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Questions about crop disease management in cotton and peanuts peak during the sweltering Dog Days of Summer.

Bob Kemerait, Plant Pathologist

July 16, 2020

4 Min Read
Brad Haire

With a good crop in the field, the Dog Days of Summer can be like leading a NASCAR race with 30 laps to go. You are close to Victory Lane, but a lot can happen in 30 laps.  Having disease in the field is like running low on fuel. You are praying for a yellow flag for gas and tires. 

In the case of row crops, a yellow flag is similar to a new or different fungicide application. Either way, careful planning and can bring the race, and the crop, home.  The most important goal in racing, or farming, is to not fall so far behind in the pack so as to not finish the race.

Questions about disease management in cotton and peanuts peak during the sweltering Dog Days of Summer. 

One is that by this time of the season, disease pressure is high from the abundance of infective spores in the environment. A second reason is that the crops have had lengthy exposure to pathogens, giving ample opportunity for diseases to develop. Third, growers are well into the heat of battle where fungicides are the weapons against disease. 

Cracks in the armor of a management program are more evident as diseases (and nematode problems) appear in the field.  Lastly, growers begin to look for the light at the end of the tunnel and will have questions about finishing the season without letting diseases take their yield and profit.

Spot Diseases

Spores of peanut leaf spot and cotton target spot diseases build continuously throughout the season and poor control early leads to a snowball effect later. The spores that cause the very first spots on the leaves may come from debris remaining from previous crops. Once spots form, new spores will be born which will produce more spores which will produce more spots and more and more spores and more and more spots.  Because of this proliferation of infective spores, plants are under increasing attack and disease becomes more and more likely.  

By August, most peanuts will be lapping the row middles and much of the cotton crop will have a dense canopy of foliage. The crops are more susceptible to diseases at this stage because of the prolonged leaf wetness and increased humidity within the canopies and because as the crops transition from vegetative growth to reproductive growth, they can become more susceptible to disease.  By August, the plants have been under assault from various diseases for nearly three months and some disease is bound to occur, no matter the choice of fungicide program. 

The most urgent question from growers once August rolls around is how to move forward if disease problems are apparent in a field. There are some problems that just can’t be fixed.  You can’t fix tomato spotted wilt in peanuts now. You can’t fix nematode problems.  You can’t fix bacterial blight or Fusarium wilt on cotton. These problems had to be managed before you closed the seed furrow. 

Other problems that can’t be fixed, no matter how good a fungicide may be, is too much defoliation from leaf spot or target spot or areolate mildew. These diseases build like freight trains; perhaps slow to start, but very difficult to stop once they reach full speed.

Where peanut growers discover disease that has not advanced too far (by the way, visible from a windshield at 35 mph is likely “too far”) there are several potentially effective options. The first is to tighten the spray window for the fungicides, perhaps spraying every 10 -12 days rather than every 14 days. 

Curative Fungicide

Another option is to ensure that the fungicide you are spraying has some curative activity. Fungicides with curative activity will not fix spots but they can keep recent, still-invisible, infections from becoming spots. 

Lastly, growers can switch from one fungicide to one recognized to be more effective against a specific disease. But in doing so, growers must remember that no fungicide, no matter how good, can work miracles.  Some of our most promising fungicides are best at keeping diseases from developing, not from stopping a disease that is already burning.

August is a time when many growers begin to plan for the end. For most peanut growers, the all-important 90-day application for control of white mold will be made in August. Depending on the fungicide program, and if white mold is well-managed, there may be no need for further control of this disease. If white mold still threatens, then additional sprays should be made. 

Peanut growers should plan to protect their peanut crop until within approximately 2-3 weeks of harvest, and possibly within 4 weeks under low disease pressure. Cotton growers should consider protecting their crop from target spot until the 6th week of bloom and protecting their crop from areolate mildew until within 3-4 weeks of planned defoliation.

About the Author(s)

Bob Kemerait

Plant Pathologist, University of Georgia

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