Plum trees are midsized deciduous trees that flower in the spring and produce abundant, edible fruits in the summer and autumn. When compared to other fruit trees popular in the home orchard, such as apple and pear, plums present relatively few disease and pest problems. Two fungal diseases -- black knot and brown rot -- afflict the plum tree. Black knot causes masses of warty black growths to form on branches and twigs and affected trees can decline and die in several years if not treated. Brown rot begins as soft brown spots on the fruit that progress until brown spores cover the fruits and spread onto nearby twigs and branches.
Inspect trees in the winter to identify infected twigs and branches. Look out for small, light brown lumps on current or 1-year-old growth. These swellings turn olive green and velvety the next year before progressing into the noticeable black swellings that usually attract homeowners' attention.
Prune out any infected twigs or branches, cutting 15 to 20 cm (6 to 8 inches) below the knot to remove any fungal tissue growing inside the branch. Pruning should occur in the winter or early spring at the latest. Once buds begin to break, the fungus releases its spores and infection spreads even further throughout the tree.
Burn or bury any infected tissue that you remove to prevent infection from spreading further.
Refrigerate or process fruit immediately after harvest, even if it does not appear to be infected. Even damage as slight as an insect bite leaves plums susceptible to airborne brown rot spores.
Pick up any rotted fruit from the ground around the tree and destroy or discard it.
Remove any "mummies" from the tree: shrivelled, spore-covered fruits that can remain on the tree for years, producing spores that prolong the infection.
Prune out infected twigs or branches during the winter. Brown rot can spread to the twigs and branches, forming cankers -- sunken brown spots that may develop a gummy texture -- and leaf dieback. If you observe leaf dieback in the summer, tie string on the branches to mark them for pruning in the winter.
When treating black knot in your plum tree, inspect other nearby trees, including wild trees, that belong to the prunus genus. This includes wild plum, prune and cherry trees. Fungicide treatments do exist for black knot but are often available only to commercial growers. Contact your local extension office for more information on treatment options available in your area. Fungicide sprays can prevent the development of brown rot in plum. Trees are usually sprayed when the buds begin to show colour in the spring and again shortly before harvest. Your local extension office can provide you with plum spray schedules and a list of fungicides approved for use in your area.