Diseases of the Rosebud

Fungal Disease

• The common windborne fungus Sphaerotheca pannosa causes a breakout of powdery mildew that swells, distorts and stunts growth and leaves a white mold on young leaves, shoots and rosebuds. Botrytis flower and cane blight attacks young shoots and rosebuds and newly pruned canes. The fungus Botrytis cinerea leaves small spots that appear as red-ringed water spots on white rosebuds. The flowers turn brown and develop a light tan to gray mold, causing the buds to droop and not open. Cankers (Crytosporella umbrina) attack weaker plants at wound sites or at the join of a young shoot and cane, appearing as small, red stem spots or brown or black cane discoloration that grows in size and can reach all the way to the rosebud as it kills the entire plant. Downy mildew (Peronospora sparsa) causes yellow angular lesions on leaves, stems and flowers and turns brown to purplish. In some cases, spore masses are visible on the underside of the leaves and canes.

Viral Disease

• "Rose mosaic is a complex of several viruses that cause similar symptoms in rose plants," most commonly the Prunus necrotic ringspot virus, according to the University of Florida Institute of Food and Agricultural Sciences Extension. Symptoms may appear sporadically depending on weather and growing conditions and vary by virus strain. Signs of infection can include chlorotic lines, ring spots and mottled leaves. The virus interferes with flower production, plant growth and longevity.

Bacterial Disease

• Although crown gall (Agrobacterium tumefaciens) is a soil-borne bacterial disease of the root or stem tissue, the galls block water and nutrient flow to the top of the plant, which weakens and stunts the rosebuds. The small swellings usually start at ground level on the stems and roots and weaken the plant and cause discoloration as they enlarge, killing the plant. Once an infection takes hold, it is nearly impossible to get rid of the disease, and the bacteria can linger in the soil for up to 20 years. Therefore, the best prevention is to carefully inspect rootstock before purchasing and start with disease-free plants. Avoid planting in areas where there has been a previous crown gall infection for at least two to four years; remove soil to root depth and replace with sterilized soil. Rotate planting with a resistant grass crop.

Management

• Good sanitation in the garden is the most effective preventive measure. Sterilize all tools with alcohol or bleach before working on rosebushes. Plant the roses with the recommended spacing for your variety to ensure good air circulation and prevent the moist, humid conditions that encourage the disease spores to grow. Avoid overhead irrigation, and do not let the leaves get wet when you water. Weed regularly, and suppress weed growth by mulching with dry mulch rather than manure or wet organic materials. Prune out any sign of damage or infection immediately, and burn them away from the planting area to prevent carryover infections. The black spot fungus (Diplocarpon rosae) weakens rose plants and makes them more susceptible to other diseases. Therefore, pruning and discarding all leaves during the winter and spring strengthens the plant's resistance to many rose diseases. If the severity of the infection makes chemical control necessary, spraying with a fungicide such as captan, chlorothalonil, neem oil, triforine (Funginex), propiconazole (Banner Maxx), or myclonbutinol (Systhane, Eagle or Immunox) every seven to 10 days throughout the growing season or at the first sign of infection keeps many infections under control.