Rainy spring weather is what minor fungal diseases need to cause leaf infections. There are a number of fungal diseases that lead to various colored, shaped and sized spots on plant leaves.
Fungi require moisture on leaf surfaces for a certain amount time to infect leaves. With increased spring moisture, there is likely to be an increase in leaf spot diseases leading to some leaf drop later this season.
While infections have been occurring, the signs of leaf spots may not show up for another week or two. If a plant is susceptible and there is a high enough rate of infection to cause leaf drop, leaves usually will not begin dropping until June or later.
Discolored leaves and leaf drop can be a concern, but unless foliar diseases repeatedly cause severe defoliation of a plant, they have minor long-term impacts on otherwise healthy and established plants.
A common reaction when brown, reddish or orange spots are seen on leaves is to apply a fungicide for control. Fungicides work best by preventing infections rather than curing them. They need to be on the leaf surface prior to disease development.
Once leaf spots or blights and leaf drop begins, it is often past the time when fungicides will be effective. Applying them too late in the season can be a waste of time and money and an unnecessary use of a pesticide.
So what is plant owner to do? On plants with a history of serious leaf drop due to fungal disease, fungicides are best applied just as plants are leafing out in spring. If a plant has no history of leaf diseases or leaf spotting is minor, there is no need to apply a fungicide.
While fungicides applied after a disease is noticed can prevent new leaf infections (if the weather is still rainy or overhead irrigation is used), they will not cure current infections; and late applications will have missed the main infection period which is usually during rainy spring weather.
If you are concerned about a plant dropping leaves, focus on how much foliage the plant has retained. The remaining leaves, even those with leaf spots, are still doing the job of photosynthesis. Unless over 50-60 percent of leaves have dropped off fairly early in the season, leaf drop is not of great concern.
The best management practices for foliar diseases is to select resistant plants, avoid overhead irrigation or irrigate in the morning if overhead irrigation must be used, and use sanitation by raking and removing fallen leaves.
Some common landscapes plants known to have foliar leaf diseases for which resistant plants should always be purchased include roses, fruit trees, crabapples, peonies and turfgrass.
Almost all of these have resistant varieties or cultivars available for purchase. To avoid problems with minor foliar diseases, select disease resistant plants for your landscape or orchard.
Look for roses labeled as black spot or mildew resistant. For fruit trees and crabapples, select cultivars listed as resistant to apple scab, cedar apple rust and fire blight as well as others. And select mildew or blight resistant peonies.
Kelly Feehan is a community environment educator for Nebraska Extension-Platte County.