Apple mosaic virus is one of the oldest known and most widespread apple viruses. The same virus can cause line pattern symptoms in plum and rose mosaic disease. Apple mosaic virus is related to Prunus necrotic ringspot virus. Apple trees infected with apple mosaic virus develop pale to bright cream spots on spring leaves as they expand. These spots may become necrotic after exposure to summer sun and heat. Most commercial cultivars are affected but vary in severity of symptoms. ‘Golden Delicious’ and ‘Jonathan’ are severely affected, whereas ‘Winesap’ and ‘Mclntosh’ are only mildly affected. Except in severe cases, a crop can still be produced by infected trees; yield reductions vary from no reduction to 50%. In some cultivars, bud set is severely affected. For more information, see the article on Apple Mosaic Virus.
Answer provided by Alan R. Biggs, West Virginia University.