Leaf spot remains the No. 1 disease worry for North Carolina peanut farmers, and North Carolina State University Extension Plant Pathologist Dr. Barbara Shew is urging folks to be prepared this year.
“We’ve been in a situation for the last couple of years where everything has been very favorable for diseases to develop. Long periods of high relative humidity are very favorable for leaf spot development. Temperatures from 60 to 90 degrees are favorable for leaf spot,” Shew said at an Extension peanut production meeting in Williamston.
“We can have periods of high relative humidity even during dry spells. For the past couple of years, we continued to have high humidity and warm over-nights even at times when it was not raining. This is favorable for leaf spot,” she added.
For several years now, favorable conditions for leaf spot began early and remained well into the fall with spray dates right up to Sept. 30 throughout North Carolina. “If you put the cycle of leaf spot starting earlier in the year and lasting later, we’ve ended up with a lot of extra time for leaf spot to develop,” Shew explained.
In addition, because of humid conditions and hurricanes, big populations of leaf spot pathogens have been building up statewide. “Because of hurricanes and late digging and pod loss and in some cases, fields being abandoned, we’ve had a lot of volunteers out there, and volunteers are an excellent way to keep the pathogen surviving over a longer period of time than you would have otherwise,” Shew said.
Moreover, Shew laments that the popular variety Bailey is not as resistant to leaf spot as it once was.
“When we first had Bailey, it was really kind of spectacular how well it did against leaf spot. That’s not so much the case anymore. It’s not because something has happened to Bailey itself; Bailey is probably the same, but what has happened is that the leaf spot pathogens have adapted to Bailey, and now they are more able to cause disease on it than they used to be.”
For control, North Carolina State is recommending farmers use different fungicide spray programs to try to control the pathogen. Shew emphasizes that it is important to get fungicides on both upper and lower leaf surfaces and it is important to get the fungicide into the lower canopy.
“Leaf spot starts down low, so you need to get the fungicide where the fungus is,” she said.
Farmers need to know their leaf spot risk. “If you had problems controlling leaf spot in the past, you have a higher risk in the future. Rotation continues to be really important,” she said.
Shew advises farmers to start fungicide applications no later than R3 and they need to consider keeping leaf spot control programs later into the season.
Moreover, most fungicide groups do not provide curative ability for leaf spot. Group 3 (DMI) fungicides do offer some curative ability, but it is limited to a relatively short period of time after infection. The rest of the fungicide groups do not offer curative ability.
“My experience and opinion is that the benefit of spraying earlier than 45 days after planting is doubtful in North Carolina, except in very poor rotations. Peanuts planted in June or in the most southern parts of the state also may need to be sprayed a bit earlier than 45 days.
“Don’t skip sprays unless indicated by advisory. Don’t stretch intervals unless indicated by advisory or specifically recommended by advisory or specifically recommended for a given product and rate. Don’t stop programs too soon; 105 days after planting is too soon in most years,” Shew said.