Left: chasmothecium of a powdery mildew fungus on oak. Right: urediniospores (top) and teliospores (bottom) of Southern Corn Rust.
Just as plants respond to seasonal cues, fungi will also undergo growth changes as the season progresses. Many fungi form overwintering, or survival structures, and produce different types of spores.
Common Corn Rust pustule. Note the few remaining bright orange urediniospores, and the predominantly dark brown teliospores in the pustule.
Two rust fungi frequently affect corn in Illinois: Common rust and Southern rust. Rust pathogens can be quite complex, and some will produce up to 5 different spore types on two hosts in one season. The round, brilliantly orange-colored urediniospores are usually produced during the growing season, giving the disease its name. As the season comes to an end, both our corn rusts form teliospores instead. They are a much darker color compared to the urediniospores, making the pustules on the leaves appear dark brown instead of the orange or rust color they were for most of the season. These teliospores are better adapted at surviving harsh conditions. However, when they germinate next spring, they will not infect the young corn plants in the fields. Instead, they will produce another type of spore which will infect Oxalis species. Spores produced on this alternate host can infect corn, though most of the spores causing infections in the Midwest originate in Mexico and are blown north in wind currents.
Oak powdery mildew. Chasmothecia are visible as small, black dots on the underside of this infected leaf.
Powdery mildew, a common pathogen on numerous hosts, produces an overwintering structure known as a chasmothecium (an older and still used term is cleistothecium). This structure is thick-walled and contains high amount of melanin, which protects the spores inside from UV light and gives the structure its dark brown color. The spores produced within the overwintering structure are also different from the spores the fungus produces during the summer: in summer powdery mildews form conidia, asexual spores that can be quickly produced and spread to infect a new host. As autumn approaches, the fungus produces ascospores, sexual spores that will overwinter and start a new infection next year. The chasmothecia sit like little brown marbles on the surface of the infected plant tissue, often leaves. After the plants senesce, the plant tissue collapses (or the leaves fall to the ground in the case of woody perennials) and the spores within the chasmothecia stay safe and snug for the winter. In spring, the chasmothecium will break open and the ascospores will spill out, ready to start the cycle of disease all over again.
For an end-of-season wrap-up of common trends seen at the Plant Clinic this year, you can read my article in the last issue of Home, Yard, and Garden Pest Newsletter for 2015 by clicking HERE.