The northern plum tree is a subspecies of the common Japanese plum which grows only in the northern island of Hokkaido, Japan. It produces sour plums, like its parent species, but rarely grows over 15 feet tall. Because it grows in much colder northern climates, it has a thick layer of bark and does not flower until spring, unlike the parent species which flowers in winter. During the summer, the northern plum tree can be susceptible to a number of leaf diseases.
Anthracnose is a fungal disease. It survives the winter in fallen leaves, which later produce exterior pale growths that let off spores. Theses spores drift on the wind and take root in the bark of the plum tree. From there, the fungus spreads in the bark to the exterior branches and leaves. Though the health of the northern plum tree is not endangered, the leaves grow in stunted and discolored. They fall off prematurely and the tree does not flower or produce fruit. The fallen leaves then provide the breeding ground for the disease's spread.
Phyllosticta Leaf Spot
Phyllosticta leaf spot is caused by the fungus Phyllosticta minima. The fungus infects the interior of the leaf and creates tan or brown dark spots. The edges of the spots blacken. These tan and brown spots dry out and fall away, producing ragged holes. Later, the blackened edges bubble out to look like peppercorns. This is caused by the fungus growing in preparation to produce spores. This disease is resistant to fungicides and can only be managed by cutting away affected leaves and branches.
Pseudomonas Tip Blight
Pseudomonas is a species of bacteria which, when it infects northern plum trees, produces a disease called tip blight. The bacteria creates a light-green and yellow discoloration running up the leaf veins. This is the only outward indication of tip blight. Aside from this, the disease causes the tips of new shoots to die, new buds to die and flowers to die. The ends of twigs and branches will slowly become hollow and break away. Specialized bacteriacides can treat the problem.