Production Systems of Fruit Tree Nurseries to Control Graft-Transmissible Diseases.

Abstract

Graft transmissible diseases of fruit crops are very important because of the large production losses and the transmission of pathogens to propagated clones. Virus, viroid and prokaryote organisms have been found as the causal agents of many diseases. However, unidentified pathogens still remain in many fruit crops. ELISA, electrophoreses and PCR can detect the characterized pathogens rapidly, but plant indexing is necessary for undermined pathogens. To obtain pathogen-free plants, a heat-treatment is effective to eliminate various pathogens from infected plants, but usually not viroids. Apical shoot-tip-culture or grafting can eliminate viroids as well as other pathogens, except for capillovirus. Methods combining pre-heat treatment and subsequent shoot-tip culture/grafting have been demonstrated for the elimination of known pathogens. Pathogen-free stock plants are usually preserved in protective screen houses. However, additional propagation and nursery production have been accomplished in fields in Japan. Thus, periodic checks or indexing of the mother stocks are needed. Pre-inoculation of bud-sources with protective mild strains of virus has been proved effective to control citrus stem pitting disease caused by aphid-vectored CTV. Disinfection of cutting tools is recommended to prevent viroid transmission, especially in nursery production.