Q. I have a Kousa dogwood on which the leaves are curled and brown at the edge. Is this a disease and is there a control for it?
A. The problem is likely leaf scorch, a common physiological problem typically caused by environmental conditions. Leaf scorch occurs when water loss through transpiration exceeds the ability of a plant to take up water through the root system. In many plants, this causes leaves to curl and become necrotic or dead along the edges of the leaf or at the leaf tip.
Leaf scorch can be caused by transplanting, root damage because of excavation, soil compaction, over fertilization, high soil salts, and environmental conditions. The environmental conditions include hot weather, low soil moisture, and windy conditions. Leaf scorch as a result of environmental conditions is being seen a lot in the landscape right now due to low soil moisture and hot windy days. The symptoms may look similar to some diseases such as anthracnose or bacterial leaf scorch. Making the distinction between disease or an environmental condition is important. Physiological leaf scorch will typically be uniform across the plant or parts of the plant with greater exposure. Symptoms on the leaf will also be uniform, with leaf roll and necrotic or brown edges of the leaf and tip. Consider submitting samples to the UMass Plant Diagnostic Lab if unsure: https://ag.umass.edu/services/plant-diagnostics-laboratory.
Leaf scorch can occur on just about any plant, however, it is commonly seen on dogwoods, maples, rhododendron, and viburnum. If leaf scorch is caused by environmental conditions, including low soil moisture, water your plants deeply once a week or every other week to alleviate plant stress. Using a mulch over the root zone and reducing competition for soil moisture from other plants like grasses can also help.
Q. There are several species of plants in my yard that are covered with a black mold-like substance. Is it damaging my plants?
A. The black mold like substance is referred to as sooty mold. Sooty molds are types of fungi that feed on honeydew, the excrement of piercing sucking insects. Honeydew has lots of nutrients supporting the growth of the fungi.
Though sooty molds are not plant pathogens, the molds can impact plant health by reducing photosynthesis. In most cases however, the sooty mold is an aesthetic issue. When honeydew is fresh, it may be be washed off; however, once the mold has started to grow, removal from plants is unlikely. Sooty mold can also cover other objects like cars, fences, windows, etc. To remove from hard, non-plant surfaces, soapy water and brushing are necessary.
To control sooty mold, the insect producing the honeydew must be identified. Insects that commonly produce honeydew include aphids, mealybugs, or scale insects. The insect may be on the plant experiencing the sooty mold or on other nearby plants. Often times the sooty mold is found on understory plants and the insect can be found feeding above in the canopy. On Cape Cod, there has been a widespread occurrence of lecanium scale. This scale insect creates copious amounts of honeydew and thus sooty mold, and can be found feeding on many species of trees and shrubs. However, white oaks seems to be the favored host, leaving the plants underneath covered in sooty mold. Once the insect has been identified, the decision about appropriate management can be made.
Russ Norton, Horticulturist, Cape Cod Cooperative Extension