Mulberry leaf spot is a prevalent fungal disease that affects mulberry trees, which are valued for their attractive foliage and edible fruit. This condition can lead to noticeable aesthetic damage and, in more severe instances, can impact the overall health of the tree and its fruit production. Understanding this disease is important for maintaining the vigor and yield of mulberry trees.
Identifying Mulberry Leaf Spot
Mulberry leaf spot appears with distinct symptoms. Initially, small, dark brown, or reddish-purple spots emerge on leaves in early spring. These spots are often irregular in shape and may be surrounded by a pale-green or yellowish halo. As the disease progresses, the spots enlarge, and their centers may turn whitish or gray, sometimes developing dark brown or necrotic margins. Over time, these lesions can merge, affecting larger portions of the leaf and potentially causing a “shot hole” appearance as dead tissue falls out. Severely affected leaves may turn yellow and drop prematurely, often in late summer.
Causes and Favorable Conditions
Mulberry leaf spot is primarily caused by fungal pathogens, most commonly Mycosphaerella mori (also known as Phloeospora mori) and Cercospora moricola. These fungi produce spores that spread to new hosts, with disease severity fluctuating annually and becoming more pronounced during wet summers. The fungi thrive in specific environmental conditions conducive to their development, including high humidity, prolonged periods of leaf wetness, and moderate temperatures ranging from 24-28°C. Spores are typically dispersed by wind and splashing water, allowing the disease to spread rapidly. The fungus can survive over winter in fallen leaves and on young parts of branches, providing a source of infection for the following spring.
Impact on Mulberry Trees
When infections are severe, significant premature defoliation can occur, especially in late summer. This early leaf drop reduces the tree’s ability to perform photosynthesis, which weakens the tree. While the disease is rarely fatal for mature trees, repeated or chronic infections can stress the plant, making it more vulnerable to other pests or diseases. For young trees, repeated infection may delay establishment and growth. In fruit-bearing mulberries, a severe infection can reduce the fruit yield and quality in the current season and potentially impact the subsequent year’s crop.
Prevention and Management
Prevention and management of mulberry leaf spot involve cultural practices and, in some cases, chemical treatments. Collecting and disposing of all fallen leaves in autumn, and any prematurely dropped leaves during the growing season, helps remove overwintering fungal spores. Burning, burying, or consigning these leaves to green waste can disrupt the fungus’s life cycle.
Improving air circulation around and within the mulberry tree canopy can significantly reduce leaf wetness. This can be achieved through proper spacing when planting new trees and by regular pruning to thin out dense branches. Avoiding overhead irrigation, which wets the leaves, and instead watering at the base of the tree can also help.
Selecting mulberry varieties known to have some resistance to leaf spot, such as certain Morus alba cultivars, can be a proactive measure if planting new trees. For existing infections, pruning out affected branches can reduce the fungal inoculum.
While cultural controls are often sufficient, chemical treatments like fungicides may be considered for severe cases or for young, susceptible trees where the disease impacts establishment. Fungicides should be applied according to label instructions, typically at the first sign of disease, and are generally seen as a last resort due to potential environmental impacts. Fungicides are often not recommended for fruit-bearing mulberries if the fruit is intended for consumption, as specific products for edible crops may not be available.