Rain can cause fungus on roses
Last updated 4/25/2020 at 1:27am
Frank Brines, Master Consulting Rosarian
Over the past couple of weeks, many areas have gotten above average rainfall. While it is welcome, it presents problems for gardeners when it arrives over such a short period of time. The "good" is that one doesn't have to pay for water that contains more salt, and the rains leech salts out of the soil, some of which comes from fertilizers. Rain also replenishes the natural aquifers and lakes.
But so much moisture creates an environment for fungi, especially powdery mildew, rust and botrytis. Many roses have serious botrytis fungus this year as the result of too much moisture trapped in the blossom. There doesn't seem to be any treatment beside removing the infected blossoms.
So far, I have been able to minimize the powdery mildew and rust with scheduled spraying. There are many treatments available at the local garden store. Read labels and compare information to select that which fits the use best.
Rust appears on the underside of leaves, and like its name, it looks like rust. It is actually an eruption of tiny spores that group together and can kill a plant if not controlled. When it becomes overwhelming, it is best to just remove all of the infected leaves and discard them in the green waste. Like all rose debris never let it remain in your garden or yard – spores that accumulate on the soil may be quickly splashed up onto the lower leaves by water and can spread onto more leaves and plants. Proper pruning that provides good air circulation through the bushes should help to prevent mildew and rust by minimizing moist conditions within the bush.
Currently the Hoplia beetle is damaging blossoms, most notably on light-colored blooms. They burrow into the petals but are easily controlled by flicking them off into soapy water. My first experience with the Hoplia beetle was 2019 when I removed nearly 100 insects from my garden. Their appearance doesn't seem to be influenced by temperature except that they hibernate in the soil after mating.
Roses love food and water. Larger blossoms are produced when well hydrated and the rain has helped to accomplish that. I maintain a well-balanced soil by providing minerals and micro nutrients every two weeks. I use the wet/dry method, that is, I apply a mix of nutrients and fertilizers dissolved in water for one application, and two weeks later I apply a dry mix. I'm always careful to keep the concentrations at recommended levels to prevent burning. Make sure the roses are well hydrated before feeding and never fertilize a dry plant. I admit that the wet weather has made wet/dry fertilizing problematic – so much rain makes the wet method impractical as the typical garden bed is already saturated. When that's the case, I substitute a dry mix for the wet one until the soil is no longer saturated.
That's it for now. Be sure to visit Rose Haven Heritage Garden, 30592 Jedediah Smith Road, in Temecula. The cross street is Cabrillo Avenue. For more information, visit http://www.TemeculaValleyRoseSociety.org.