FRUIT VIRUS RESEARCH AND THE INTRODUCTION OF VIRUS-FREE 

 TREES TO FRUIT INDUSTRY IN THE NETHERLANDS 



Dr. Ir. H.J. van Oosten* 

 Research Station for Fruit Growing, Wilhelminadorp 



After World War II, research on fruit tree viruses expanded 

 rapidly throughout the world. Initially, most of the research was 

 conducted in England and the USA. In 1953, virus research with 

 fruit trees was started in the Netherlands by the Plant Protection 

 Service. This agency established that several viruses known to 

 occur in apple and pear in other countries were also present in the 

 Netherlands. Later, the Institute of Phytopathological Research in 

 Wageningen, the Netherlands, started more basic fruit virus research. 

 They found better indicator plants for some viruses than those which 

 were generally used, with quick and distinct symptom expression af- 

 ter infection. The Plant Protection Service and the Institute of 

 Phytopathological Research were also involved in the production of 

 virus-free trees. The value of virus-free plant material for the fruit 

 industry was examined by the Research Station for Fruit Growing at 

 Wilhelminadorp while the propagation of the promising virus-free 

 material was organized by the General Inspection Service for Woody 

 Nursery Stock. Thus, 4 organizations are involved in the fruit 

 virus research, evalution of virus-free material, and the release 

 of the new material to the fruit industry. This paper will briefly 

 review why the virus program was started, research results, and the 

 organization for the release of virus-free material. 



Virus-infected Trees 



It has been shown that several important diseases of fruit 

 trees are caused by viruses: rubbery wood, mosaic, chat fruit, 

 proliferation, star crack, and rough skin in apple; vein yellows, 

 ring mosaic, and stony pit in pear. Especially in the USA incom- 

 patibility of some cultivars with important stocks like Virginia 

 Crab and Northern Spy were found to be caused by viruses. In some 

 cases, yield losses up to 501 were reported. But several viruses 

 also were detected that gave no symptoms on the leaves, stems or 

 fruits of the main cultivars (varieties). These viruses were 

 called the "latent" viruses. When each of these "latent" viruses 

 occurred separately, no effect on growth and production could be 

 detected, but when more than one was present (as often is the case) 

 tree growth and yield reductions could be measured in trials. Thus, 

 it was concluded that many viruses were harmful to apple and pear 

 trees and that the problem was serious enough to warrant investi- 

 gations of ways to eliminate viruses from apple and pear. 



*Researcher on virology, clones, rootstocks and interstocks 



