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ROSE CARE FOR JUNE
by Barbara Picarelli, Consulting Rosarian
by Barbara Picarelli, Consulting Rosarian
Supposedly the rains are over and now we can enjoy our roses, along with the rust, black spot, and other ordinary diseases nature has to throw at us. What to do? Well, I just went out today and picked off as many diseased leaves as I could, pruned back some roses and mulched and fertilized.
Basically in the summer it’s time to enjoy the roses at their first flush of bloom (which is very late this year) and then worry about all the disease that is now appearing. I’ll focus only on the disease portion of the usual problems, and leave the insects for now. I shall use a very simplified form of discussion of the disease and its process.
I have powdery mildew on some of my roses, and this is a prevalent problem in California. The most common symptom is twisted distorted leaves covered with a powdery whitish material. The fungus can over winter as dormant mycelium or as a resting spore cleistothecia. As conditions warm up the spores become active producing conidia and the cleitothecia produce ascospores. These are windborne and carried to young susceptible plant parts. The conditions are normally having a 71degree temperature with a 98% humidity. Temperatures above 90 degrees inhibit spore formation. After spores form they directly penetrate the epidermis inside the plant cell, and take the nutrients from the plant.
Sanitation is the best form of control, with dormant pruning and thorough clean up under the plants. For those who chose to spray a fungicide is the treatment of choice, and works best in the early formative stages of the disease, with use every 7 to ten days. It is not very effective in the later stages of disease. A lime sulfur spray is a good protectant spray but must be used with caution in very warm weather as it can severely burn foliage.
Black spot is another serious problem especially with perfect conditions of warm days and cool humid nights. Black spots are on the leaves as well as some canes. Generally the spots have a sooty appearance with feathery edges, and can lead to defoliation of the plant. Again this is a disease that over winters on infected leaves and canes as a mycelial and with the increased spring temperatures the fungus produces conidia which is moved by air currents or splashing water, onto new and developing leaves. This fungus develops with free water and takes about 7 hours for infection to occur. Usually the lower and inside leaves of the plant are more likely to be infected as these areas have a longer time for water to be present.
Again, dormant pruning and thorough cleanup can do much to reduce the fungus. A well aerated soil with good drainage will also help reduce the presence of free water, as well as pruning so the bush has adequate air movement to the inside. Should you need to wet the leaves, do so as early in the day as possible so the plant will have time to dry. Spraying with a protectant spray in early spring may be beneficial, on a one week interval, or after a rain.
And finally for this issue is our dreaded and hated Rust! This fungus is probably the most easily identifiable disease. Severe outbreaks occur when cool temperatures and high moisture are found, especially during the prime growing season. The first symptoms are found nearer the ground and toward the inside of the plant. There is up to nine species of rust, and it is a complex disease having up to five different spore stages in its life cycle.
Commonly recognizable by it’s bright yellow orange spore on the underside of the leaves in the early stages to the black aeciospores at maturity. Once the orange spores develop underside of the leaves, they soon become noticeable on the upper leaf as well with a stippled sort of look. The next stage is a typical red brown or dark orange spore and it can repeat several times during the growing season in a 10-14 day interval. As fall approaches the black spores develop and leaves, canes and flower parts can all be susceptible to the fungus.
Again the fungus over winters on fallen leaves and canes. Since they are also a wind borne and the spores are tiny it is easy to have residual fungus residing on the plants or soil. Sanitation is one of the prime methods to control this disease, but fungicidal sprays can also be used. For us non sprayers, pruning the center of the plant to allow for good aeration is important. I have on occasion when the infestation is super severe, defoliated all my bushes and pruned, then allowed the plant to grow foliage again.
I suspect nature has the upper hand.
Information from the ARS Consulting Rosarian Manual
A YEAR OF ROSE CARE:
January
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Last Modified: 6/8/06