Over the past week or so Iowa State University Extension plant pathologist Alison Robertson has received e-mails, calls and samples of two leaf spot diseases of corn that occur in Iowa from time to time - Holcus leaf spot and Physoderma brown spot. You may be seeing these diseases in fields this year. In the following article, Robertson explains what you need to know about them.
Holcus leaf spot is caused by a bacteria Pseudomonas syringae. Symptoms are light tan (sometimes almost white), round to oval spots. They may appear to be water-soaked at the margins or have a light brown border occur on lower leaves.
The spots are initially about one-fourth inch in diameter, but often grow larger and coalesce or merge together into irregular spots and streaks of dead tissue. Later the lesions dry out, turn light brown and have a papery texture.
Looks similar to other leaf diseases
Holcus spot is often confused with eyespot, a fungal disease caused by Aureobasidium zeae. Eyespot lesions are much smaller (about 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, wet, windy conditions early in summer favor infection and 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.
How should you manage Holcus leaf spot? Don't worry about it, says Robertson. This disease is mostly cosmetic and does not result in yield loss. Fungicides are not effective against this bacterial disease.
Another disease is Physoderma Brown Spot
Physoderma brown spot is another unusual disease that's showing up on corn this summer in Iowa. It is caused by Physoderma maydis, an organism closely related to diseases such as Pythium and the pathogen that causes crazy top.
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 purple 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 (mid-July) to be seeing this disease. Southern rust usually occurs in mid-to-late August in Iowa when temperatures are much warmer. Furthermore, the pustules of southern rust produce thousands of orange spores that can be wiped off the upper leaf surface with your finger.
What kind of conditions favor Physoderma brown spot? It infects leaf tissue when free water collects in the whorl and temperatures are between 70 to 85 degrees F, thus resulting in the bands of infected and non-infected leaf tissue.
How can you manage this disease? 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 to 60 days after germination and become more resistant to infection with age. Of the fungicides sprays available, only Headline lists Physoderma leaf spot on the label. However, infections in Iowa are usually not severe enough to warrant a fungicide application.
For more, visit www.extension.iastate.edu/CropNews/2008/0718robertson.htm.