Frogeye, target spot, and blue mold seem to be at the top of many tobacco growers’ minds these days. While blue mold may be of the most concern, and the disease most in need of immediate action, frogeye and target spot can also be detrimental to yield and leaf quality. Fungicides that manage frogeye and target spot differ from the fungicides that will manage blue mold, so a confident diagnosis is key to managing these leaf spot diseases. Here are a few reminders about these fungal leaf spot diseases, along with management recommendations once these have been identified.
Frogeye
Figure 1: Frogeye leaf spot on lower leaf of burley plant. (Photo: Kenneth Seebold, UK)
- Caused by Cercospora nicotianae
- Frogeye often occurs on lower leaves
- Relatively small circular lesions with white center surrounded by tan to dark brown ring, with yellow area around the ring
- Prevalent in warm, rainy conditions; very common
- Fungicides containing azoxystrobin (Quadris, Satori, AzoxyStar, Azoxy 2SC) will manage frogeye leaf spot
Target Spot
Figure 2: Extensive target spot on burley tobacco (Photo: Kenneth Seebold, UK)
- Caused by Thanatephorus cucumeris; anamorph Rhizoctonia solani
- Target spot also is more common on lower leaves
- Lesions may become very large and may be limited by leaf veins. Dark brown rings within the lesion, with yellowing outside the brown areas
- Common under rainy conditions; also a common float bed disease
- Fungicides containing azoxystrobin (Quadris, Satori, AzoxyStar, Azoxy 2SC) and mancozeb (Manzate Pro-Stick) will manage target spot.
Blue Mold
Figure 3: Blue mold lesions on burley tobacco leaf (Photo: Mesner, 2000).
- Caused by Peronospora tabacina
- May be seen on lower to mid leaves
- Begins as subtle yellow-orange lesion, progressing to tan. Outside borders of lesion are poorly defined
- May occur under rainy conditions, particularly with winds from the south. Disease is likely to start in shaded or low areas with high humidity
- Many fungicide options, including mandipropamid (Revus), dimethomorph (Forum), acibenzolar-S-methyl (Actigard), fluopicolide (Presidio), azoxystrobins (Quadris, Satori, AzoxyStar, Azoxy 2SC), mancozeb (Manzate Pro-Stick).
