Alabama Cotton: Weather Ripe for Disease Development, Stemphylium Leaf Spot Active

    Target spot in cotton. Photo: University of Tennessee

    Warm, wet weather will continue to drive disease pressure in August, especially target spot and areolate mildew in cotton. As of August 11, target spot has been confirmed in several fields in south and central Alabama. Although areolate mildew has not yet been reported, it won’t be too far behind.

    Symptoms of target spot and areolate mildew will begin in the lower canopy and infection can result in premature defoliation. Target spot pressure is normally heaviest in southwest Alabama, and symptoms include irregularly- sized leaf spots with concentric rings. In contrast, areolate mildew is easily identified by a white mildew covering parts of or entire leaves.

    My advice is for producers to continue being proactive in protecting their crop from disease by scouting and making timely fungicide applications. Fungicides will not only protect your cotton crop from target spot and areolate mildew but also offer yield protection. As our cotton agronomist, Dr. Steve Brown, puts it “Alabama’s cotton crop is currently all over the board in terms of growth stage and potential.”

    So, producers should consider fungicide applications if their crop has good yield potential and is between 1st and 6th week of bloom and disease is present or conditions remain conducive for disease development. However, keep in mind that target spot can be difficult to control with fungicides if 25 to 30 percent of the leaves are already gone.

    Producers should continue to monitor for weather events and anticipate how they may impact management decisions. See the ACES Cotton IPM Guide for fungicide rates and recommendations.

    In other disease news, cotton leafroll dwarf virus (CLRDV) has been detected in our sentinel plots in Brewton, Fairhope, and Prattville. Leaf bronzing/reddening has been the most common symptom observed in these plots. So far, CLRDV incidence has been highest in cotton varieties PHY 500 W3FE and PHY 480 W3FE followed by DP 1646 B2XF and lowest in DG 3615 B3XF, which is the same trend we observed last year.

    In Alabama, yield losses can vary from 0 to 30% and later planted cotton seems to be more at risk for yield impacts. Unfortunately, we do not have any management recommendations to offer at this point in the season. For more information on the disease and management, please refer to the ACES publication ANR-2539-A new virus disease in Alabama Cotton.

    Stemphylium Leaf Spot

    Stemphylium leaf spot is starting to show up in North Alabama cotton. Stemphylium starts as small brown lesions, and as they enlarge, they can lead to premature defoliation. There are several cotton leaf diseases that have similar visual symptoms as Stemphylium. Stemphylium and Cercospora leaf spot start in the top of the plant where many wet weather diseases such as Corynespora target spot and Ascochyta blight start lower in the canopy.

    AgFax Weed Solutions

    It is important to correctly diagnose which leaf disease you are dealing with because Stemphylium is the secondary problem brought on by potassium (K) deficiency. Fungicides do not correct K deficiency or protect against secondary pathogens that infect leaves affected by K depletion.

    Potassium adds strength to the leaf cells and the lack of K in the leaf cells makes them weak and susceptible to secondary fungal infections such as Stemphylium, Alternaria and Cercospora leaf spot.

    Stemphylium often occurs in dry land fields under drought stress or in fields where roots are restricted due to a hard pan. Anything such as nematodes or Fusarium that damages roots can also impede the uptake of nutrients. Most of Northeast Alabama has had good soil moisture this year and although the crop is late, most fields are setting a heavy boll load.

    Bolls, which are K sinks, draw K from the leaves, and as K is depleted, affected foliage becomes susceptible to Stemphylium, Alternaria, and Cercospora leaf spots. Short season varieties can sometimes be more susceptible to Stemphylium since they often have an intense demand for K in a short time.

    Stemphylium can also be a problem under irrigation or fields with a high yield potential. Stemphylium leaf spots start to appear around the fourth week of bloom with a heavy fruit/boll load which corresponds to a heavy demand for K. The roots of the plant also start to decline then due to competition for carbohydrates by developing bolls.

    This adds to the challenge of taking up enough soil potassium. Dry weather may also promote leaching of K under heavy irrigation as the dry soils pull the water and K through the soil profile more quickly. There have also been a few cases of Stemphylium documented where high soil magnesium levels competed with K for uptake in the plant.

    Growers should soil test fields with Stemphylium to make sure there is adequate K. Petiole testing can help detect K deficiencies up to two weeks in advance, especially as the crop moves toward peak bloom. Foliar K may lessen the damage if K deficiencies are caught early enough (before the fourth week of bloom) in irrigated fields or dry land fields with adequate moisture.

    Unfortunately, if K deficiency is detected late in the growing season (sixth week of bloom), foliar K sprays will likely not be a benefit. I would not make foliar fertilizer applications to fields under drought stress as the plant may not be able to take it up and there is an increased risk of foliar burn.

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