May 2013 C&CI • Disease & Pest Control • 39
No single solution to coffee leaf rust outbreak
2012. CIRAD, the French agricultural research organization estimates that it has caused serious losses in yields in 2012/2013 (almost 20 per cent), and says the damage to coffee trees could affect harvests up to 2016. “The total financial losses resulting from the 2012/2013 epidemic look set to reach several hundred million Dollars, with extremely serious social consequences in the agricultural sector,” said CIRAD in a recent statement on the issue.
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Coffee leaf rust is caused by a fungus, Hemileia vastatrix. It attacks the leaves of coffee trees and can cause substantial defoliation. Its history is easy to trace: in 1869, it wiped out coffee growing in Ceylon (now Sri Lanka). Since its introduction into Central America in 1976, the fungus has caused severe epidemics in Costa Rica in 1989, Nicaragua in 1995 and Colombia from 2008 to 2011. In 2012, the epidemic reached regional proportions, severely affecting every country in Central America and also South America, including Peru.
Several hypotheses As CIRAD noted, hypotheses relating
to climate change, crop management, and even the evolution of a new ‘race’ of the pathogen have been put forward to explain the intensity of the 2012 epidemic. According to CIRAD, rainfall levels in the region may have been of a level that prevented the spores being washed away but were sufficient for their germination. The hot weather in 2012 shortened the disease’s ‘latency period’ and significantly increased the intensity of the epidemic, it said. It also fostered attacks in high- altitude zones, which are generally assumed not to favour the disease due to their usually low temperatures. In such zones, growers do not generally apply preventive treatments against leaf rust. Treatments were thus launched relatively late in 2012, often not until the damage was already irreversible.
Scientists at CIRAD believe the wind also played a major role, spreading leaf rust over large distances. The constant
supply of this inoculum from outside growing areas made management of the epidemic even more complicated. CIRAD says it also believes that crop management techniques, such as the trend towards growing coffee in full sunlight, rather than in shade, in order to enhance yields, may have also played a
Scientists at CIRAD believe the wind also played a major role, spreading leaf rust over large distances. The constant supply of this inoculum from outside growing areas made management of the epidemic even more complicated
veritable epidemic of coffee leaf rust has been spreading through Central America since
Opinions differ about the cause of the coffee leaf rust (roya) outbreak that is so badly affecting coffee farmers in Central America, but a consensus has emerged that climate change probably had a large role to play
Coffee leaf rust on a coffee tree in western Bolivia (photo: Neil Palmer, CIAT)
part. Joint studies by CIRAD and CATIE in Costa Rica have shown that shade serves to reduce leaf rust attacks, since it regulates the number of cherries a plant produces. It also intercepts dew, a source of water that fosters spore germination, and helps protect soil against acidification, a condition that has been shown to foster attacks. “Some producers were unable to fertilize their plots at the end of the year due to the prevailing dry conditions, and leaf rust more severely affected these insufficiently fertilized plots,” said CIRAD. “Excessive delays in applying fungicides also had an adverse effect,” and as it also noted, the overall context of falling prices did not encourage producers to take preventive measures against leaf rust, and in any event, nobody expected the disease to be so serious.
CIRAD says it believes that the possibility that the pathogen has evolved cannot be ruled out (although other experts are sceptical about this). CIRAD says it believes that climatic conditions and crop management may not be sufficient to account for the level of the outbreak, and says it is possible that some strains may have become more aggressive or adapted to more extreme climatic conditions. “Work by CIRAD and its partners since the 1990s has shown that epidemic intensity is linked to high fruit loads, which make coffee tree leaves more sensitive to leaf rust,” it said. “The process can be corrected through fertilization, which ensures good growth and thus helps dilute the disease by steadily bringing in new, healthy leaves. This prevents defoliation and premature branch death. “This research also demonstrated the importance of nutritional factors in general, fruit loads, fertilization, and also soil quality,” CIRAD said. “Plots with highly acid soils are more severely attacked than others.” Jacques Avelino, a researcher at CIRAD,
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