Stone fruit rust is a wet weather fungal disease and if left uncontrolled can cause severe premature leaf drop, causing reduced yield, and fruit quality reductions. The disease is sporadic in most regions of Australia because it requires both warm and wet conditions for long enough periods for it to develop.
Plant Part: Leaves, branches and fruit.
Season: Spring - Summer
Symptoms: Rust produces deep yellow angular spots, about 2 mm wide on the topside of leaves and a mass of rust red powdery spores opposite the spots on the underside of the leaf. In severe infections the leaves will fall prematurely reducing fruit production. Leaf symptoms are commonly seen in late Summer and Autumn. When a tree is affected and it prematurely loses its leaves, it can weaken the tree, exposing the trunk and branches to sunburn.
Control: Removal of the infected leaves and the infected twigs will help in controlling the fungus. A preventative fungal regime is recommended - A spray of copper oxychloride at bud swell will help. If the tree is still showing signs of infection, you can also apply a spray of wettable sulphur (Manutec) at petal-fall, 4 weeks after petal fall and then 8 weeks after petal fall. (always read the product instructions).
Prevention: Avoid overhead watering, since the spores travel by rainfall and splashing water. Where possible, avoid planting your fruit tree in a valley as humid air can collect in this area. Keep weeds, cuttings and plant debris cleaned up and dispose of this material away from the tree to keep from spreading spores accidentally. Check out the Care instructions in the annual winter pruning section. Applying a copper spray at bud swell and repeating if necessary is a great preventative.
References: NSW Department of Primary Industries, Bayer, Images from UF/IFAS Plant Diagnostic Center M.Borden, Herbiguide.com.au, and the FST team.
