A Perspective on Leaf Rust

by Brew Impact on Feb 06, 2020

A Perspective on Leaf Rust

While in Guatemala last year, we spoke at length with Ricardo Zelaya on how he was managing Roya two years on, and some of the methods he uses to combat it.

When leaf rust, or Roya, broke out in Central America in 2013 its effects were devastating on yields and quality. Farmers through El Salvador and Guatemala had to drastically change their farming methods to combat this new fungus, which was more destructive than any fungus they had encountered before. The fungus eats away at the leaves, and draws nutrients away from the fruit production, affecting fruit quality and quantity. The affect of Roya on the taste of the coffee is unique, we tasted samples of coffee that were completely ripe-looking, but the coffee tasted as though the fruit and seed was unripe - hollow, thin and peanut-like.

Currently Roya is being managed reasonably well in Antigua, some farms are managing it better than others. Although more recently it has seemed to be manageable, it certainly is clear that Roya is in Guatemala to stay - there seems to be no chance of eliminating it completely. The growth of the fungus also seems to come in waves, depending on the weather. In 2013 during March, April and May, they say almost no growth of Roya and no new growth in Roya. In June 2013 Ricardo trialled a new application of systemic fungicide - one product that contained both a short term and a long term fungicide. The application of this is done via a dry product that is applied to the ground around the tree, then when it rains this product is carried down to the trees roots, and then into the tree. The benefit of this type of application is that it is applied to the root system, and so the fungicide will be carried through the plant and protect the entire plant. More specific and simpler fungicide applications, like spraying, will only protect the actual leaves or stem that the product is applied to. Ricardo's application of fungicide this year will be one application to the roots, and then three rounds of spraying. To get the same level of fungus control with spraying alone, approximately six passes of spraying are required. The downside of the root application of this fungicide is that it takes about 45 days to begin to work, but it will be more effective and more cost efficient than spraying.

(The trees shown above have been effected by leaf rust. This is identifiable by the spindly, bare branches that are missing the lush green leaves of a healthy coffee shrub.)

Ricardo suspects climate change is playing a big part in the spread of Roya throughout Central America. He bases this on a couple of changes since 2004; he has seen that the sun has been more intense in recent years, apparently giving off more radiation that in years past. He has also seen an increase in the range of temperature, from the hottest days to the coolest nights recorded at the farm during the rainy season. The Roya is triggered by this fluctuation between low and high temperatures. In the past when the maximum and minimum were more similar, the Roya would not have been as reactive as it is now. The difference in temperature stimulates the growth of Roya. The humidity does not seem to play a big part in the proliferation of Roya, not so much as the changes in temperature though. In the past during the rainy season, the lows at night would get to 16C, but now there have been nights getting to lows of 13C or 12C in July or August. Along with the differences in temperature ranges, Ricardo has also noticed an increase in rainfall over fewer days in Guatemala. So the volume of water that is falling has increased, but the number of instances has fallen. In the past a “big rain” was 25mm of rain, and now a “big rain” is more like 45mm or 50mm of rain.

Managing Roya has become part of the routine maintenance for Ricardo Zelaya, the hope that Roya will fade away is gone.