Leaf Diseases

LEAF DISEASES (I) By Bailey Walten 1990 Newsletter Many of our ornamentals and shade trees are susceptible to various leaf diseases that can cause spotting, blotching, curling, wilting and premature defoliation. The majority of these symptoms are caused by different fungi with their severity varying with current weather conditions. Cool wet springs enhance the activity and spread of fungus as spores are readily carried from plant to plant by wind and splashing rain. Some visible indications that fungi are active are: sooty or powdery masses on foliage or erupting from pustules; fuzzy mold growth on blighted parts; tiny, dark pin pointed-sized fruiting bodies in affected tissues and slimy, pussy blisters. The most common fungus diseases which concern us are apple-scab, leaf spots, anthracnose and powdery mildew. In most cases preventive fungicidal sprays aren’t warranted because little harm is done to the plants affected or it’s not cost effective as the control obtained is marginal. When we do make recommendations for fungicidal sprays, they should be done preventively as the foliage emerges and at two week intervals with a minimum of two to three applications.”Apple Scab” primarily affects certain varieties of apple, crabapple and pyracantha and can be successfully controlled with three sprays in the Spring, preventing Summer defoliation. “Anthracnose” on Sycamore, Oak, Ash, Maple and Dogwood affects the new growth in unusually wet or cool springs. If severe defoliation or stunting of growth occurred last season, it would be advisable to treat this spring. “Leaf spotting” diseases can be quite conspicuous on Hawthorn, Maple, Mountain Ash, Dogwood and Laurel. There are control treatments recommended, however, we have found them ineffective. “Powdery Mildew” is prevalent on Lilac, Crape myrtle and some varieties of euonymus. Though this white powdery mold can become unsightly by late Summer, it doesn’t seriously harm the plant so spraying is not typically suggested. The most important factor to remember in treating controllable diseases is to treat them preventively. Once symptoms have appeared, it’s usually too late to get good control. Trees and shrubs which are susceptible to leaf diseases withstand and recover from their attack if their vigor is maintained with regular fertilization. The best and most practical way to control disease problems is to plant disease resistant varieties of plant material. CONTROLLING DISEASES IN THE LANDSCAPE (II) By Bailey Walten 1996 Newsletter Plant diseases are caused by viruses, bacteria, fungi, or nematodes. Unfortunately, they are much more difficult to control than insect pests. Indeed, there are many diseases (especially viruses and bacteria) that cannot be treated because no cures have been discovered as yet. In those situations it is better to plant resistant cultivars or control the vectors where relevant, eliminate nearby alternate hosts, and practice good sanitation by prompt removal of diseased plant parts during dry weather (disinfect the pruning tool between cuts) and cleaning up old leaves, twigs, and fruit that may harbor pathogens. Many fungus diseases are at least partially controllable with pesticides and we have many fungicides in our arsenal. The three main types of fungicides are protectant, curative, and systemic. Protectant materials have to be applied before the pathogen attacks the plant and several follow-up treatments are necessary to maintain protection (especially during damp weather which promotes disease activity). Curative fungicides can be applied after symptoms are first noticed and still obtain control though some damage will have already occurred. Systemics are absorbed by the plant, move about in the tissues, and usually are longer acting than non-systemics. Some diseases that we regularly treat preventively are scab on crabapples, Discula Anthracnose on dogwoods, anthracnose of oak,ash, and sycamore, Phomopsis blight on juniper and Volutella on boxwoods and pachysandra. Timing of applications is critical and we do our best to schedule them at appropriate intervals as weather allows. Dutch Elm Disease is one we treat systemically with trunk injections. Controlling this disease is very difficult and expensive and no one can warrant long term results. There are many fungus diseases that attack turf but relatively few demand spray treatments. Because grass grows so rapidly, it will usually recover from minor damage. Dollar spot, brown patch, summer patch, Fusarium, and Pythium often need to be controlled chemically to prevent severe root dieback causing significant turf loss. Stress conditions such as high temperatures and humidity can greatly increase fungal activity. Proper mowing, watering deeply once or twice a week during drought periods, and maintaining good soil fertility will alleviate stress and help the lawn survive the summer when diseases are worst. Controlling diseases in the landscape is no easy proposition. Outdoor plants are subject to all the negative environmental factors that exacerbate pathogenic activity. Recognition of a problem and prompt diagnosis is crucial; therefore we rely heavily on the customer to alert us to symptomatic changes in their plant material so the right control measures can be taken. We can advise you about implementing good cultural practices, recommend disease-resistant varieties, and when appropriate, apply the most effective chemicals at the right times to obtain the best controls.