City streets harbor hosts for the dreaded roya de cafe rust

As coffee producers and the government seek to stem an epidemic of coffee rust, there is a fifth column that can harbor the disease.

Coffee rust fungus has swept through plantations in Central America and Mexico and caused serious problems for producers. Coffee rust, known as roya de cafe (Hemileia vastatrix),  is the worst seen in Central America and Mexico since the fungal disease arrived in the region more than 40 years ago. Guatemala has joined Honduras and Costa Rica in declaring national emergencies over the disease.

Lawmakers have appropriated money for sprays and other responses to the disease.

But on the streets of San José and other Costa Rican communities the coffee plants have been placed as ornaments. Some of these exhibit symptoms of the rust, which is spread by the wind. No matter how many times growers spray their plants, the disease remains untouched on the ornamental hosts.

There is not a lot of incentive for city workers to do anything about the situation. Their salary does not depend on the coffee the plants produce.

John Vandermeer, a University of Michigan ecologist, said earlier this year that over the last 20 to 25 years, many Latin American coffee farmers have abandoned traditional shade-growing techniques in which the plants are grown beneath a diverse canopy of trees. In an effort to increase production, much of the acreage has been converted to sun coffee, which involves thinning or removing the canopy and a greater reliance on pesticides and fungicides to keep pests in check.

Vandermeer said he suspects that the shift to sun coffee may be contributing to the severity of the latest coffee rust outbreak. The move to sun coffee results in a gradual breakdown of the complex ecological web found on shade plantations. One element of that web is the white halo fungus (Verticillium lecanii), which attacks insects and also helps keep coffee rust fungus in check. The ornamental plants on city streets are almost always in the sun. This may support Vandermeer’s assessment.

There is, however, an upside. Some ornamental plants show no sign of the disease even though they are in close proximity to heavily diseased bushes.

These may provide some clues as scientists seek out resistant variations of the coffee plant.

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