Apricot rust is a serious disease caused by the fungus Tranzschelia discolor. Untreated apricot rust will cause defoliation and lowered fruit production in an apricot tree. One infection will not kill the tree, but several occurrences over several years may make the tree susceptible to other diseases.
The top portion of the leaves on the apricot tree become splotched with yellow patches that run together. Rusty brown spots will develop on the underside of the leaf opposite the yellow spots. Shoots of the tree may come under attack, the bark splitting where dead patches appear. Fruit on the tree will show signs of spots, which are easily depressed, with a red center and greenish border.
Although all apricot trees are susceptible to rust, certain varieties are resistant to the disease. Planting resistant varieties will prevent occurrence. Planting in valleys should be avoided, as the fungus that creates apricot rust carries on the wind, which valleys trap.
Sanitation processes prevent the conditions that propagate rust disease. Pruning should be done at the end of the harvest; remove dead or diseased branches. Diseased branches should be removed as soon as they are found and destroyed by fire to prevent the spread of spores. Removal of fallen leaves reduces the chance of fungi growing in the decay. Green leaves left on the tree after harvest require manual removal.
Fungicide application in spring prevents the development of fungi during the warm, moist months before bud production. Spraying is required before the buds begin to flower and is repeated every 7 to 10 days, 7 when the weather is wet and 10 when the weather is dry.
New trees require planting in areas that are free of disease and away from old orchards that have been infected. Never plant a new tree in soil that once held a diseased tree, as the fungus can live in the soil for several years.