- 5 - 



It has led, for example, to the complete replacement of mother trees 

 of 'Golden Delicious' by the two best sources. No. 2 and No. 5. 

 These are now generally known as clone A and B, respectively. Dif- 

 ferences in production between virus-free sources of 'Anjou' pear 

 were recently reported by Westwood and Cameron (1974) . Thus the 

 virus story will not end with the removal of viruses by heat treat- 

 ment of a cultivar. The choice of the source of a cultivar before 

 heat treatment is also of importance. The comparison of different 

 sources of one cultivar will therefore have the attention of our 

 research station in the next years. 



Table 2. The skin smoothness of 4 different virus-free sources of 



'Golden Delicious' on M.26. Mean for the 1971 through 

 the 1974 harvest seasons. 



Source No. Smooth fruit (I) 



2 33. 4a^ 

 5 28.7ab 

 1 24.2bc 



3 20.6c 



^Means followed by the same letter are not significant at the 5^ 

 level. 



Propagation 



When the virus and pomological tests are completed, the collab- 

 orating organizations jointly decide which tip-grafts of a cultivar 

 are worth multiplying for release to the fruit industry. For root- 

 stocks, the virus tests are the main basis for the decisions because 

 of the long time necessary for pomological evaluations. The General 

 Inspection Service then establishes mother trees and stoolbeds of 

 the newly selected material. The nurseries also are allowed to 

 start new stoolbeds of virus-free rootstocks but under control of 

 the General Inspection Service. The nurseries, however, have to 

 buy their bud- and graft-wood of virus-free cultivars from the Gen- 

 eral Inspection Service. 



To give an impression of what's going on, some data are pre- 

 sented in Table 3 about the release of apple bud and graftwood in 

 the past season (summer, 1974 and winter 1975). It should be noted 

 that this season there were no virus-free M.9 rootstocks available 

 for budding. (M.9 is the most important rootstock in the Nether- 

 lands.) The virus-free buds were therefore budded on other virus- 

 free rootstocks and on virus-infected M.9. 



Virus-free M.9 rootstocks are rapidly being multiplied under 

 the control of the General Inspection Service. Some nurseries have 

 started new stoolbeds with all the new virus-free layers that have 

 become available. These stoolbeds are now coming into production 



