Most fungal leaf spot diseases are common infections in wheat and other cereals. Symptoms are similar and can be observed as lesions varying in color from brown to purple to gray. Common fungal leaf spot diseases include:
Septoria Leaf Blotch
Septoria leaf blotch, caused by the fungus Septoria tritici
, typically develops on wheat on the lower leaves in early spring
. It moves to upper leaves under cool, wet weather conditions when temperatures range from 59 to 77 degrees F. Irregular spots will be reddish-brown in color and are spread randomly over leaf blades.
The greatest yield losses happen when infection attacks the flag leaf and two leaves below in late spring or early summer before flowering. Yield reduction can be as high as 20 to 30 percent, according to The Ohio State University.
This fungus, caused by the Pyrenophora tritici-repentis fungus, produces lesions measuring 1/5 to 3/5 inches with a dark brown center. Early in the season, tan spot lesions often have a distinctive yellow border. A reddish discoloration, called “red smudge,” can form on the seed coats of infected kernels. These infected kernels are usually plump, not shrunken, but the discoloration in the grain may prompt market discounts.
When wet weather lasts more than 24 hours, tan spot spores germinate and infect surrounding plants. Each wet weather episode releases new spores, and symptoms become apparent within five to seven days.
Tan spot is considered to be the most destructive leaf spot disease found in all wheat classes throughout the growing season, according to North Dakota State University research. Tan spot is among the foliar leaf diseases that can cause reduced wheat test weights and yield losses of up to 50 percent.
The net blotch fungus, caused by Pyrenophora teres, is restricted to barley crops and is abundant during cool, damp periods. The disease first appears on seedling leaves as oblong brown spots. These blotches are often confined between the veins and marked internally with a hallmark netting appearance. Infections occur on leaves throughout the growth cycle, from seedling until near maturity. The lesions may cover a large portion of the leaf, reducing or destroying its photosynthetic ability.
Stagonospora is caused by Parastagonospora nodorum and Parastagonospora avenae f. sp. triticea. This fungus initially causes small, water-soaked lesions on the lower leaves of the plant that become yellow and, eventually, red-brown. As the disease advances, the lesions form a gray to gray-brown center encompassing brown specks. After flowering occurs, wet weather can contribute to the development of lesions on the glumes, frequently beginning at the tip. Whole areas of the glumes may eventually become covered with dark purple to dark brown lesions with ash-gray areas. Severe leaf blotch infections damage crops by resulting in lightweight, shriveled kernels.