A large number of latent viruses have been identified in apple. More common latent viruses include apple chlorotic leaf spot virus, apple stem pitting virus, and apple stem grooving virus. These viruses also cause diseases in other fruit crops. Apple chlorotic leaf spot virus causes pear ring pattern mosaic and has been found in all pome and stone fruit species. Apple stem pitting virus is the causal agent of pear vein yellows.
As the designation "latent" implies, these viruses are symptomless in most commercial cultivars. However, they may cause symptoms in certain cultivars, scion/rootstock combinations, and ornamental varieties. Symptoms of apple chlorotic leaf spot virus include chlorotic leaf spots, leaf distortion, chlorotic rings and line patterns, reduced leaf size, and stunting. On 'Virginia Crab', apple stem grooving virus produces symptoms such as chlorotic leaf spots, stern grooving and pitting, union necrosis, and swelling of the stem above the graft union. Like apple stem grooving, apple stem pitting virus symptoms are associated with 'Virginia Crab' and include stem pitting of xylem that generally stops at the graft union. Apple stem pitting virus also causes fluted (grooved) fruit in 'Virginia Crab' and epinasty and decline in 'Spy 227'. Although they cause no marked symptoms in commercial apple cultivars, latent viruses may have a detrimental effect on the growth and cropping of some cultivars.
In general, apple viruses other than tomato ringspot virus (which causes apple union necrosis and decline) are transmitted only through grafting. If a tree is healthy, with virus-free rootstock and scion, that tree will remain healthy in an orchard situation. If an infected tree is present in the orchard, many viruses have been known to spread slowly to adjacent trees through natural root grafts.
Original text prepared by E. V. Podeckis and R. Welliver. The original version of this article appeared in The Mid-Atlantic Orchard Monitoring Guide (NRAES-75) and is reproduced with permission from the Natural Resource, Agriculture, and Engineering Service, Cooperative Extension, 152 Riley-Robb Hall, Ithaca, New York 14853-5701, U.S.A. (607) 255-7654. It has been edited for presentation here by Alan R. Biggs, West Virginia University.