Leaf blotch

Leaf blotch, caused by the ascomycete fungus Diplocarpon mespili, is a common and destructive disease affecting various members of the Rosaceae family. The pathogen primarily targets the foliage, disrupting photosynthetic processes and often causing premature defoliation, which stresses the host plant and reduces its long-term vigor.

The disease has a wide host range, frequently infecting quince, hawthorn, medlar, pear, mountain ash, and serviceberry. While the leaves are the primary point of attack, in severe cases, the infection can also manifest on the leaf stalks and green shoots, causing necrosis and further hindering the development of the plant.

Symptoms typically begin as small, circular, reddish-brown or black spots on the leaves. As the infection progresses, these spots expand in size and may coalesce, forming large, irregular dead patches. Under magnification or close inspection, tiny black fruiting bodies known as acervuli can be observed within the necrotic tissue, serving as the source for secondary infections.

The development and spread of Diplocarpon mespili are heavily dependent on environmental conditions. The fungus thrives in warm, humid weather, with rainfall and overhead irrigation significantly facilitating the dispersal of conidia (spores) from leaf to leaf. The pathogen survives the winter in fallen infected leaves, making them the primary reservoir for the following season.

Managing leaf blotch requires a holistic integrated pest management approach.

• Sanitation is crucial: rake up and remove all fallen leaves at the end of the season to eliminate overwintering inoculum.
• Improve air circulation through proper pruning to help the canopy dry out faster after rainfall.
• Apply preventative fungicide sprays during the growing season, particularly during periods of frequent rain or high humidity.
• Choose resistant cultivars if available to minimize the need for chemical intervention.