SPREAD OF NECROTIC RING SPOT AND SOUR CHERRY YELLOWS VIRUSES IN NIAGARA PENINSULA ORCHARDS
In the Niagara Peninsula of Ontario necrotic ring spot (NRS) and sour cherry yellows (SCY) are the two principal virus diseases of sour cherry. Since 1951 most trees grown in this area have been propagated from virus-free budwood but about 4% of the rootstock used carries virus.A combination of symptoms known as ’shock’ is the usual reaction to infection with either virus. However, in some trees the first sign of infection is only an etch, which is usually considered to be a secondary symptom of NRS. In a few trees the primary symptoms are very mild trace reactions strongly suggestive of trees that have been infected for a long time. Indexing on peach seedlings is generally not a good indicator of current season infection. It is however very accurate in detecting virus in trees that have been infected for more than 1 year.The rate of virus spread in young orchards is dependent upon the age of the orchard, the proximity of older diseased trees, and the amount of disease within the orchard. Necrotic ring spot virus (NRSV) spreads very slowly in orchards under 4 years old but can spread very rapidly in orchards over 4 years old. Most rapid spread of NRS occurs after 20% of the trees in an orchard are infected. Sour cherry yellows virus on the other hand does not spread rapidly until after the 10th year. Both viruses can spread over a considerable distance, NRSV at least 800 yd and SCYV about 100 yd, but most infections of both occur within 50 ft of a known source. There is no indication that any plant other than sour cherry serves as a source of inoculum.