We all know how different the weather pattern has been this year. Foliar plant diseases develop when weather conditions are right, allowing the pathogens to grow and infect our plants. Our extended cooler spring temperatures and abundance of rainfall allowed those early spring foliar diseases more time to develop.
Even disease-resistant plants have been showing signs this year with the higher-than-normal disease pressure. A disease-resistant plant may show signs of the foliar disease, but the infection does not progress any further in a normal year. For example, flowering ornamental crabapples that have genetic resistance to Apple Scab or Cedar Apple Rust have shown signs of both of these foliar diseases this year. Foliar diseases of tomatoes also have been high this year. Foliar disease of tomatoes starts at the soil line and spores get splashed up onto foliage by rain and watering. Once on the lower leaves, the disease progresses upward and outward. At the very first sign of the disease, a protective fungicide is suggested. (Remember, to always thoroughly read and follow label instructions.) While we have seen these leaf diseases on resistant varieties this year, they are not as bad as susceptible varieties.
With conditions changing from cool and wet to hot and dry, the early spring diseases can only continue from already infected plant parts. As hot and dry patterns develop, then other plant diseases have the opportunity to show up. For example, in the lawn, we have the "patch diseases" developing. We know them by other older names, like melting out, dollar spot, brown patch, etc. They can have very distinct patterns like rings, or solid patches, with failing or a general thinning of the lawn. As with the other foliar diseases, prevention is the key. If you have had trouble in the past, you can expect it again so be prepared two to three weeks before the lawn gets in trouble and begin to treat it. If you have to re-seed or overseed from a disease, be sure to use high quality disease-resistant grasses.
In addition, certain landscape plants and perennials will routinely have leaf diseases and, when possible, "upgrade" your garden with disease-resistant plants of that same flower or shrub. An example of a perennial that always gets powdery mildew is Phlox. Culturally we can grow them in full sun and thin the stalks in the spring for better air circulation and sun penetration, but eventually they will get powdery mildew. The shrub known as Ninebark is commonly impacted by powdery mildew, and the newer cultivars are better. Walking the yard in the fall, you will see mildew on many plants as they begin to go dormant, which is pretty normal that time of year. It is when they are infected early in the year that influences long-term health and appearance for the whole season. Daylilies and iris will both have foliar diseases as the season moves forward. In many cases, this is related to overcrowding, and/or the sun-shade pattern changing.
University of Illinois Extension has a great many web sites on gardening, and a great one to learn about plant diseases is "Focus on Plant Problems at http://urbanext.illinois.edu/focus/