Two major fungal diseases of roses are black spot and powdery mildew. Black spot appears just as its name would indicate, and the spots sometimes have a yellow halo surrounding them. The disease starts on the lower leaves and progresses upwards as water or rain splashes fungal spores upward from infected leaves, canes or infected leaves left on the ground from the previous year. Leaves must remain wet for several hours for infection to occur, and symptoms can arise within 72 hours of infection if the weather is warm. Repeated infection from black spot increases the stress on the plant and opens it up to other diseases and attack from insects -- and can potentially cause the eventual death of the plant depending upon the severity of infection and defoliation.
Powdery mildew is easily recognized by the white powdery substance that develops. Infection of young or newer leaves can appear as stunted, distorted growth and is then followed by the easily recognized powder that forms on the leaves. Powdery mildew is introduced to new plants via fungal spores carried by wind or by plants infected in previous years as the fungus overwinters on plants. Infection occurs when humidity is high and we have warm days and cool nights.
To help minimize potential rose diseases, selecting resistant varieties is a good place to start. A note on disease resistant plants: If the weather is perfect for development of diseases, even resistant plants can still show signs of infection especially if they are under stress. You can also take other proactive steps to assist in minimizing disease chances including:
º Water from the bottom to minimize water splashing on leaves and encouraging infection and avoid late day/evening watering as leaves won't dry off quickly enough as the temperature reduces in the evening.
º Provide good air circulation, including spacing plants properly and proper pruning so that air can flow through the plant more readily allowing leaves to dry off quicker.
º Mulching to minimize soil/water contact with leaves. The University of Illinois booklet, Pest Management for the Home Landscape, even recommends removing and replacing mulch if black spot occurred the previous year to minimize infections the following year from overwintering spores.
º Clean up and destroy and infected leaves. Do not compost leaves infected by either as composting will not destroy the fungal spores.
º Prune back rose canes showing signs of infection in the spring. Canes infected with black spot begin with purple spots turning to black making it easy to determine which canes are infected.
º Use fungicides to help control as needed. Contact your local Extension office for recommendations.