Diseases of the Satsuma Plum Fruit

Black Knot Disease

• Black knot is a common and serious fungal disease that can significantly reduce the fruit bearing capabilities and life spans of Satsuma plum trees. This disease earned its name because it causes hard, black knots or swellings to develop on smaller branches. As these knots spread, they gradually girdle and kill the branches. If left untreated, the knots produce fungal spores that carry on the wind to infect new plum trees. Numerous black knot infections cause Satsuma plum trees to lose vigor, blossom poorly and become unproductive. Cornell University recommends controlling the spread of black knot by pruning out all developing knots from infected trees before the new buds appear in the spring. Cut affected branches at least 4 inches below the knot and apply wound dressing. Immediately burn or bury all prunings to avoid contaminating other trees.

Brown Rot Disease

• Brown rot disease often affects Satsuma and other Japanese plum varieties growing in the mid-Atlantic area. Although brown rot frequently infects shoots, spurs, branches and blossoms, growers typically aren't aware of the disease until the plum fruits develop symptoms. On Satsuma plums, the brown rot initially looks like small lesions covered with tan or gray fungal spores. As the lesions mature, they turn light brown in color. Wet weather conditions can promote rapid growth of the fungal spores and the plum fruit can rot within just a few hours. Most of the rotted fruit falls to the ground, but some plums remain on the tree and host the fungus throughout the winter. North Dakota State University suggests promptly pruning out all infected twigs and remaining fruit. Immediately bury or burn the prunings so the disease doesn't spread.

Perennial Canker

• Satsuma plums are also susceptible to perennial canker infections. Infected branches develop oval-shaped lesions that enlarge and eventually girdle and kill the wood. The canker fungus produces small, black fruiting spores inside of the canker, and these spores ooze out in the spring to be spread about by wind and splashing rain. North Dakota State University reports that you can help control this virus by planting your Satsuma plums in well-draining locations. Prune your trees in the spring only after the growth has begun so that the pruning wounds heal faster. Promptly cut out any cankers and severely infected branches and apply a fungicidal dressing to any wounds. Burn or bury the prunings immediately.

Bacterial Spot Disease

• Bacterial spot disease often affects Satsuma plum trees grown in drier regions of the U.S. This disease usually attacks the leaves, but sometimes affects young shoots and fruit. The leaf spots initially appear as water soaked areas on the underside of the leaves, but later turn black or dark brown in color. The middle section of the infected area often falls out of the leaves and a red ring develops around the remaining hole. Infected leaves typically drop, causing the Satsuma plum to defoliate prematurely, lose vigor and produce smaller plums. Infected plum fruit generally develop reddish-brown to dark brown sunken spots. West Virginia University recommends preventing bacterial spot disease by applying a copper compound right before the new spring growth. You can also apply antibiotics once a week once the blossoms start falling.