BP-26-W
Three major diseases of roses which cause the greatest concern to Indiana rose growers are black spot, powdery mildew, and crown gall. Knowledge of the symptoms and how to manage these diseases is necessary for successful rose growing.
Black spot (Diplocarpon rosae) is the most common and damaging disease of roses in Indiana. As the name suggests, black, circular spots form on leaves during spring and summer. Infected leaves eventually yellow and drop prematurely. Severe infection may cause some canes to become completely defoliated, thereby making them more prone to winter injury.
The fungus that causes black spot thrives under wet conditions. Therefore, infection occurs primarily during spring (May and June); it is this period when fungicide sprays are most needed to prevent black spot. Effective materials are:
Common Name Trade Name -------------------------------------------- chlorothalonil Daconil 2787 myclobutanil Immunox propiconazole Banner Maxx thiophanate-methyl Cleary's 3336, Domain, Fungo-Flo thiophanate methyl + mancozeb Zyban, Duosan triforine Triforine, Funginex --------------------------------------------
These chemicals are often included as a component in commercial rose pesticide formulations. Check label to determine chemical composition. 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. Continue to spray according to label directions. Note: Triforine or thiophanate-methyl are recommended for rose varieties which are also highly susceptible to powdery mildew.
Prevention of black spot is helped by following good sanitation procedures through the year, i.e., rake and destroy infected leaves. It is also helpful to maintain plants so they have an open center. This allows good air circulation and rapid drying of foliage. If plants have excessively dense growth and are planted too closely, air circulation is impeded and black spot may be more severe.
Powdery mildew (Sphaerotheca pannosa) ranks second to black spot in terms of occurrence and injury caused. The mildew fungus grows over the upper leaf surface and is easily recognized from the white, powdery patches formed. Severely infected leaves become brittle and wither and die. Infected stem tissue has a withered, spindly appearance and may also be coated with the white, powdery growth of the fungus.
Unlike black spot, powdery mildew does not require wet conditions for infection to occur. Mildew does best when the temperature is cool and relative humidity is high. Often mildew is a mid- to late-season problem, not becoming apparent until July or August. However, be on the lookout for mildew throughout the growing season. Fungicides need to be applied at the first sign of powdery mildew. Immunox, Banner Maxx or Triforine are recommended for rose varieties which are highly susceptible to powdery mildew.
Powdery mildew is most severe in heavily shaded areas. Avoid such locations when planting roses or other plants susceptible to mildew, i.e., lilac, zinnia, dahlias. Follow suggestions for good air circulation as outlined under blackspot.
Unlike black spot and powdery mildew, crown gall (Agrobacterium tumefaciens) is a soilborne bacterial disease which infects root and/or stem tissue rather than leaves. Galls are most frequently seen as round, rough-surfaced growths near the plant's crown or on roots. These "crown" galls will eventually girdle the plant, resulting in poor growth and eventual plant death.
Crown gall is a difficult disease to control once a plant is infected. It is best to rogue diseased plants as soon as they are detected. Where possible, remove and discard all soil in which the root system is growing and replace with sterilized soil. If crown gall is a recurring problem in an area of your yard, it is best to rotate the area with a nonsusceptible grass crop for 3 or 4 years.
Where a gall is restricted to stem tissue only, cut off the entire cane and immediately seal the cut surface with pruning paint. Always sterilize cutting tools with rubbing alcohol after each cut .
To prevent crown gall, inspect all new rose plants before purchase for signs of swelling near the bud union or on roots. During and after planting, take special care to avoid any and all wounds, especially near the soil line. Provide good winter protection to prevent bark cracking or other cold injury to bud union area. Never use crown gall-infested soil to mound over bud union for winter protection.
Reference to products in this publication is not intended to be an endorsement to the exclusion of others which may be similar. Persons using such products assume responsibility for their use in accordance with current label directions of the manufacturer.
REVISED 5/01
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