Update on Suspected Areolate Mildew in North Carolina

— Written By Lindsey Thiessen
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Recently, cotton producers have been concerned with areolate mildew (Ramularia areola syn. Ramularia gossypii) in North Carolina cotton. This disease is not common to North Carolina, but recent wet weather and its presence in South Carolina suggests it could have made the leap into NC cotton as well. We have heard several counties in eastern NC seeing this disease. We are actively investigating the presence of this disease in NC, and physical samples will help determine its identity and distribution. Should you suspect the presence of this disease, contact your local county agent and samples should be sent to the NC State University Plant Disease and Insect Clinic for confirmation. To submit a sample, send several detached leaves in a plastic bag, along with a damp paper towel to prevent desiccation (if mailing a sample, leave the paper towel dry). If the sample can’t immediately be sent to the clinic, it should be refrigerated until it can be sent.

Symptoms of this disease including white mildew covering parts of or entire leaves. These symptoms appear at the lower portion of the canopy and work their way upwards, in some cases very rapidly. Hyphal mats or white strands of the fungus can be observed when examined closely.

There’s little information about this disease in the United States. It has been observed in Georgia, Alabama, and, as of this year, South Carolina. There’s not much information to suggest that it causes significant yield losses, but a fungicide application (e.g. Headline, Quadris, Topguard, etc.) may be warranted with continued conducive environmental conditions.

Other leaf spots are also more prevalent at the moment, including target spot, cercospora leaf spot, ascochyta, and bacterial leaf spot. We rarely have conditions necessary to warrant fungicide applications to reduce losses from fungal leaf spots in North Carolina. Depending on the growth stage of the cotton and foliar disease present, a fungicide application may be warranted with continued wet weather. Based on recommendations in other states, if cotton is currently within the 6th week of bloom or earlier, and the disease is confirmed and prevalent in large proportions of the field with a weather forecast conducive to further disease development (cloudy/damp/wet conditions and saturated soils), a fungicide application may be necessary. For fields that are beyond the 6th week of bloom, fully cutout with fairly mature bolls, and within 4 weeks or so from defoliation, a fungicide application may not be warranted. Before any fungicide sprays, it is important to submit samples to the disease clinic to confirm the presence of this disease and rule out other less destructive pathogens.

Lastly, if the NC State University Plant Disease Clinic confirms areolate mildew in one or more of your fields, this presents a great opportunity for on-farm research, since this disease is relatively new to NC and there is little data to determine when fungicide applications may be warranted. If you are interested in participating in a simple field trial, please contact your county agent immediately so that we can establish a simple field trial to help answer these questions on your behalf.