General Recommendations: This is a broad group of plant diseases, caused by fungi, that is commonly found on many garden plants. Leaf spots are classically thought of as tan to brown, local lesions with distinct borders or delimited borders on leaves or flower parts. When the spots become numerous and begin to touch each other the disease is called a blight or blotch or less commonly, scorch. This category can be confused with anthracnose diseases, many of which are also leaf spots.
Most fungal leaf spot diseases are active during wet times of the year such as spring or early fall, but only a few are important enough to warrant aggressive control measures. Management for these diseases should begin by controlled watering practices to avoid getting above-ground parts wet. Also plant with adequate space between plants to hasten drying, pick up or pick off infected material, and apply fungicides before infection begins. Fungicides are often used as the sole method of reducing leaf and flower spot diseases. When evaluating whether to use a fungicide or not, professionals will weigh the value of the plant and the likelihood of the disease-causing serious damage to the plant against the cost of the fungicide and the expense of application. In most cases, fungal leaf spot diseases are not significant enough to warrant fungicide applications. Cultural practices to lessen the inoculum distribution and reduce the chance of infection is typically all that is necessary to manage leaf spot problems. Most fungal leaf spots cause only cosmetic damage or occur late in the season, and thus do not warrant control. Small plants or plants repeatedly defoliated by disease may require a preventive treatment.
Control of Leaf & Flower Spots, Blights and Blotches:
1. When possible, purchase disease-free plants of resistant cultivars. Certain popular types of plants that are known to be susceptible to leaf spots have been bred for disease resistance. This is certainly true for many vegetables, roses, peonies, grapes, tomatoes, and apples, to name a few. Ask local nursery dealers if resistant cultivars are available.
2. Collect, compost, or bury all fallen leaves in autumn and again in spring before new growth begins. This reduces the amount of inoculum from fallen leaves. Most leaf spots that occur in early spring can cause premature defoliation and growth decline.
3. Prune to increase air circulation around woody shrubs and trees.
4. Maintain plant vigor by proper planting, by not handling plants when the foliage is wet, by avoiding overhead watering, and by fertilizing on the basis of a soil test. Protect tender plants during winter, and control weeds.
5. Use a fungicide when the value or health of the plant is going to be reduced by fungal leaf spot infections. Fungicides need to be applied before the first sign of infection and continued at 5- to 10-day intervals throughout the period of host susceptibility. This is especially true when the rainfall pattern is such that leaves and flowers stay wet for prolonged periods (usually greater than 48 to 72 hours). Thorough coverage is critical.