Fungal spores which were formed on infected fallen leaves or on canes are splashed to the new lower leaves of roses in the spring by rain or sprinkler irrigation. The spores must remain wet for several hours in order for the spore to germinate and invade the leaf. If temperatures are warm following infection, symptoms may become visible in 3 days. New spores can form in as few as 10 days after infection. Thus under some conditions the fungus can infect a plant, begin producing new spores, and spread to other leaves within two weeks.
Leaves - dark spots up to 1/2 inch in diameter have "feathery" edges when some rose cultivars are infected. Spots are smaller with more distinct edges in other cultivars. These spots may have a yellow halo surrounding them. Infected leaves yellow and fall.
Canes - small purplish spots form on the current years growth.
Because the fungus spore must have free water on the leaf or cane in order to germinate and penetrate, take steps to insure that plant surfaces dry quickly or are not wetted.
- Avoid dense plantings.
- Maintain good weed control so that air circulation around the plants is good.
- Avoid wetting the plants when irrigating.
- If plants are misted or sprinkler irrigated, only do so when the plant surfaces will dry quickly.
Because the fungus survives from year to year in infected material,
- rake and destroy fallen leaves.
- prune out canes that have symptoms of black spot disease.
When feasible, plant resistant cultivars. David Thompson, Bebe Lune, Coronado, Ernest H. Morse, Fortyniner, Grand Opera, Lucy Cromphorn, Sphinx, Tiara, Carefree Beauty, and Simplicity are considered resistant. Teas, hybrid teas, hybrid pertetuals, Pernetianas, Austrian briers, and polyanthas are usually very susceptible while rugosa hybrids, moss roses, and wichuraianas are more resistant. The American Rose Society recommends the following varieties as resistant: Bride's Dream, Canadian, White Star, Cary Grant, Dainty Bess, Duet, Electron, Elina, Elizabeth Taylor, Helmut Schmidt, Keepsake, Lady, Lady Rose, Lady X, Las Vegas, Mikado, Nantucket, Olympiad, Otto Miller, Pascale, Polarstern, Precious Platinum, Princess of Manaco, Pristine, Sheer Bliss, Sunbright, Tansinnroh, and Uncle Joe.
Apply a fungicide with a spreader-sticker added so that young tissues are protected. In the spring as daily temperatures reach the high 50F range, begin applying a fungicide every 2 weeks. As the leaves emerge, make fungicide applications each week. Cease spraying when day temperatures average in the 80F range or when weather is dry. Resume spraying in the autumn when temperatures moderate and rainfall or dew regularly occur.
Prepared by Gary W. Moorman, Professor of Plant Pathology