Diseases of maize in the wet lowland tropics and the collapse of the Classic Maya civilization

Abstract

The Classic Maya civilization was centered in lowlands of the Petén in northern Guatemala, and collapsed mysteriously in the ninth century AD. Abandoned were rich agricultural lands carved without metal tools out of a tropical rain forest, lands that had been farmed with increasing intensity for six to sixteen centuries. The Maya evidently resettled in highlands to the south or in less productive dry lowlands to the north. No reoccupation occurred of the Petén farms, homes or ceremonial centers until their discovery in the past two centuries. Sustained crop failure of maize (Zen mavs L.) due to an epidemic of the planthopper-borne virus, maize mosaic virus (MMV), is proposed as a primary contributing cause of the collapse. Major diseases and pests of maize in the tropics are assessed for their relative significance in and near the Petén vs. the highlands, and the viruses are highlighted. Maize mosaic virus is a devastating virus disease transmitted by the corn planthopper, Peregrinus maidis, an insect restricted to tropic lowlands. Maize and teosinte are its only definitively known hosts. Thus the disease has been serious only where maize is grown more-or-less continuously through the year in wet or irrigated tropics (e.g., Caribbean Islands, Venezuela, Hawaii, Tanzania, Australia). It is reported here for southern Mexico and the Petén of Guatemala. Resistance in maize occurs only in one known form, the gene Mv. that confers a high level resistance but not immunity. Resistance data are presented for 63 of the 67 races of maize thought to have evolved in the Northern Hemisphere. The Mv gene is shown to occur in all seven of the races of maize evolved in the Caribbean, but in none of the primitive Mexican or Central American races. It is proposed that maize mosaic virus originated in northern South America at or about the time maize was brought into the Caribbean by the Arawak around the time of Christ. The sympatric origin or selection in maize of the Mv resistance mutant in this region is assumed to have led to its incorporation in all seven Caribbean maize races. It is conjectured that viruliferous leafhoppers were blown from the Caribbean into the Petén around the eighth century allowing the disease to become epidemic in susceptible maize races such as Nal-Tel and Tepecintle, grown by the Petén Maya. Sustained failure of maize production due to MMV would have characterized areas of intensive maize cultivation, particularly where it was year-round. The disease would have been less severe in areas with a long dry season, as to the north of Yucatán and it would not have occurred in the highland areas to the south and west, areas to which surviving Maya presumably migrated. © 1980 New York Botanical Garden, Bronx, NY 10458.