Oats in Iowa suffer from two major diseases, crown rust and barley yellow dwarf virus. Crown rust typically appears as orange leaf pustules. It also causes yellowing and death of the leaves. Severely affected plants are stunted and produce little grain. The disease usually first appears in May and can remain active throughout the rest of the season. Spores can be windblown long distances, but in Iowa most initial infections are believed to be the result of spread from buckthorn shrubs.
Buckthorn is the alternate host of the crown rust fungus, Puccinia coronata. Overwintering teliospores germinate and produce basidiospores that infect buckthorn. In turn, aeciospores from the buckthorn plants infect oats. Once the disease has spread into an oat field, it can move quickly from plant to plant by the urediniospores, which are the familiar orange spores.
Under wet conditions, crown rust can reduce yields severely. In 1993, rust was a major factor in the very poor oat crop. Yield loss due to rust is affected by planting date. When oats are planted late, they are smaller when first infected, and the disease has a longer period of time to develop on the plants and spread to more plants.
Barley yellow dwarf virus causes extreme stunting and discoloration of leaves. Leaf blotches that are yellow, orange, or red develop until entire leaves are involved. Plants along the edges of fields are usually affected first. Severely affected plants produce little or no grain. The virus is spread by several species of aphids, which are more active under warm conditions in the late spring and summer. Plants infected in the seedling stage are the most severely damaged. Early planted oats typically suffer less yield loss, because they are older during the period of the greatest aphid activity. The virus can infect all cereal grains, corn, and grass weeds. Barley yellow dwarf virus is not considered an important corn disease in Iowa.
April 20 is the average planting date for oats in Iowa. If oats are planted in cold wet soil, they will be affected by seedling blights, but if the soil is dry enough, earlier planting will help reduce losses to crown rust and barley yellow dwarf.
I advise planting oat cultivars that have resistance to these diseases, because both diseases are potentially severe each year. Some previously resistant cultivars are now susceptible to rust, but there are still pure-line (Dane, Don, Hazel, Sheldon, Horicon, Newdak, Prairie, Bay, Settler, Troy, Valley, INO9212, and INO9201) and multiline varieties (Webster) that are resistant. No cultivars are highly resistant to barley yellow dwarf, but several (Hamilton, Webster, Hazel, Horicon, Newdak, Ogle, Prairie, Troy, INO9212, and INO9201) have adequate resistance. See the Iowa State University Oat Yield Test results for further information on resistance ratings to both diseases.