No matter where you are in the United States, you could be dealing with rust on your sweet corn. But all corn rusts are not alike, and differentiating between the two types – southern rust and common rust – is important. Both are caused by the Puccinia species of fungi: southern rust by Puccinia polyspora and common rust by Puccinia sorghi.

Although the two do share similar traits, they can be differentiated by the size, color and shape of their spores. Southern rust is the more aggressive of the two, yet both can occur simultaneously on the same leaves. Southern rust pustules are orange, small and dense. They can be seen on the upper leaf surface and on other plant tissues. Common rust has cinnamon-colored pustules, larger in size and more scattered on the leaf, occurring on the top and bottom of the leaf surface.

These rust fungi have several life cycle stages, which have to occur for them to proliferate and infect corn plants. Both rusts are obligate parasites, requiring a living host to complete their complex life cycles. On corn plants, the urediniospores will reproduce asexually, causing repeated infections of the plant when conditions are right.

The rusts also have sexual stages of reproduction, which occur on another host species. For common rust, that host is Oxalis spp. But for southern rust, the host remains unknown. The resulting aeciospores, which develop on the alternative host, become windborne and travel to corn plants, where the urediniospores can then develop.

These urediniospores are formed in a structure known as a uredinia, or more commonly, the pustule. These are the “rust” spots that signal an infection has occurred. Sweet corn, popcorn and seed corn are most impacted by rust. Because rust spores are carried up from south to north on wind currents and the younger juvenile leaves of the corn plant are more susceptible to initial infection, later planted corn can be more severely impacted than earlier plots.

Speaking in a video presentation, Dr. Snook Pataky, professor emeritus of plant pathology, University of Illinois, explained that regardless of any resistance a corn hybrid may have, juvenile leaves are always more susceptible to the rusts than are adult corn leaves. There is a “vegetative phase change,” which occurs in-between the six- to eight-leaf stage of maturity and the adult leaves, which then form.

The epidermal layer is different in juvenile leaves than in adult ones. The leaf wax and hairs, cell wall structure and cuticle thickness all change. More mature corn plants are not as susceptible to infection by rust spores. Juvenile leaves can be severely infected, while adult leaves may show little rust. But the inoculum present in the juvenile leaves can reinfect the plant as new leaves are forming and can also serve to infect later-planted corn crops.

The urediniospores of southern rust do not, as of yet, overwinter in the United States, although common rust spores do survive the winter in southwestern regions. Southern rust can, at times, spread to northern regions, particularly during hot weather years. In most years, northern growers will contend with common rust alone.

Symptoms and proliferation

Once the corn has been infected with rust spores, the spores will need to germinate. Uredinia form about one week after initial infection for common rust and about two weeks for southern rust. Under the proper conditions, these structures release thousands of spores, which will re-infect the plant. In wet conditions, this germination can happen within one hour. The leaf whorl, where moisture is retained, is often the site where the spores can readily germinate.

Common rust prefers temperatures between 68 and 80 degrees Fahrenheit, but will germinate more slowly outside of this range. For southern rust, ideal conditions for urediniospore germination occur under warm, wet and humid conditions, as this is a true tropical climate disease.

Southern rust (left) and common rust on corn

Any time there is adequate moisture, existing uredinia will germinate more spores. This results in portions of the leaf showing symptoms, then some non-infected tissue and another band of infection when conditions were right as leaves emerge from the whorl. Urediniospores of southern rust can also form on the inner portion of husk leaves and on leaf sheaths.

“Because the leaf tissue is infected down in the whorl, we oftentimes see bands of infection,” Pataky said.

In dry conditions, the infection doesn’t happen, but the next rain, heavy dew or irrigation event produces another band of infection. This reinfecting of the leaves during conducive conditions can impact yield. In sweet corn, common rust will cause a 0.6 percent decrease in yield for every 1 percent increase in leaf area infected, Pataky said. If 60 percent or more of the plant is infected at tassel, the corn may not set ears. With common rust, most of the yield reduction is due to poor kernel fill.

Southern rust is more aggressive than common rust, with its dense uredinia that form on various plant tissues. Studies indicate that the yield of sweet corn can be reduced to 45 percent when a field is inoculated with Puccinia polyspora. Yield reductions are directly related to the leaf area infected.

Control of rust

“Reductions in yield are associated with the severity of the infection. And the severity of infection is associated with the host growth stage of initial infection” Pataky said. “The earlier the infection, the more severe the disease will be.”

To combat rust disease, resistant hybrids can be planted. Partial resistance is quantitatively inherited and allows some uredinia to form. Rp-resistance is a hypersensitive response where uredinia generally cannot form on the plant. Selecting hybrids with resistance, planting less resistant hybrids earlier in the season and the selective use of several approved fungicides can decrease infection rates.

Rust infects field corn, too. But resistance is trickier here, as field corn is grown so extensively that it is likely the disease would rapidly overcome any resistance. In sweet corn, a few common rust strains are able to overcome some of the more than 25 different Rp-genes, particularly if these genes are widely utilized, Pataky said.