Black spot is a common fungal disease on roses, especially hybrid tea roses. The disease usually starts as irregular black spots on leaves. Leaves turn yellow and drop. Repeated defoliation weakens the plants, leading to poor bloom. The fungus overwinters on fallen leaves and in cankers that can develop on canes. Infection occurs throughout the growing season when conditions are warm and humid. If you grow susceptible rose varieties in areas where black spot is a problem, you'll almost certainly have to spray fungicides to keep the problem under control. However, you can avoid the need to spray by choosing disease-resistant roses and growing them in a favorable location. Plant roses in a sunny area with good air circulation. Avoid wetting the foliage when watering, and remove spotted or yellowing leaves from the plants. In fall, clean up all diseased leaves and remove diseased canes. Fungicide sprays for black spot are preventive, not curative. That means you need to apply them before the disease develops. Spray a systemic fungicide such as triforine (Funginex) every 7-10 days to prevent black spot from developing. Follow label directions carefully.