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The incidence of maize stunt diseases, or achaparramiento , rose in Nicaragua from an insignificant level to national crisis proportions, then declined to insignificant levels again, all in the space of 10 yr. The 3 pathogens that cause achaparramiento, corn stunt spiroplasma, maize bushy stunt phytoplasma, and maize ray ado fino virus are transmitted by the corn leafhopper, Dalbulus maidis (De- Long & Wolcott). Vector populations did not change sufficiently over time to explain the epidemic pattern. Nor did changes in the varieties of maize that farmers used cause the differential expression of disease symptoms during this period. The most likely explanation of the epidemic pattern is an abrupt 3- to 4-yr increase in irrigated maize production during the dry season, when maize was not grown traditionally. Irrigated maize provided a temporal bridge for the pathogens, which then were transmitted to rainy season …