ST. Paul, Minn. (July 8, 1997)—Love roses? Unfortunately, so do fungi. But if your roses are on the sick list these days, don't despair. America's plant doctors, members of the American Phytopathological Society (APS), can help you grow remarkable roses!
"Keep an eye open for fungus, especially black spot. Black spots on upper leaf surfaces reaching one-half inch in diameter and surrounded by yellowing, often of the entire leaf, indicate the presence of the infamous fungal disease known as black spot of rose," Cindy Ash, APS Director of Scientific Services, said.
Here's how to avoid black spot:
- Remove and destroy cankered canes.
- Water only at the base of each plant and early in the day.
- Promote good air circulation by providing adequate spacing between plants.
- Separate highly susceptible varieties.
- Remove all old leaves this fall and mulch underneath roses to cover any remaining leaves; this blocks the dispersal of spores.
- Apply a fungicide (chemical used to protect plants from certain disease causing fungi), if necessary, when leaves begin to expand --and periodically thereafter-- as weather conditions and disease development dictate. Check the label for one of the following active ingredients: horticultural oil, copper hydroxide, myclobutanil, chlorothalonil, thiophanate methyl or mancozeb. Always read the entire label before using any pesticide, no matter whether it is organic or synthetic and apply according to directions.
How does black spot affect roses? Infected leaves are shed and the plant loses its food manufacturing ability (ie. goes on a crash diet). In addition to leaf spots, small, purplish-red --nearly black-- raised irregularly shaped cankers (dead infected areas) form on first year canes. Tiny black bumps can be seen in the leaf spots and cankers if you look closely with a magnifying glass. These are asexual structures containing spores (seeds of the fungus). These spores are readily spread with water and if the water remains on plant parts for extended periods, the spores germinate and start new leaf spots and cankers.
The American Phytopathological Society is a professional scientific organization of 5,000 members dedicated to the study of plant diseases and their control. For more information, visit APSnet.