Early Rust Symptoms in Corn

Symptoms

• Common rust appears as a series of elongated or oval pustules on corn foliage. These pustules are usually reddish brown, and scattered over both the top and bottom leaf surfaces. They can be very small, and are difficult to notice in the early stages of the disease, particularly in fields that aren't heavily infected. Over time, the pustules may rupture, releasing dusty red spores that can infect other leaves.

Life Cycle

• Corn rust is caused by the fungus Puccinia sorghi. This organism has a complex life cycle that involves five different types of spores and two separate hosts. Most of the spore types occur in wood sorrel, a type of small herbaceous plant in the Oxalis genus. When the fungus reaches a certain stage, it produces growths called cluster cups, which release spores to infect corn crops. These windborne spores are aeciospores. The fungus completes its life cycle by releasing urediniospores, which spread the disease to other corn crops. According to the University of Wisconsin, this disease's early life cycle occurs only near Mexico.

Control

• This disease is difficult to treat or control once established. However, according to the University of Massachusetts-Amherst, some fungicides work to keep rust infections down. The University agriculture extension recommends spraying when 80 percent of leaves have 1 or more pustules. Farmers can also control rust by planting all corn fields at the same time, maintaining good air circulation in the field and choosing rust-resistant varieties.

Considerations

• While corn rust has been a serious problem in the past, most modern corn cultivars resist this condition. Common rust no longer causes significant losses in field corn unless conditions especially favor the disease. Sweet corn is somewhat more susceptible, but most varieties still resist this fungus. The disease prefers prolonged cool temperatures between 60 degrees and 74 degrees Fahrenheit, as well as very humid conditions. Observe corn crops carefully during long spells of cool, wet weather.