third week and fifth week of bloom

“What we found is the results varied from year to year because of the disease onset occurring earlier in 2015 that first and third week of bloom shot looked a little better than the rest where in 2014 it was the later applications for the third and fifth week,” she said. “So we’re thinking it might be the third week of bloom, depending on a field-by-field basis.”

Significantly different yields

The 2014 trial’s Phytogen entry was the only one that produced yields that were significantly different,” she said, noting that those were at a 90 percent confidence level statistically. “Normally we like to have a 95 percent confidence level.

In the other years and varieties, we didn’t have significant differences, but we did see a trend where the fungicides were protecting the lint.”

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