Those leaves with the striking green and yellow marbled pattern you spy on the branch of your neighbor’s apple tree dangling over your fence may look like a cool new designer cultivar, but don’t be fooled. More than likely, it’s the result of a mosaic virus.
Viruses cause many important plant diseases and are responsible for huge losses in crop production and quality in all parts of the world. There are over 2,000 known viruses and about one-fourth cause disease in plants. These submicroscopic, non-cellular particles are among the smallest and simplest entities that can cause disease.
Mosaic viruses affect a wide range of edibles – apples, beans, celery, corn, cucumbers, figs, peppers, spinach, and tomatoes are some of the more common ones. They can also infect ornamental plants like delphinium, gladiola, marigold, petunia, and one of the most notable, roses.
Symptoms of a mosaic virus vary with the type of plant, the type of virus, and environmental conditions, usually developing during cool weather in spring. Most often, no new symptoms appear after hot weather arrives. Puckered leaflets, and variegated patterns of yellow and green on the leaf, fruit or flower are common symptoms. Some infected plants show no sign of the disease while others are actually grown for their ornamental value, like the variegation types of heavenly bamboo.
Once a plant becomes infected with virus, it usually remains throughout its life. There is no cure or treatment for virus-infected landscape plants, and none is generally needed. In addition to proper cultural care to improve and maintain plant health, the most effective management option is to purchase plant material that is of high quality and certified virus-free or virus-resistant.
Back to that apple tree. Since all varieties of apple are susceptible to apple mosaic, that may be what you observed, especially if it was after a cool spring and the variety happened to be ‘Jonathan’, ‘Golden Delicious’, or ‘Granny Smith.’ While it shouldn’t hurt the tree in the long run, the fruit yields are likely to be reduced.