Diseases of apple trees are the most common reason for poor apple yields in commercial and home orchards. All the well-known apple varieties are susceptible to a number of foliar and other tree diseases. It is essential for apple trees for be regularly sprayed with reliable disease control methods to reduce chances of infestation.
Apple scab is a fungal disease caused by the fungus Venturia inaequalis. The fungus causes irregular, circular leaf spots with feathery margins. The leaf spots are brown to olive green in colour. Severe infestations are referred to as sheet scab and cause premature leaf drop. The spots spread to the fruit as well, where the skin around the lesions ruptures to reveal a dark layer of spores. Young fruit is most likely to get infected.
The spores of the fungus overwinter in the fallen infected leaves and are activated during the wet season. The spores are also transported by air. Cultural control methods include water management, avoiding over-watering the trees and making sure the trees have an open canopy that provides good air circulation. Fungicidal controls include the use of myclobutanil, mancozeb and captan.
Cedar-apple rust is a common foliar and fruit disease of apple and crabapple. The disease is caused by the fungus Gymnosporangium juniper-virginianae and appears in the form of leaf spots that lead to leaf drop especially during dry seasons. If left uncorrected and if trees keep getting prematurely defoliated for several years in a row, it considerably weakens the tree. The formation of fruit buds is also affected.
The juniper is the primary host for the fungus. Four- to eight- day-old apple leaves are most likely to get infected. Earliest signs are yellow to orange spots on the upper leaf areas. As infection progresses, the spots increase in size and often have a black border. Controls include the use of resistant varieties such as Red Delicious, Spartan, Priscilla, Liberty and Empire. Fungicide controls include the use of immunox and captan.
Powdery mildew is a fungal disease that infects the young green tissue in apple trees. The disease is caused by the fungus Podosphaera leucotricha. The disease is evident by the chlorotic or yellow spots or patches on the upper leaf surface and a white, powdery substance on the underside. The infected leaves curl, crinkle or roll upward along the edges. In severe cases, leaves start to fall off, reducing the productivity and vigour of the tree.
As it progresses, powdery mildew destroys the vegetative shoots and the flower buds. The disease also spreads to the fruit, leading to russetted fruit. The use of recommended fungicides are critical for the control of the disease. This helps to reduce the number of spores forming on the new developing tissues and prevents the infection of fruit. Commonly used fungicides include sulphur, triforine and potassium bicarbonate.