By Pat Dray
The Garden Spot
Pat Dray.
If you’ve noticed a white, chalky substance or black spots on the leaves of some of your outdoor plants, it’s most likely a fungal disease.
The majority of plant disease is caused by fungal infection. Research at the Michigan State University Extension confirms that fungal pathogens are behind 85 percent of all plant disease. These diseases include black spot, rust, downy mildew, powdery mildew, blight, dollar spot, red thread, and many others.
Field studies on plant pathogens have demonstrated that the growth of fungi is favored by high moisture and moderate temperatures, so these hot and humid summer days are the ideal environment for the reproduction of mold spores – and thus fungal growth. Remember, your plants need to breath, or transpire, to carry water from the roots to the leaves where it can evaporate. When relative humidity levels are too high or there is a lack of air circulation, a plant cannot make water evaporate (part of the transpiration process) or draw nutrients from the soil, setting it up for fungal diseases to take hold.
The best solution is to choose disease resistant plants to start with. Start by doing an online search for blight resistant varieties and choose those plants when possible. Since that ship has already sailed for this growing season, consider it this winter when you’re pouring over seed catalogues.
Prior to planting – even for shrubs this fall – make sure that any debris in your garden such as decaying leaves or foliage from past seasons is cleaned out and not put in your compost pile. Keep the concept of right place and right space in mind when planting for good plant health. Spacing is important, since good airflow between plants will keep the humidity lower. Make sure that you thin out any seedlings to the recommended spacing. Thinning out plant growth will encourage airflow around leaves and stems and help to prevent fungal disease.
Next, be sure to irrigate wisely. Overhead watering can disrupt powdery mildew spores, but it also encourages water-spread pathogens. Water close to the ground to reduce wet leaves, and water early in the day so excess moisture dries by nightfall. Drip irrigation is ideal since it waters plants at the roots while reducing water usage, as the water is not evaporating while airborne.
You should be closely inspecting your plants for any signs of illness. If you see any problems, prune infected plant parts promptly and dispose of the debris. Again, don’t compost it since you’ll only be spreading diseases when you use the compost next year. Always cut back into healthy tissue so no disease remains.
When you suspect disease, sterilize your pruners after each cut by wiping them with a common household disinfectant or rubbing alcohol. Otherwise well-intentioned snips may spread the problem. When you’re pruning healthy plants, prune the center of the plant or shrub so that air circulates freely rather than just giving the plant a “haircut.”
Happy gardening.
Pat Dray is a past president of the Orange Garden Club.