The Economics of Living with Citrus Diseases: Huanglongbing (Greening) in Thailand

The economic losses due to Huanglongbing (greening disease) in Thailand are extreme despite the fact that methods are available for control of the disease. This paper discusses the economic and financial impact of greening disease in Thailand and losses incurred by farmers. It also outlines a program for prolonging the life of their citrus trees, thus greatly increasing their financial returns. The devastation of citrus in Thailand caused by Huanglongbing (HLB) (greening disease) was described by Roistacher (14) as follows: "In my lifetime of involvement with citrus virus and virus-like diseases, never have I witnessed such severe destruction of citrus as seen in this consultancy visit through Thailand. This destruction of citrus by the greening disease has been ongoing in Thailand for over 30 years and has been well documented by others. I believe there is a tendency to accept and accommodate to the disease and to live with it. I observed that after a period of destruction by the greening disease, the trees become unproductive and unprofitable after 5 to 8 years". The seriousness of this disease and its destruction of citrus in Thailand was mentioned and reviewed by all consultants who visited that country and observed the disease (1, 8,9,12,14,15,16,17,18). This paper reports on the financial impact to farmers and their countries by HLB and suggests ways in which these losses can be minimized through programs of certification and education; and by the application of methods for disease control which are now standard practices in other citrus-growing countries throughout the world. HUANGLONGBING DISEASE IN THAILAND Citrus is one of the leading fruit crops in Thailand, second only to mango in fruit production. Its tonnage exceeds that of durian and ramboutan and is greater than the combined tonnage of jackfruit, tamarinds, banana, mangosteen, langstats and longan. Yet, primarily due to the greening disease, the average yield of 13 Tka for citrus in Thailand is low when compared to yields of 50-90 Tka in other countries (8). Schwarz et al. (17) reported that HLB first appeared in Thailand in the 1960's and the length of time between the onset of the disease and debilitation of the entire tree was about 2 yr. They reported that: "Trees are not usually killed, but decline to a point where growers uproot them when costs of production exceed returns". They further reported that of the two methods of transmission of the disease (by vector and by propagation) "the vegetative propagation of infected source trees is important in spreading the causal organism from one locality to another when infected nursery stock is moved from an infected area to one that is free of the disease". The practice of marcotting or air layering continues to this day in Thailand. Su and Tolley (18) evaluated the current status of citrus production in Thailand and reported that: "Thailand is the only large citrus producing country in the world using marcotting or air layering to propagate citrus trees. The present technique for all practical purposes guarantees production of nursery trees already infected with greening. 280 Thirteenth ZOCV Conference, 1996Procaryotes and Blight This technique must be discarded entirely for citrus propagation". They indicated that there is a continuing deterioration of the Thai citrus industry, primarily due to HLB and that the average life span of a citrus tree in the field was only 8 yr. They suggested that there was a lack of trained personnel to handle the difficult problems associated with the disease and indicated that there was an urgent need to develop a clean stock program and establish a citrus certification program for Thailand. Grenzebach (8) studied the economics of producing citrus in Thailand and calculated the financial returns from an average citrus farm. His analysis showed a 12-yr cycle of production with most citrus declining to a point of non-profitability after 8 vr as shown in Fig. 1. Roistacher "(14) observed tr;es near death from HLB in many citrus groves in the Phrae and Nan areas which were only 5 yr of age or less. The disease destroys about 10 to 15% of the tangerine trees each year in Thailand and is exceptionally destructive in the northern region were many areas have gone out of production. Considering the significant financial losses suffered by the growers and the country from this devastating disease, relatively little has been done to educate on the causes of decline or to develop a strategy for its prevention and control. VECTOR CONTROL FOR HUANGLONGBING IN THAILAND The fact that this disease has been destroying trees in Thailand for over 30 yr, or that trees decline after 5 to 8 yr, and that growers continue to plant new orchards with infected marcotted trees suggests that there is a lack of understanding by the farmers of the HLB organism and its mode of spread by its psyllid vector, Diaphorina citri Kuwayama. Much RETURNS IN $/HA PER YEAR 12 YEARS OF LIFE EXPECTANCY BASED ON GRENZEBACH (1994)