Most varieties of Japanese plums (Prunus Salicina) are hardy, disease-resistant trees that grow between 15 to 20 feet tall and produce succulent, juicy, plump fruit that starts off green then changes to yellow, maroon and deep purple-black. These fruit trees require fertile, well-drained soils to thrive and remain healthy. However, improper care and maintenance makes the Japanese plum susceptible to a variety of fungal diseases that adversely affect its health, fruit quality and quantity.
The three main types of fungi to infect Japanese plums are leaf blight, sooty mold and black knot.
Although the Japanese plum tree is not as susceptible to leaf blight as other varieties of plum trees, it is not completely immune either. Symptoms of leaf blight include deep brown to black spots that appear on the surface of leaves, and yellowing of leaves in some cases.
Insects such as mealybugs, scale, aphids and whiteflies excrete "honeydew"---a sweet, sticky substance--on twigs and leaves of Japanese plums that serves as the ideal growing medium for airborne scooty mold spores. The mold germinates into black strands that cover the area and cause discoloration.
The bacterial fungus black knot forms light brown ½-inch to 1-foot long swellings, or galls, on previous or current growth that turn olive green, hard and brittle by the next season. It usually occurs in spring and is carried by airborne spores from infected trees including wild cherry.
Leaf blight is commonly called "shot hole disease" because the spots on leaf surface penetrate the tissue to form holes. The coat of black sooty mold on foliage impedes the Japanese plum's ability to produce food. Shrubs planted under trees infected with insects that produce honeydew are at a risk of developing the fungal disease as well. According to a Cornell University publication, severe outbreaks of black knot cause high losses to commercial plum growers in parts of the Midwest and Northeast. It kills young, infected twigs mostly during the first year of infection, but larger branches take longer to display damage. Black knot stresses plum trees, makes them susceptible to infections from other pathogens and stunts growth in extreme cases.
Spray diluted lime sulfur on infected foliage to treat leaf blight. Apply 10 days after the blossoms fall, and repeat application three weeks later. In cold areas, apply a third application four weeks after the previous one; however, forego the application in dry season. Spray registered insecticide over the insects that cause sooty mold to prevent future outbreaks. No registered chemical control methods exist for home or orchard control of black knot; gardeners should adopt good cultural practices.
Prune infected leaves or twigs immediately and clean up debris around the tree to prevent the spread of fungal diseases. Use horticultural oils or lime sulfur during the dormancy period. Because Japanese plums benefit from cross pollination, frequently inspect nearby trees for signs of fungal diseases and take appropriate measures to prevent it from spreading.