What can I do about black spot on my roses? I heard that burying banana peels in the soil might help.
According to The Organic Gardener’s Handbook of Natural Insect and Disease Control (edited by Barbara Ellis; Rodale Press, 1996), there are several steps to dealing with black spot on your roses. First, avoid wetting the leaves, and do not handle the plants when foliage is wet. Prune the plants to make sure there is good air circulation. Make sure the roses are in sun, and are not shaded by large shrubs or trees. Avoid using high-nitrogen fertilizers, and only fertilize based on a soil test’s indications. If you expect an appearance of black spot (based on past experience), spray plants weekly with sulfur or fungicidal soap. Once you see symptoms, it is hard to control black spot. Remove and dispose of any affected parts of the plant (don’t compost). Make a solution of 1 teaspoon baking soda in a quart of water, and spray the infected plants well.
University of California, Davis’s Integrated Pest Management website says the following about black spot (Diplocarpon rosae):
“The fungus requires free water to reproduce and grow, so leaves should not be allowed to remain wet for more than 7 hours. (When hosing off aphids, do it in the morning so leaves have a chance to dry by midday.) Provide good air circulation around bushes. Remove fallen leaves and other infested material and prune out infected stems during the dormant season. (…) Miniature roses are more susceptible than other types, although a few varieties are reliably resistant to all strains of black spot.(…) A combination of sodium bicarbonate or potassium bicarbonate plus horticultural oil (as discussed above under “Powdery mildew”) or neem oil has also been shown to be effective in reducing black spot.”
Brooklyn Botanic Garden has information on natural disease control, including the following:
“Baking soda (sodium bicarbonate) is non-toxic, readily available, and very inexpensive. It can be effective against powdery mildew and somewhat useful against black spot. If you repeatedly spray leaves with bicarbonate, though, it will eventually reach the soil below, where it can accumulate and lead to slower plant growth. Bicarbonate can form insoluble particles with calcium and magnesium ions when it concentrates in the soil, making these important nutrients unavailable to plants. High levels can also prevent plants from absorbing iron and can lead to chlorosis.
Bicarbonate is most likely to build to damaging levels in drought-stressed areas where there is little rain to flush it away. It is also likely to build up when applied in a small area, and when used in conjunction with drip-type irrigation. Garden situations are so complex that it is hard to predict the point at which you will see adverse effects. Stop applying bicarbonate sprays, however, at the first sign of plant damage or lower quality blooms.”
Brooklyn Botanic Garden also mentions a beneficial bacterium which may provide some help:
“Preliminary research shows that the beneficial bacterium Bacillus laterosporus (sold as Rose Flora) is as effective at protecting black spot-susceptible rose cultivars as some chemical fungicides. It probably protects against black spot through competition, but this agent is still relatively new and experiments detailing its mode of action have not been completed. As a ground spray, it can help control new sources of black spot infection. As a foliar spray, it seems to be more effective when mixed with the antitranspirant sold commercially as Wilt-Pruf. The powdered formulation can cause eye irritation, so use eye protection when mixing solutions and applying.”
About the practice of using banana peels to control black spot on roses, I found the following item on Gardening Folklore from Ohio State University Extension, which suggests the peels might be a good fertilizer, but does not say they will control the fungal problem.
Excerpt:
“Placing several banana peels in the planting hole was popular among rose growers in the 18th century, but they had no idea why the peels seemed to yield healthier roses. Today, we know that banana peels contained many useful nutrients, including calcium, magnesium, sulfur, phosphates and sodium. The peels rot quickly which means these nutrients are readily available to the plant.”
Some sources recommend using compost tea or milk sprays on black spot-affected leaves, but Washington State University Horticulture Professor Linda Chalker-Scott dismisses these methods as ineffective. She also states in an article in Master Gardener magazine (Spring 2009) that baking soda sprays may only be of limited efficacy in combatting black spot. Studies have shown that it works better when combined with horticultural oil.
To sum up, I would pay attention to the cultural practices (not wetting the leaves, etc.). You can try a baking soda spray (always test on a small area of the plant first), but it may not have lasting power as a treatment. Prof. Chalker-Scott mentions that coarse organic mulch (such as wood chips) reduces incidence of black spot, so you may want to adopt this mulching practice.