Samples of two leaf spots that occur in Iowa from time to time, have been identified by Iowa State University plant pathologists, as Holcus leaf spot and Physoderma brown spot.
Holcus leaf spot is caused by the bacterium Pseudomonas syringae. Symptoms are light tan (sometimes almost white), round to oval spots, which may appear water soaked at the margins or have a light brown border occur on the lower leaves.
The spots are initially about one-fourth inch in diameter, but often grow larger and coalesce into irregular spots and streaks of dead tissue. Later the lesions dry out, turn light brown and have a papery texture.
Holcus spot is often confused with eyespot, a fungal disease. Eyespot lesions are much smaller (approximately one-eighth to one-fourth inch in diameter), very round spots with a yellow halo, distinct brown border and light colored center that appears translucent when the leaf is held up to the light. Holcus spot symptoms also can resemble chemical injury to leaves.
Holcus leaf spot is mostly cosmetic and does not result in yield loss. Fungicides are not effective against this bacterial disease.
Physoderma brown spot is caused by Physoderma maydis, an organism which is closely related to the oomycetes (such as Pythium and the crazy top pathogen).
Numerous very small (approximately one-fourth inch in diameter) round to oval spots that are yellowish to brown in color usually occur in broad bands across the leaf. Dark purplish to black oval spots also occur on the midrib of the leaf. Symptoms may also occur on the stalk, leaf sheath and husks.
Physoderma brown spot is often misdiagnosed as eyespot or southern rust. The spots of Physoderma do not have the light colored center that is associated with eyespot. As for southern rust, it is probably too early in the growing season to be seeing this disease. Southern rust usually occurs mid- to late- August in Iowa when temperatures are much warmer. Furthermore, the pustules of southern rust produce thousands of orange spores than can be wiped off the upper leaf surface with your finger.
The pathogen survives in infested crop residue and soil for up to 3 years thus the disease is more common in corn following corn fields particularly if a lot of crop residue remains on the soil surface. Corn plants are most susceptible 50-60 days after germination and become more resistant to infection with age. Infections in Iowa are usually not severe enough to warrant a fungicide application.