Much of the Southeast has recently experienced the typical summer pattern of frequent late afternoon and evening thunderstorms. Heavy rainfall combined with high humidity creates a favorable environment for plant diseases. Our wet, warm weather adds up to fungus “heaven” in our lawns, landscapes and gardens.
Keep a close watch for foliage diseases which are evident on plants by spots on plant leaves. Most foliage diseases require wet conditions and as a result are more prevalent during wet weather. Not all spots on plant leaves are caused by a foliage disease, though. So you have to be careful of the diagnosis. The main point is to be aware and watchful for disease development during wet weather.
How do fungal diseases develop?
Foliar diseases occur on most species of landscape trees and shrubs as well as turf. Most fungal diseases are dependent on moisture, especially foliage or leaf spot diseases. Many of these disease-causing fungi spread by microscopic airborne spores that require moisture to germinate, infect and colonize our plants. Most fungal leaf spot diseases require a 12- to 14-hour period of uninterrupted wetness. This is how moisture plays a role in disease development and why it’s important to irrigate on an as needed basis. Typically these diseases create a problem only with the aesthetics of the plant.
Fungi are the primary cause of most leaf diseases. As the spores are blowing around in the wind, some will land on a susceptible plant species — it’s random chance. If the leaf (or other plant part) that the spore lands on is dry, the spore does not stick to the leaf, is blown off, can’t germinate, etc., because there is not sufficient moisture. But, if the leaf or other plant part is wet (from rain, irrigation, dew, etc.), the spore can stick to the leaf, germinate and then penetrate the plant tissue. This is when infection occurs.
How can I prevent fungal diseases?
For the most part, diseases can be prevented by utilizing proper cultural practices such as variety selections, irrigation, plant and soil nutrition, pruning, and row spacing. When there is inadequate circulation of air, poor water drainage, exorbitant irrigation, and too much dampness due to rainfall, the fungi can become a problem. Not all plants are equally susceptible to these foliage diseases. It’s wise to learn the landscape and garden plants that are likely to experience disease problems as a result of our classic summer weather.
Some tips to help you manage fungal diseases in the landscape
Avoid working in your gardens when the plants are wet. When you handle wet plants, you may be spreading the spores of fungal diseases from plant to plant, increasing the chances that they will find a plant or location where conditions are favorable. Wait until the plants have dried completely before you handle them.
If you are pruning plants that are susceptible to these diseases (like roses), clean the blades of your pruners with rubbing alcohol as you move from plant to plant. This will help to prevent the spread of diseases.
Keep your gardens well-weeded. Weeds may play host to a disease that won’t affect them, but your garden plants may not be so lucky. Weeds are also a good place for insect pests to hide out. Don’t spray your garden with water and try not to wet the leaves of your plants when watering.
A fungicidal spray program can be used to prevent and reduce many of these diseases. But it needs to begin ahead of the symptoms in order to be effective. There are also cultural practices that can be helpful in managing some of these diseases.
A healthy plant naturally resists diseases. Only choose healthy seedlings. Water plants properly. Make sure planting environment is kept clean and well ventilated. Avoid excessive moisture, as it is the most favorable atmosphere for pathogens to breed. Remove rotting leaves and trash. Maintain a weeding program to reduce breeding sites of insect pests and diseases. Remove and dispose of plant parts, do not introduce them to your compost pile.
For assistance identifying and managing plant problems, visit ces.ncsu.edu where you can submit questions via the ‘Ask an Expert’ link, or contact your local Cooperative Extension center by phone: If you live in Pender County, call 910-259-1238; in New Hanover County, call 910-798-7660; in Brunswick County call 910-253-2610.
Susan Brown is the consumer horticulture extension agent with the North Carolina Cooperative Extension in New Hanover County. Contact her at firstname.lastname@example.org or 910-798-7476.