by Carol Sitarski
Master Gardener, Cornell Cooperative Extension of Allegany County
It’s raining, it’s pouring, and the fungal spores may be growing.
This spring’s rain seemed to be nonstop. Don’t get me wrong— we needed rain to replenish our water tables, but with the good comes the bad. The bad is all the fungus that may start growing on our susceptible trees and plants.
Already I have seen this happening on crabapple trees and Asiatic lilies in my area and gardens.
Crabapple trees that once had beautiful blooms and fresh leaves now have foliage that are spotted and drooping, giving the tree the appearance it is dying. Later in the season these affected trees will have fruits with circular rough spots.
All of this comes about because of a fungal disease called Venturia inaequalis. The spores from this fungal disease can winter in leaf litter or the ground under the tree and are blown about by the wind. Spores remain dormant on the ground until temperature and moisture are right, which triggers the release of the spores into the air. These spores land on both bark and leaves, but do not affect the bark, only the leaves.
After penetrating the leaf, the fungus symptoms will become evident. You’ll see a small yellow spot that grows and ruptures into a black lesion. That spreads more spores. This will not kill the tree but it will be unsightly.
Control measures include removing leaf litter from under the tree (now and in the fall). Burn the leaves or place the leaves in black plastic garbage bags exposed to direct sunlight for multiple days to kill the spores.
A fungicide containing copper or sulfur may be applied to the leaves after flower buds have fallen using a sprayer, but it isn’t guaranteed to be effective. Apply the fungicide thoroughly to the leaves.
A solution of sodium bicarbonate (baking soda) applied with a sprayer is reported to be effective and is acceptable for organic farming. It also helps with the control of powdery mildew. See the recipe below.
This is a lot of work considering that the tree might not be affected again until a very rainy spring, but it will ensure your tree remains healthy. These are preventive measures and would need to be reapplied after rain.
You may want to choose a different variety of crabapple tree. There are now several varieties of crabapple trees that offer resistance to this issue.
If you have Asiatic lilies you may have noticed this type of damage to the leaves. This most likely is a fungal disease by the name of botrytis that affects the stems, leaves and, in extreme cases, the buds and flowers. (Don’t confuse this with damage caused by the lily leaf beetle.)
Botrytis also attacks bedding plants.
Like the Venturia inaequalis fungus, botrytis starts as spores on the ground, in garden debris or under leaves. It also is spread by wind or rain splashing from the ground back up onto this year’s fresh leaves.
If the conditions are favorable, such as extreme wet weather along with warm temperatures, the fungus can spread like wildfire.
Symptoms begin as small white or brown spot that grow into a larger black spot with a lighter middle and will quickly engulf the entire leaf, stem or bud if action is not taken quickly. This disease will not affect the bulb, other than causing it not to grow larger that year. However, the bulb will die if it’s under siege from botrytis for three years in a row.
To prevent botrytis, thoroughly remove all leaves and stems in the fall and spray the ground in the spring with a commercial fungicide or sodium bicarbonate (baking soda) spray. See the recipe below.
Infection can begin after the plant is injured by hail or frost in the spring, so after an injurious event, start spraying on a two-week rotation until the plant starts to die back naturally.
Basic fungicide recipe
The ingredients for the fungicide made with baking soda are 1 teaspoon baking soda, 1 quart water and 1 teaspoon dish soap or horticultural oil. Mix the ingredients together and spray. Be sure to cover the whole plant, including stems and top and bottom of leaves. Spray every 3-5 days for plants that are infected, especially while weather conditions favor fungal diseases. Use preventively every two weeks and reapply after heavy rainfall.
Do not apply when the weather is very hot and/or sunny because this can lead to leaf burn.