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Abstract
A leaf infection index was used to express in numerical terms the severity of apple mosaic virus (AMV) symptoms. It was demonstrated that mild strains of AMV gave apple trees a high level of protection against infection with severe strains of the same virus. It was also shown that scions carrying a mild strain when worked to trees carrying a severe strain were effectively protected against invasion by the severe strain. Although there was some breakdown in protection in the first and second years after working, in subsequent years protection became more firmly established.
The cross-protection reaction provides a useful technique for detecting mild or symptomless strains of AMV. It is suggested that cross-protection could be used to protect orchard trees in areas where field spread of AMV occurs and that it could be used to regenerate apple trees stunted by severe strains of the virus.
