Epidemiological review on citrus greening and viral diseases of citrus and banana with special reference to disease-free nursery system

Virus and virus-like diseases of citrus and banana have been causing considerable damage to fruit production in tropical areas. This is due to their systemic invasion and common spread through infected bud woods, vegetatively propagated seedlings, and transmission by insect vectors. The heat-tolerant form of greening locally known as Huanlungpin in Taiwan, has seriously affected citrus trees in the tropical and subtropical regions of Asia. This caused great damage to the citrus industry by shortening tree lifespan. Most citrus cultivars, except pummelo, were already susceptible to the Asian form of greening organisms (GO) before the 1970s. However, pummelo become infected by a new GO strain in Taiwan and Southeast Asia in 1971. Citrus tristeza virus (CTV) is the most common and destructive virus in the western hemisphere, and has a worldwide distribution. Various new strains of CTV have recently evolved and invaded pummelo, sweet orange, and mandarin cultivars resistant to the old strains of CTV. The bunchy top disease is a common and destructive viral disease of banana in the Asian and Pacific regions. The banana mosaic, caused by CMV, has become a major threat after the widespread use of tissue cultured plantlets. Banana streak badnavirus and bract mosaic virus have spread to many banana areas in the Pacific regions in recent years. The virus and virus-like diseases have become one of the serious constraints for the production of tropical fruits in the Asian and Pacific regions. The epidemiological data of the aforementioned virus and virus-like diseases, which are indispensable for formulating integrated control measures, are reviewed in this paper. These vector-borne systemic diseases have been effectively controlled by integrated control measures including production and cultivation of virus-free seedlings, elimination of inoculum sources, and prevention of reinfection through IPM [integrated pest management] of vector insects. Establishment of pathogen-free nursery system is primarily important to prevent prevalence of the diseases. The precise and rapid indexing techniques are indispensible for management of virus-free nursery system. The development and application of molecular diagnostic probes for detection and indexing of virus and greening pathogens are primarily important in the production of pathogen-free seedlings