The Degradation of Starch in Apples Removed From the Tree at Different Stages of Development

The viruses cucumber mosaic (CMV), turnip mosaic (TuMV), broad bean wilt (BBWV), tomato aspermy (TAV) and tobravirus were isolated from field-grown spinach in Britain and identified by host range, electron microscopy and serology. CMV was most prevalent and widespread. Infected CMV-sensitive spinach cultivars usually showed diverse symptoms that were either associated with individual viruses or a mixture of TuMV and CMV. CMV-tolerant cultivars were symptomless when infected with CMV alone but produced diverse symptoms when infected with other viruses. In field experiments there was an association between trap catches of winged aphids and CMV incidence. Virus incidence was greatest in summer-sown crops, less in spring-sown and least in autumn-sown. Sap transmission tests showed CMV incidence to be similar in the CMV-tolerant and sensitive cultivars and that visual assessment underestimated virus incidence. A variant of a spinach CMV isolate overcome CMV tolerance with the production of severe symptoms and concomitant increase in virus content. CMV and BBWV reduced growth in a sensitive and BBWV in a tolerant cultivar.