In issues 13 and 14 of Purdue's Pest & Crop Newsletter I reported that common rust of corn was developing rapidly on some corn inbreds. It now appears that rust will be unusually severe on corn this year. The problem is not confined to seed corn and certain specialty corn hybrids. I have had several reports of heavy infection on hybrid field corn.
Common rust is what plant pathologists refer to as a polycyclic disease. The pustules that develop initially on lower leaves, as a result of infections produced by spores that blew into Indiana cornfields from southern states, produce more spores that cause more infections within a field. These infections give rise to more pustules, more spores, more infections, and so on. The number of pustules in a field tends to increase exponentially. At first, the rate of increase does not appear to be too rapid (the lag phase). Then, the diseases explodes as the rate of pustule development enters the exponential phase. The number of pustules increases in a manner similar to the accumulation of money at compound interest. However, rather than increasing at rates of 5 to 8% per year, rust pustules can increase at rates of 15 to 30% per day.
All that is needed for a rust spore to infect the leaf is a few hours of dew. Temperatures in the range of 60 to 77 ·F are most favorable for infection, but at higher temperatures some infection will occur. Recent daytime temperatures have been in the 80s, but at night, when infections usually occur, temperatures have been dropping into the lower 70s.
Intensity of rust is generally expressed as the percentage leaf area affected. This is referred to as percent severity. When severity of various leaf blights, such as northern corn leaf blight or gray leaf spot, is assessed, 50% severity means 50% of the leaf area is blighted. With rust, severity is measured a bit differently. A visible rust pustule is a mass of spores produced by the fungus in an infection site. Thread-like strands of the fungus grow within the leaf tissue and after a few days, the fungus begins to produce spores at the center of the infection site. This mass of spores erupts through the leaf epidermis and gives rise to the visible pustule (the brick red spot). The actual pustule occupies only about 1/3 of the leaf area invaded by the fungal colony. Hence, when the maximum possible density of pustules occupies a leaf, only about 33% of the leaf area will actually be covered with pustules. This density of pustules is considered to be 100% severity and lesser densities are rated accordingly.
Rust damages the plant in two general ways. The fungus is a parasite and it derives all of its nutrients from the host plant. Carbohydrates and other nutrients that would normally sustain vegetative growth and fill the grain are diverted into the fungus. Also, at the site of each pustule, the leaf epidermis is torn open. This means that the ability of the plant to regulate its water economy through stomatal action is compromised. A heavily rusted plant can show symptoms of moisture stress under hot, windy weather even though soil moisture would be regarded as adequate. A severely rusted plant suffers both from lack of nutrients moving into the grain and from lack of water.
All corn has a general resistance to rust in that mature leaves are less susceptible to infection than juvenile leaves. This year, rust began developing very early, and a lot of infection evidently occurred while leaves at each layer were in the juvenile stage. Many hybrids also have resistance that reduces the number and size of pustules or extends the time from infection until when a pustule forms. These types of resistance reduce the rate of epidemic development. To return to the money analogy, these types of resistance are somewhat equivalent to a lower rate of compound interest. A few pustules will be seen on leaves of plants with these types of resistance, but severity will remain low throughout the season.
From several phone calls I have had, it appears that some hybrids are very susceptible to rust. Some growers are wondering if a fungicide should be used. Although fungicides are commonly used on seed corn and sweet corn, they are not generally used on hybrid field corn. On both sweet corn and seed corn, effective control requires that the first fungicide application be made when rust first appears. Applications are repeated on a 7- to 14-day schedule, depending on weather and the susceptibility of the corn. Because of the phenomenal number of rust spores that can be produced in a field of corn once the disease is established, it is very difficult to halt an epidemic with fungicides. This is why applications must begin when disease first appears. It is easier to prevent an epidemic than to halt one that is already on a roll. Therefore, if hybrid corn producers are just now noticing that rust is severe, it is probably too late for a fungicide to be effective. On the other hand, if rust was noticed earlier, before it became severe, and a fungicide was applied, a second fungicide application may be justified. It is important to do some careful calculations about costs and returns. Unfortunately, it is not possible to precisely relate disease severity to yield loss. For each 10% severity of rust, yield losses may range from 3 to 8% of potential yield. The earlier rust develops, the greater the loss. Severe rust tends to shrivel grain rather than reduce grain number.
Several fungicides are labeled for use against corn rust. Various mancozeb products can be applied after silking. The label for Tilt specifies that applications not be made after silking. However, Indiana has a supplemental label, 24(c), which allows post-silking applications on corn grown for seed. If Tilt is applied to seed corn after silking, the applicator is required to have a copy of the special label in his or her possession at the time of application. If using any fungicide, the label should be consulted regarding rates, timing of application, total amount that can be applied during the season, and other important considerations.