The fungus overwinters on fallen leaves and diseases canes. The spores hang around until the weather is conducive for development and then are splashed to the newly emerging leaves and stem tissue by rain or overhead irrigation. Spores are also easily moved by air currents.
Ideal conditions for black spot to develop are leaf wetness, humidity and warm temperatures. Sounds like Ohio weather for a good part of the growing season, right?
The symptoms start out as round to irregular shaped black spots on the leaves. These spots are surrounded by yellowing of the leaf. The spots coalesce and the leaves eventually drop when they are heavily damages.
Black spot doesn’t usually kill a plant, but it can weaken it. A weakened plant may be more susceptible to winter injury or death.
Our weather conditions are so conducive for black spot to develop that rose growers know to keep fungicide sprays on their plants all season if they want to prevent black spot. Otherwise, planting resistant varieties is an option.
Many of the shrub and rugosa type roses show good resistance and breeders are constantly working on varieties that are resistant.
Many are seeing holes in their leaves or other types of feeding damage. Japanese beetles skeletonize the leaves. This means they eat all of the green tissue and leave the veins, which eventually turn brown.
Other holes or types of feeding damage that is really common on roses in the Miami Valley are caused by three different species of sawflies. These small non-stinging wasps lay their eggs on the rose plants and the larvae cause the damage.
The roseslug, bristly roseslug and curled rose sawfly feed on the leaf surfaces (upper or lower surface depending upon the species) and skeletonize the leaves. They tend to eat just the upper or lower layer and leave a very thin layer of leaf tissue that looks like a cellophane window.
We call this effect “windowpaning.” Eventually the entire tissue falls out, leaving a hole.
Control for this pest and the timing depends on which species you are seeing. The bristly roseslug continues to feed all season while the others tend to feed earlier in the season.
These are not caterpillars so don’t use pesticides that control caterpillars. These are sawflies and will only be controlled by those products that have sawfly listed on the label. Look for products labeled for roses that contain bifentrhin, carbaryl, cyfluthrin and imidacloprid.
The final issue we have seen on roses is a virus that you don’t want to have on your roses. Rose rosette is the name of the disease, and it’s caused by a virus transmitted by an eriophyid mite.
Symptoms include great distortion of the new growth, excessive thorn production and elongation of the new shoots and abnormal reddening and discoloration of the leaves.
If diagnosed in your roses, remove the plant, roots and all, immediately and consider using a regular miticide program to prevent spread of the mite.
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