Areolate Mildew Confirmed in North Carolina

— Written By Lindsey Thiessen
False Mildew on Cotton

False/Areolate Mildew caused by Ramularia areola on cotton. (Photo courtesy of Terri Thomas)

Areolate mildew (Ramularia areola syn. Ramularia gossypii) has been confirmed on samples collected from Halifax County, North Carolina. This is likely not the only field given recent wet weather conditions. 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.

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.

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

Based on recommendations in other states, if cotton is currently within the 6th week of bloom or earlier, and the disease is 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.