Unusual foliar diseases showing up in Midwest corn
Over the past week, I have received e-mails, phone calls and samples of two leaf spots that occur in Iowa from time to time, 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 caused by Aureobasidium zeae. 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.
Warm (75-85 degrees Fahrenheit), wet, windy conditions early in the season favor infection and the development of Holcus leaf spot. Symptoms often appear suddenly after a heavy rain, but then do not spread to new leaves. Corn is most susceptible prior to tasseling.
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 zoospores of P. maydis infect leaf tissue when free water collects in the whorl and temperatures are between 70-85 degrees Fahrenheit, thus resulting in the bands of infected and non-infected leaf tissue.
The pathogen survives in infested crop residue and soil for up to three 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. Only Headline lists Physoderma leaf spot on the label; however infections in Iowa are usually not severe enough to warrant a fungicide application.