Zonate Leaf Spot

By Jim Stack, former UNL Extension Plant Pathologist and current Kansas State University Extension Plant Pathologist

Pathogen

Gloeocercospora sorghi D. Bain & Edg. Fungal structures: conidia and sclerotia. Zonate Leaf Spot is common throughout the sorghum producing areas of the USA occurring in high rainfall years or periods of high rainfall. Zonate Leaf Spot was severe in several South Central Nebraska counties in 1998; a double epidemic of Sooty Stripe and Zonate Leaf Spot occurred in many fields. The pathogen survives as sclerotia in plant residue. The Zonate Leaf Spot pathogen also infects other grass species including corn and millet; these other hosts may serve as a reservoir of inoculum.

Conidia

Disease Symptoms

The first visible symptoms are the appearance of small non-diagnostic lesions on the lower leaves. These lesions may occur anywhere on the leaf. As the lesions mature they become circular or target shaped on the interior of the leaf and semicircular on the leaf margins. Some zonate lesions do not have a target appearance and can be confused with physiological spotting or genotype-environment interactions. On some hybrids, lesion appearance and size is variable. Lesions elongate, run together, and whole leaves may be blighted. When the weather is favorable the disease progresses up the plant and lesions may occur on all leaves of the plant.

>Circular

Semicircular
Zonate Lesions	Lesion Apperrance
Blighted	Disease Progress
	Favorable Weather Conditions The exact conditions considered ideal for disease development are uncertain; however, moderate to high temperatures with periods of high relative humidity are associated with epidemics. The leaf wetness requirements are not known. Wind and rain disperse the conidia. Management Due to cost, fungicide applications for the management of Zonate Leaf Spot are not feasible in grain production. Residue management by crop rotation is the most feasible disease management option.