Virus-tested Trees 



The initial research of the Plant Protection Service showed 

 that some harmful viruses (like rubbery wood in apple and vein yel- 

 lows in pear) were widely spread in Dutch fruit orchards. Latent 

 viruses occurred in most trees, especially in those on the most im- 

 portant rootstocks. Therefore, the Plant Protection Service ini- 

 tiated a program to find trees free of the most important viruses. 

 Thousands of trees from commercial orchards were tested on indica- 

 tor plants but only a few trees proved to be free of the most impor- 

 tant viruses. The virus-free trees were used to build up a new 

 stock of mother trees and rootstocks. This plant material was 

 named "virus-tested," because of the absence of some important vi- 

 ruses (rubbery wood, mosaic, proliferation and fruit attacking vi- 

 ruses in apple; vein yellows, ring mosaic and stony pit in pear). 

 However, the problems were how to introduce this material into the 

 fruit industry and how to guarantee and to maintain the absence of 

 some viruses. In the old system, every nurseryman had his own 

 mother trees of cultivars and stoolbeds of rootstocks and the Gen- 

 eral Inspection Service controlled only the authenticity of the 

 cultivars and rootstocks. Now it was thought that the General In- 

 spection Service should also control the health of the plants. 

 Therefore, it was decided that growing of "virus-tested" mother 

 trees should be centralized on a special plot under the control of 

 General Inspection Service. Since about 1965, hundreds of "virus- 

 tested" mother trees of most cultivars have been planted. All 

 nurserymen are obliged to buy their bud- and graftwood from these 

 mother trees. The virus-tested rootstocks are propagated in com- 

 mercial nurseries but are strictly controlled by the General Inspec- 

 tion Service. This system has worked quite well. 



Virus-free Trees 



About 1960, a new technique became available for the produc- 

 tion of trees free of all known viruses: heat treatment. Plants 

 are grown in a climate chamber at 37°C for 6-8 weeks and very small 

 tips (5 mm or less) of new growth are grafted to apple and pear 

 seedlings, free of all known viruses. Usually 95% of the tip-grafts 

 made on the seedlings survive. Generally, several sources of each 

 cultivar are treated. The tip-grafts produced are usually free of 

 any virus contamination. However, it is necessary to test every 

 tip-graft on indicator plants to be sure of the absence of viruses. 

 These tests for viruses causing symptoms on leaves and stems usually 

 take 3 years, while those for viruses causing symptoms on fruits 

 may take 6 years. 



The Research Station for Fruit Growing at Wilhelminadorp ob- 

 tained some trees resulting from the tip-grafts for pomological 

 tests and the General Inspection Service was allowed to build up a 

 stock of heat-treated material. However, generally no material 

 was released until the completion of the virus and pomological 

 tests. 



