Q: Last summer, the mums in my garden developed some sort of black spot disease. I took out the diseased mums and didn’t plant anything on those spots. However, now some of my other mums have developed the same black spot and it’s spread to my strawberry plant. I noticed the leaves on it had black spot. Is my strawberry plant now diseased or is there anything I can do to get rid of it? Is my soil now diseased? What can I do to prevent this from happening next spring?
A: The good news is that your mums, which are members of one plant family, did not pass their black spot onto your strawberries. The bad news is, strawberry can develop its own black spot disease that, although similar to the spots on your mums, is particular to a different plant family.
It’s a bit complicated, so first, let’s discuss the black spot disease on your mums. Mums are a member of the Asteraceae family, a plant family with more than 24,000 species of plants worldwide. Members of the Asteraceae plant family can be susceptible to the disease that causes black spots known as pseudomonas cichorii, or bacterial leaf spot. Plants in this family include flowers such as ageratums, asters, cosmos, dahlias, marigolds, zinnias and more. Vegetables include artichokes, lettuce and endive, among others.
Pseudomonas cichorii can live on infected plants; in the contaminated soil; in leaf, stem and flower debris; infected seed; and also in the pots they were grown in and on garden tools. The plants cannot be cured. Remove the entire plant as soon as you notice the disease, and remove those nearby to prevent spreading of the disease. Regularly clean up fallen leaves and other plant debris. Disinfect all garden tools that come into contact with the disease using a bleach/water solution.
Dig out the rest of the mums with the black spot. The soil is not diseased; you can safely plant any other plant in the space as long as the plant is not a member of the Asteraceae family. After a few years, you can try mums again, but only those chrysanthemums that claim resistance to bacterial leaf spot. Consult The Ohio State University (OSU) Fact Sheet HYG-1219-92, “Growing Chrysanthemums.”
Now let’s talk about strawberries. Strawberries are a member of the Rosaceae family, a plant family with more than 2,000 species of plants worldwide, especially fruits such as peach, apple, cherry, as well as flowers such as anemone, coneflower, clematis, roses and more. Members of the Rosaceae plant family can be susceptible to the fungal disease that causes black spots known as Mycosphaerella fragariae.
Leaf spots first appear as circular, deep purple spots on the upper leaf surface. These spots enlarge and the centers turn grayish to white on older leaves and light brown on young leaves. A definite reddish purple to rusty brown border surrounds the spots. On fruit, surface black spots may form under moist weather conditions. The spots form on ripe berries around groups of seeds. The spots are about one-fourth inch in diameter, and there are usually only one or two spots per fruit.
The fungus overwinters as spores on injured leaves. It infects the plant and produces more spores on both sides of the leaf surface that spread the disease during early summer. Spores are spread by splashing rain. Middle-aged leaves are most likely to develop the fungus, but damage also occurs on stems and runners, as well as leaves.
Leaf spot is best controlled by planting resistant varieties of strawberries. The following June-bearing varieties are resistant to both leaf spot and leaf scorch: Allstar, Canoga, Cardinal, Delite, Earliglow, Honeoye, Jewell, Lester, Midway and Redchief. The ever-bearing varieties, Tribute and Tristar, are tolerant to leaf spot and leaf scorch.
Here are some gardening practices to help limit leaf spot on strawberries:
- Remove older leaves and infected leaves from runner plants before planting.
- Space the runner plants according to the planting instructions.
- Plant strawberries in light, well-drained soil with full sun exposure and good air circulation.
- Weed routinely to increase air circulation and reduce drying time for wet leaves.
- Regularly clean up fallen leaves and other plant debris.
- Disinfect all garden tools as directed above.
- Remove any additional infected leaves you spy after harvest.
- Focus on planting resistant varieties to eliminate the need for fungicide control.
For more horticultural information, call the OSU Summit County Extension Hotline Tuesdays from 9 a.m. to noon at 330-928-4769, ext. 3, and request Fact Sheet HYG-3015-08, “Strawberry Leaf Diseases.”
Please note fact sheets are sent out free; there is a fee for bulletins. Many bulletins are available online at ohioline.osu.edu and can be printed from home or accessed at the public library.
Dayle Davis is a freelance writer and avid perennial gardener, with a B.A. in communications and course work in botany, geology and wildflowers. Davis is a master gardener emeritus under The Ohio State University’s Horticultural Extension. Readers can send in questions regarding lawn and garden issues, which could be featured in a future edition, as well as about the OSU Master Gardener program. Questions can be emailed to firstname.lastname@example.org, faxed to 330-665-9590 or sent to Leader Publications, 3075 Smith Road, Suite 204, Akron, OH 44333. Please do not send any leaves, seeds or other organic material. Inquiries about area garden clubs or groups should be sent directly to the particular organization in question.
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