Coffee leaf rust (Hemileia vastatrix) is a fungus that is widely feared among coffee growers. Its full life cycle is not completely understood, but direct contact must be made by the fungus, or the fungi spores, with the leaf to infect it. Scientists also know that the presence of free water is required, and a temperature between 15 and 30 degrees Centigrade. The rust attacks the stomata cells of the plant – those cells that help the plant “breath” – and leaves a powdery, yellowish-orange dust. The fungus attacks the youngest leaves of the coffee plant, sometimes the fruit, and very rarely the older leaves. Physiologically, the rust impairs the plant’s ability to photosynthesize (and therefore produce energy to develop coffee berries), as well as causes premature defoliation (loss of leaves) and reduced flower production (flowers = coffee fruit!). Additionally, new growth is severely limited – due to lack of energy – and the new branches are what will be next year’s coffee crop.
The Salvadoran Foundation for Coffee Research recently estimated that coffee leaf rust is plaguing between 30 and 50% of the coffee plantations in the country. Of those, the majority are seeing well over 10% of their crops damaged by the pathogen. They are estimating minor losses in coffee harvest this year, but a full 28% lower harvest for the 2013-2014 season. The many coffee cooperatives are asking the Ministry of Agriculture & Livestock (MAG, for its Spanish name) to look into this issue and take action; coffee is a cash crop of the country and they feel the government needs to lend a hand in overcoming this plague. At the end of last week, the Salvadoran Coffee Counsel and the MAG completed a comprehensive study of 215 coffee farms in six different zones of the country. Their estimate was that the coffee rust is affecting 10% of the coffee in the country, but on those farms that sprayed fungicide, their infection rate averaged at 2%. They plan on conducting a follow-up study to determine if the rust is spreading, as well as obtaining a count on the acreage affected.
Controlling this rust from spreading is not an easy feat; complete eradication is widely deemed impossible, but controlling a catastrophe is within reach. Fungicides are only somewhat effective; copper oxychloride – a chemical-fungicide mixture – is the most widely used and most effective. In places such as El Salvador that have long dry seasons, the non-chemical solution of removing affected leaves should be used extensively. Without water, the spores cannot spread, and so the extent of the fungus can be severely limited if the effort is made during the dry season. Full removal of the affected leaves from the plantations is necessary. The MAG says they are negotiating the import of copper sulfate and slaked lime as treatments – a common fungicide used to combat a variety of leaf rusts. Other methods of fighting the spread of the rust is to remove shade trees (a bad blow for the birds and other biodiversity), or to plant resistant cultivars (but “resistant” does not mean immune, so the fungus is still there, just spreading much more slowly). However, over time, the extent of rust damage between coffee farms that cut their shade trees, plant resistant cultivars, and use fungicide compared to those that do not take those actions is minimal. One future possibility involves biological control: there are at least two different species of fungus that act parasitically on H. vastatrix. One fungus species, (Lecanicillium lecanii), seems to work mutually with an ant (Azteca instabilis) to control the spread of the coffee leaf rust. This biological control is only in trial phase, and many more hurdles need to be overcome before it is a practical and common solution to the leaf rust problem. For now, coffee farmers in El Salvador can take comfort in knowing that the dry season is just around the corner, and along with the rain, so too the leaf rust’s damage wanes.