ABSTRACT Lethal yellowing (LY), a disease caused by a phytoplasma, is the most devastating disease affecting coconut (Cocos nucifera) in Mexico. Thousands of coconut palm trees have died on the Yucatan peninsula while plantations in Central America and on the Pacific coast of Mexico are severely threatened. Polymerase chain reaction assays enable identification of incubating palm trees (stage 0+, phytoplasma detected but palm asymptomatic). With the development of LY, palm trees exhibit various visual symptoms such as premature nut fall (stage 1), inflorescence necrosis (stages 2 to 3), leaf chlorosis and senescence (stages 4 to 6), and finally palm death. However, physiological changes occur in the leaves and roots prior to onset of visual symptoms. Stomatal conductance, photosynthesis, and root respiration decreased in stages 0+ to 6. The number of active photosystem II (PSII) reaction centers decreased during stage 2, but maximum quantum use efficiency of PSII remained similar until stage 3 before declining. Sugar and starch concentrations in intermediate leaves (leaf 14) and upper leaves (leaf 4) increased from stage 0- (healthy) to stages 2 to 4, while root carbohydrate concentrations decreased rapidly from stage 0- to stage 0+ (incubating phytoplasma). Although photosynthetic rates and root carbohydrate concentrations decreased, leaf carbohydrate concentrations increased, suggesting inhibition of sugar transport in the phloem leading to stress in sink tissues and development of visual symptoms of LY.
