Apple and pear trees are prone to infection with a range of viruses, of which the most important in South Africa are Apple mosaic virus (ApMV) and Apple chlorotic leaf spot virus (ACLSV). Both viruses can exist in trees for long periods without showing visible symptoms, i.e., they are latent (especially ACLSV). Visible symptoms of ApMV occur with virulent strains of the virus or in sensitive cultivars, such as Golden Delicious and Granny Smith, and in old orchards. The rapid emergence of visible impacts can be triggered by conditions leading to stress in the orchard, such as drought and disease. Where trees are affected, fruit tends to be smaller, leading to yield reductions and price reductions. Both viruses are spread by grafting with infected plant material. The virus incidence can be managed by planting only certified virus-tested trees, removing infected material from orchards, using strict sanitary practices, and using only certified material for re-planting and top-working.
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