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Weather and Plant Diseases

Posted by Richard Hentschel -

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 rainfall allowed those early spring foliar diseases more time to show up. Disease resistant plants have been showing up with disease this year anyway 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 have also been high this year. Foliar disease of tomatoes start at the soil line and spores get splashed up onto foliage by rain and watering. Once on the lower leaves the disease progresses upward in the plant. At the very first sign of the disease, a protective fungicide spray is suggested. We have seen these leaf diseases on varieties that are resistant this year anyway, just not as bad as susceptible varieties.


With conditions changing from cool 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. 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 failing or a general thinning of the lawn in spots. 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 2-3 before the lawn gets in trouble and begin to treat. If you have to re-seed or over seed from a disease, be sure to use high quality disease resistant grasses.

Certain landscape plants and perennials will routinely have leaf diseases and where 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, thin the stalks in the spring, plant them where there is good air circulation, but eventually they will get powdery mildew. The shrub we know as Ninebark is also commonly impacted by powdery mildew. Walking the yard in the fall you will see mildew on many plants as they begin to go dormant. It is when they are infected early in the year that impacts long term health and appearance for the whole season. Daylilies and Iris will both have foliar diseases as the season moves forward. In a lot of cases this is related to being overcrowded, the sun/shade pattern changing. Extension has a great many web sites on gardening and one you can find a whole lot about plant diseases, .

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