Thursday, September 23, 2021

Look For Wet-Weather Diseases

By Amanda Huber,
Southeast Editor

target spot
Target spot symptoms include irregular-sized leaf spots with concentric rings — photo by Dr. Bob Kemerait, University of Georgia

Managing the cotton crop in many Southeast fields is presenting a challenge in 2021. Weather conditions shaped a planting window from April to June. Even into midseason, rain has prevented timely spraying and caused waterlogged soils in some areas.

Cotton responds to oversaturation of soils in different ways. Plants may become chlorotic, reduce nutrient uptake and, therefore, shoot growth. Typically when soils dry, the crop rebounds and begins to grow again. However, extended leaf wetness can lead to foliar disease.

Wet-Leaf Foliar Diseases

University of Georgia Extension plant pathologist Bob Kemerait says, “Rain and increased humidity create favorable conditions for infection and spread of target spot. Additionally, heavy rains can leach potassium from the root zone, thus increasing the risk for Stemphylium leaf spot and Cercospora leaf spot. Ascochyta ‘wet weather’ blight can also produce spots with concentric rings.”

Although each of these diseases produces leaf lesions, it is the placement on the cotton plant that helps differentiate between them.

Kemerait says Cercospora leaf spot, which is associated with potassium deficiency, and Stemphylium leaf spot appear first in the upper leaves.

“Target spot disease, or corynespora, begins as a small spot and develops into white lesions with the characteristic, target-like concentric circles on them. At this state, the plants begin to defoliate rapidly.”

Scouting Is A Must

Auburn University Extension plant pathologist Amanda Strayer-Scherer says areolate mildew is another fungal disease that begins in the lower canopy and can cause premature defoliation.

“Target spot pressure is normally heaviest in southwest Alabama, and symptoms include irregular-sized leaf spots with concentric rings. Areolate mildew is easily identified by a white mildew covering parts of or entire leaves.”

Fungicides should be applied preventatively for target spot starting at the first or third week of bloom followed by a second application at the third or fifth week.

“Alternatively, you can apply fungicides at the first sign of target spot or areolate mildew and apply a second application as needed. However, keep in mind that target spot can be difficult to control with fungicides if 25% to 30% of the leaves are already gone. Priaxor, Revytek, Elatus and Miravis Top should do well against both diseases,” she says.

Improve Application Coverage

Kemerait says it can be a challenge protecting the crop against a disease such as target spot where fungicide coverage is needed deep in the canopy.

“Getting sufficient fungicide coverage can be improved by increasing spray volume, increasing spray pressure and making the first fungicide application early enough before the canopy of leaves is fully closed.”

This is one reason farmers are advised to scout the crop often checking the lower canopy for tell-tale symptomatic leaves.

“If target spot is not identified after scouting, growers may delay a fungicide application and scout again in the coming weeks. Fungicides are not needed after the sixth week of bloom.”

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