avoid mistakes with 

 industry. Three tre 

 free M.9 rootstock. 

 quality are studied 

 each tip-graft for p 

 value of these tests 

 already been detecte 

 'Red Boskoop' , the ' 

 obtained after heat- 

 grafts from the same 



- 4 



the material which may be released to the fruit 

 es of every tip-graft are made, using the virus- 

 Characteristics of growth, production and fruit 

 for about 3 years. This program for testing of 

 omological characteristics started in 1972. The 



is beyond doubt. Some unexpected events have 

 d. For example, in the important Dutch cultivar 

 Schmitz-Hubsch' a mutant with striped fruits was 

 treatment at 2 different institutes. Other tip- 

 source of the cultivar had normal fruits. 



Table 1. The growth per tree (meters) of virus-free and virus-in- 

 fected trees of 'Golden Delicious' on M.9 on fresh and 

 replant soil. 



Virus status 



Growth per tree (meters) 



Fresh soil 



Virus-free 

 Virus-infected 



Replant soil 



Virus-free 

 Virus -infected 



1972' 



2.9 

 2.4 



3.2 

 3.2 



1973' 



6.3 

 4.2 



4.8 

 3.2 



1974' 



28.1 

 18.2 



11.9 



7.9 



In the nursery 



After planting in the orchard 



C. Comparison of different virus-free sources of one cultivar 

 (variety) . 



It is well known that different sources of a cultivar may dif- 

 fer in growth, production and fruit quality. There are two main 

 reasons for this: (a) the sources differ in virus-contamination; 

 and (b)genetic variation. 



In the Netherlands, the virus-induced variation in the culti- 

 var 'Cox's Orange Pippin' is well knovm. In almost every orchard 

 of this cultivar, poor and heavy bearing trees occur. But, there 

 seems to be a genetic variation also. Campbell (1974) found that 

 the virus-free English clone of 'Cox's Orange Pippin' was less pro- 

 ductive than some virus-infected commercial sources of the cultivar 

 These virus-infected sources will be heat-treated and it is hoped 

 that a heavy-bearing 'Cox's* clone can be found. Similar work is 

 now being done in the Netherlands with the most important cultivars. 

 In one trial, differences in smoothness of the fruit skin has been 

 found among the 4 virus-free sources of 'Golden Delicious' (Table 

 2). These results were very important to Dutch fruit industry. 



