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Disease Cycle and Epidemiology

Experimental evidence implicates the planthopper Myndus crudus as a vector of the lethal yellowing phytoplasma (Figure 14). The planthopper is an insect with piercing and sucking mouthparts, and feeds on the contents of the plant host vascular system. The insect spreads the phytoplasma during feeding activity as it moves from palm to palm. The phytoplasma is not known to survive outside either its plant or insect hosts. The geographic range of lethal yellowing is limited in the United States to the subtropical southern third of Florida because the planthopper is not considered cold hardy.

Figure 14

Inoculation of a susceptible plant initiates infection that is followed by a prolonged latent (incubation) phase estimated between 112 to 262 days. About 80 days prior to symptom appearance, the growth of infected palms is stimulated. This is followed by a period of gradual decline, and growth ceases about 1 month before the end of the incubation phase.

Following an initial disease outbreak, further spread of lethal yellowing is characterized by a “jump-spread” pattern, indicating dissemination involving an airborne vector. Spread occurs among susceptible palms within a localized area, resulting in a random pattern around an active focus of disease (Figure 15) that eventually claims most susceptible palms within the locality (Figure 16). Beyond this primary focus, further spread may occur in jumps of a few to 100 km or more, thus establishing new disease foci. Differences in the rates of spread of lethal yellowing at different geographical locations have also been noted. In Florida (United States), spread of the disease from the cities of Miami to Palm Beach, a distance of about 128 km, occurred within 3 years. In Jamaica, however, movement of the disease from the west to the east end of the island, a distance of approximately 238 km, took about 60 years.

Figure 15

Figure 16

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