(left) Low power image of sporulating acervuli on the underside of a tart cherry leaf; (right) Close up of the formation of an acervulus on the underside of a tart cherry leaf.

Photograph courtesy Phillip Wharton
Department of Horticulture, Michigan State University

Host: Prunus cerasus, Tart cherry
Disease name: Cherry leaf spot
Pathogen name: Blumeriella jaapii 

Tart cherries (Prunus cerasus), which are sometimes called sour or pie cherries, are best known as the key ingredient in desserts. Today, there are about 36,000 acres of cherry trees in Michigan. Michigan is the leading producer of tart cherries in the US, producing 200 to 250 million pounds of tart cherries each year. Cherry leaf spot, caused by Blumeriella jaapii (Rehm) Arx, is the most important disease of tart cherry in Michigan and throughout the northeastern United States and Canada. All the tart cherry varieties grown in the U.S. are susceptible to cherry leaf spot and currently the disease must be chemically controlled. When not properly controlled, cherry leaf spot can cause leaf chlorosis and premature leaf defoliation. Fruit on trees defoliated by leaf spot before harvest show poor coloration, are low in soluble solids and are less firm than fruit on healthy trees. Early defoliation can also result in reduced winter hardiness and subsequent flower bud and tree death due to low winter temperatures. One of the long term goals of the MSU tart cherry breeding program is the development of new varieties that have stable resistance to cherry leaf spot.

The cherry leaf spot fungus B. jaapii, infects young tart cherry leaves during rainy weather.  The fungus enters the leaf through stomata found in the lower leaf surface and grows within the intercellular spaces in the leaf.  Given the optimum conditions for growth (a temperature between 15-20 °C accompanied by rainfall or high humidity), the first symptoms become visible after 4-5 days, as small white dots on the underside of the leaves.  These spots develop into brown lesions after 7-8 days.  As the lesions develop, light pink to white masses appear in the center.  These are the fruiting bodies (acervuli) of the fungus and contain masses of spores.  Figure A shows the appearance of lesions on the underside of a tart cherry leaf as viewed from above with a dissecting microscope.  The pinkish-white horn shaped structures are masses of spores exuding from the leaf.  Figure B is a x50 magnification image of a lesion from the side, showing the masses of spores exuding through the leaf surface and forming a horn shaped structure.  The spores are embedded in a mucilaginous matrix which is highly water soluble.  During wet weather, the matrix is dissolved by rain drops and the spores are dispersed to surrounding leaves by the action of splashing rain and air currents.  The spores then germinate and start the process of infection over again.