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How to Treat a Black Spot on Shrubs
How to Treat a Black Spot on Shrubs. Black spot fungus (pseudomonas bacteria diplotheca) is one of the most common plant diseases. Aptly named, it causes black, purplish or brownish-black lesions or blisters on the leaves of shrubs and flower petals. Lesions are sometimes ringed with yellow and are usually 1/16 to 1/2 inches in diameter....
Black spot fungus (pseudomonas bacteria diplotheca) is one of the most common plant diseases. Aptly named, it causes black, purplish or brownish-black lesions or blisters on the leaves of shrubs and flower petals. Lesions are sometimes ringed with yellow and are usually 1/16 to 1/2 inches in diameter.
Although black spot fungus thrives in warm, wet weather, it can survive cooler temperatures by living in infected plant litter. Composting this litter can re-infect plants, weakening them in future seasons. Although a widespread malady, black spot fungus can be treated quickly and easily.
Things You'll Need
Fungicide
Baking soda
Dish soap
Trim infected leaves. Don't compost them; throw them in the trash and discard. The fungus can also be transmitted through wet clothing, so be careful not to touch other shrubs while working with infected leaves.
Spray the infected shrubs with a fungicide every week. Effective fungicides include copper fungicide, chlorothalonil, myclobutanil, sulfur, triforine, propiconazole and thiophanate methyl.
Take a more eco-friendly approach. Instead of the fungicide, use neem oil, or prepare a solution of baking soda, water and dish soap by mixing 2 tbsp. of baking soda, 1 gallon of water and 2 drops of dish soap. Mist the plants with this solution once a week. This solution changes the pH level of the leaf surface, making it inhospitable for black spot fungus. (The dish soap helps the solution stick to the surface of the leaves.)
Tips & Warnings
Consider planting your shrubs in an area where they will receive ample sunlight and good air circulation. Ideally, shrubs should not touch one another. While this is not always possible for aesthetic or other reasons, it is one of the best ways to prevent black spot fungus from spreading.
To prevent black spot from occurring, particularly if symptoms appeared in previous years, apply a fungicide to your shrubs once a week from late winter to early spring.
Black spot fungus can infect plants in as few as seven hours, with legions appearing in as few as three days. The fungus is transmitted through water. Avoid late-night watering; instead, water during the day when it can evaporate from the leaves more quickly.
