One doesn't need an agronomy degree to know that our rainy June probably increased the likelihood of diseases in 2011. Any discussion of disease in corn automatically draws attention to the most familiar foliar disease topic – gray leaf spot. Would you please allow this agronomist the chance to discuss two less well known diseases though – diseases that are also favored by rainy weather?
Common Corn Smut, while not usually of economic importance, is an unsightly disease and it appears all too frequently in central Illinois. Masses of white, gall-like growth form on leaf tissue and these white growths often crack open to show a black interior. The earliest symptoms of smut appear similar to "white, bubbles or blisters" that run in a line down the majority of the leaf blade.
Ustilago zeae is the pathogen responsible for this disease. The fungus starts out as dark-colored hardy spores (teliospores). During moist periods when temperatures don't exceed 90 degrees, secondary spores are produced that are splashed or blown by wind onto the plant. These spores are either male or female, they each form hair-like structures, and those hair-like structures (hyphae) enter the leaf through small naturally occurring holes on the leaf surface or through wounds. When the two cross within the plant, chemicals are released that cause rapid cell growth – thus the bubble or gall-like symptoms observed on the plant.
Exaggerated cell growth is followed by further growth of the fungus. This time fungal hairs, termed mycellium, grow between the cells. The enlarged cells are invaded and killed by these structures which then use the contents of the cell for the nourishment of the fungus. More dark-colored teliospores are produced within the engorged cells which results in the galls dark colored interior. The production of such teliospores may result in several "generations" of infection throughout the summer. Common smut is only injurious to yield if a majority of the field shows smut symptoms above the ear leaf, and it is, once again, usually a cosmetic disease. Light rain favors outbreaks of smut as do periods of rapid growth – rain and rapid growth are two staples so far in 2011.
Sclerophthora macrospora, the fungus responsible for causing crazy top, also infects corn during rainy periods. The disease overwinters in corn residue as a thick walled spore (termed an oospore). When saturated conditions persist for a day or two, secondary spores that can actually swim emerge, come into contact with leaf tissue, penetrate it, and produce other fungal structures that grow within the vascular system of the plant. Wind and/or flooding can increase the likelihood that spore-infested soil enters the whorl of the plant which increases the likelihood of crazy top symptoms. Chemicals associated with the fungus are responsible for the exceptional and atypical cell growth associated with this disease.
A wide range of plant material can host Sclerophthora. Oats, barnyard grass, foxtail, sorghum, crabgrass, and wheat are just a few of the plants on which the disease can survive. Corn is most susceptible when it is less than a foot tall, and symptoms typically develop in areas of a field as opposed to an entire field. True crazy top symptoms will resemble an explosion of "leaf-like" growth, "crammed" tightly together that replaces the tassel of the plant. Stunting and excessive tillering can occur due to the infection, and ears may be both more numerous and more "leafy" than what is typical for that hybrid. As with common smut, this disease – while an oddity – is usually not deemed to be a significant yield reducer.
Source: Matt Montgomery, County Extension Director