Further studies on European rasp‐leaf and leaf‐roll diseases of cherry trees

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Abstract
SUMMARY: Cherry leaf‐roll, raspberry ringspot and arabis mosaic viruses occurred in high concentration in cherry sap extracted from buds or from young leaves in the spring. The viruses could be detected serologically in crude cherry sap which was highly infective to herbaceous test plants. During summer and autumn infectivity of leaf‐roll virus decreased in cherry sap, while a virus inhibitor increased.Each virus was transmitted by inoculation with sap from herbaceous host plants to young Prunus seedlings and from these to mazzard F12/1 and Bing sweet cherry plants by grafting. Cherry leaf‐roll virus caused leaf rolling and a few very small enations on the undersurfaces of Bing leaves. Raspberry ringspot virus and one strain of arabis mosaic virus induced small enations characteristic of mild rasp‐leaf disease. When plants infected with these viruses were also infected with Prunus necrotic ringspot virus, the symptoms were only slightly enhanced whereas they were very severe when prune dwarf virus was added.Rasp‐leaf disease may increase by the spread of prune dwarf virus in orchards where latent infection with raspberry ringspot virus had occurred previously. In other orchards, trees may develop the disease as raspberry ringspot virus spreads into trees already infected with prune dwarf virus.