Field investigation on citrus huanglongbing (HGB, greening) disease caused by insect-borne liberobacter was carried out in Thailand from 1992 to 1993. The disease and its vector Diaphorina citri were present in all the citrus orchards surveyed in 12 districts throughout Thailand. Electron microscopic examination confirmed the presence of causal organisms in sieve cells of leaves with symptoms. Many viruliferous psyllas that transmitted the pathogen on young citrus seedlings were collected in HGB-infected fields. Many orchards had been destroyed due to the disease, while some pummelo orchards were still productive. HGB disease spread from infected trees to healthy pummelo trees. Sour lime, Citrus aurantifolia, was the major HGB-carrier and inoculum source. Circulation of infected nurseries or bud-sticks through growers promoted the inter-field transmission of the disease. The disease was more prevalent in flat or plain fields than in those surrounded by hills or wind breakers, due to the enhancement of vector dissemination by the wind. Rough lemon, Calamondin, Som-pan and Ladu mandarins were tolerant, while large numbers of sweet orange, mandarin and tangelo trees were susceptible. The tree growth and yields were markedly reduced by the disease when the trees were infected at young ages. There were 2 types of citrus trees in the relation of yield to canopy volume after infection, those with high and low yields. Grapefruit, some tangelos and Som-pan mandarin trees were highly productive, while large numbers of mandarin and sweet orange trees showed a low productivity.