In our research plots and in grower fields, we are beginning to see target spot and frogeye pressure. Target spot is caused by Thanatephorus cucumeris (more commonly known as Rhizoctonia solani) and frogeye is caused by Cercospora nicotianae. These are both true fungi, not oomycetes (water molds) like the black shank and blue mold pathogens. Foliar fungicide applications may be necessary for effective management of these diseases.
Both target spot and frogeye start in the oldest, lowest leaves on the plant. Early lesions look very similar between the two diseases, however, target spots tend to expand in rainy conditions, while frogeye lesions will stay smaller than a dime. Frogeye will have white centers, surrounded by a thin tan ring with yellow on the outside (Figure 1); target spots will be primarily brown. Larger target spot lesions develop rings within the spot, and sometimes the centers will fall out (Figure 2). Target spot near the stem end of the leaf can lead to total leaf loss.
Rainy environmental conditions will speed up development of both diseases. Raindrops splash spores from the pathogens into the upper leaves, allowing these diseases to progress higher on the plant.
If tobacco growers have not already done so, consider making a Quadris application. Going through available data from multiple states, in most years spraying Quadris 4 to 6 weeks after setting tobacco reduces target spot. This is most effective if drop nozzles are used, so the chemical is applied where the disease begins. Growers may consider increasing the Quadris rate to 10-12 oz / A for additional protection against blue mold, if there is local concern about this disease. Quadris “burn” may result if plants are sprayed at high rates under stressful environmental conditions, so growers are encouraged to take precautions. Always follow the product label for safety information and allowable uses.
- 2016 Fungicide Guide for Burley and Dark Tobacco (PPFS-AG-T-08)
- Burley and Dark Tobacco Multi-state Production Guide (ID-160)
By Emily Pfeufer, Extension Plant Pathologist