Apple mosaic is caused by at least two plant viruses. The viruses curl and discolor apple foliage and can reduce fruit yield and tree growth.
Identification
On infected apple varieties that are more sensitive to the viruses, leaves turn pale yellow or white along the veins and develop large pale spots. The spots later turn brown and the leaves drop prematurely. On apple varieties less sensitive to the viruses or when apple trees are only infected with mild strains of the viruses, small, yellow or white spots develop on the leaves.
Life cycle
Apple mosaic is transmitted by budding and grafting of apple trees. It also spreads through root grafts of healthy trees with infected trees. Root grafts can naturally form when trees of the same species grow near each other.
The severity of apple mosaic symptoms varies with the apple variety and strains of the viruses. Symptoms are more pronounced after a cool spring.
Damage
Apple mosaic curls leaves and discolors them white or yellow then brown. Infection slows tree growth and reduces fruit yield by as much as 25% when strains of the virus are mild and by up to 50% when the viral strains are severe. All varieties of apple are susceptible to apple mosaic. Some such as Golden Delicious, Granny Smith, and Jonathan exhibit more severe symptoms.
Solutions
The occurrence of apple mosaic can be minimized by using virus-free plants obtained from a reputable, commercial supplier. Once trees are infected, there is no way to eliminate or manage apple mosaic except to remove the tree and replace it.
Adapted from Integrated Pest Management for Apples and Pears and Pest Management Guidelines: Apples, University of California Statewide Integrated Pest Management Program (UC IPM).
