physoderma brown spot

 Pathogen

Physoderma brown spot is caused by a chytrid oomycete fungus, Physoderma maydis (syn. P. zeae-maydis), closely related to oomycete or water mould fungi, such as downy mildew. This disease is usually a rare, mild disease of corn that we see most years to a low extent in Nebraska. However, during some summers, when the weather conditions are favourable for developing this disease, the disease may appear more and cause fear because the symptoms are like other serious diseases.

Symptoms

Symptoms of Physoderma brown spot can be confused with other diseases. Lesions are small and round to oblong, yellow to brown, and can appear on the leaf blade, stem, leaf and bark. On the leaf blade, these small lesions can resemble those caused by rust, such as early southern rust. However, examining these lesions under high magnification reveals that they are not rust pustules. Also, the obvious difference is that the brown spots of Physoderma usually appear in different groups on the leaf, especially at the base of the leaf. These lesions appear differently in the centre than on the rest of the leaf. In the midrib leaf, these lesions tend to be dark in colour and sometimes larger, so their difference in appearance in this area from the edge of the surrounding leaves is a clue to the identification of this disease. As the disease progresses, the lesions increase in size, merge with neighbouring lesions to form larger lesions, and darken in colour from chocolate to red or purple.

Epidemiology

The pathogen produces brown sporangia packed within infected cells (picture). Each sporangium produces about 50 amphipods that need light and water to germinate and infect the plant. The infection usually occurs in areas where water accumulates during rains and irrigation, which is why lesions appear in groups throughout the leaf. Like several others in Nebraska, this pathogen lives in crop debris and may be more prevalent in continuous corn and high-residue fields, where tillage practices are reduced use there.

Administration

Although the resistant virus is found in areas where the disease is a persistent and serious problem, it is considered of little importance in the Midwest and seed companies do not recommend resistance to it. Hybrids in this area. Fungicides are prescribed for controlling Physoderma brown spot, but the disease is rarely serious and should not require treatment. However, correct identification of the disease is important to avoid unnecessary treatment due to misdiagnosis.