The coffee leaf rust epidemic in Central America that began in 2012 took the entire industry by surprise. There were many unusual factors to the epidemic that left farmers, agronomists, and researchers scrambling for answers.
Typically, the amount of fruit on a coffee plant can explain 50% of the coffee rust variability (when a plant produces a lot of fruits, it stresses the plant and it is more susceptible to rust). However, during the 2012-13 epidemic, even young plants with no fruit or immature plants in nurseries were severely affected. Similarly, contrary to expectations, coffee rust did not decrease with increasing altitude. In Guatemala, for example, severity of rust attack was equally high in farms ranging from as low as 400 to 1400 m. (This situation is consistent with increased temperatures at higher altitudes.)
Not all farmers were affected equally. Two factors in particular seemed to have an effect on the severity of rust attacks, even on farms right next door to one another: fertilization and shade. Farmers across the region who applied appropriate fertilizers were less affected by rust. This makes sense—a well-fed plant is likely to be more resilient to stressors like disease. But the effects (both direct and indirect) of plant nutrition on both the severity of rust infection and the capacity of the coffee plant to recover from rust damage, are not well understood.
In addition, shade is also known to affect coffee rust development, though shade can have antagonistic effects. During the recent epidemic, rust infestation was similar in plots under shade or at full sun exposure (in spite of the lower fruit load normally reached under shade). However, the amount of defoliation and dead branches was higher at full sun than under shade. Coffee plants at full sun were particularly stressed (low rainfall and high temperatures) during the epidemic in 2012.
A combination of shade and fertilization seemed to be the basis of successful management of coffee rust epidemic that began during 2012-13. But defining the specific approaches that worked, and understanding why they worked, is critical for providing accurate guidance to coffee producers in the future.
For example, different heights of shade and leaf shapes could affect how water drips down to coffee plants underneath, either helping or hindering the spread of leaf rust spores. By studying different kinds of shade, we can identify the shade attributes that are most relevant for fighting coffee rust.
WCR is studying these complex interactions—between shade, fertilization and rust—on 48 plots in Acatenango, Guatemala, a region severely hit by coffee rust in 2011-12 and at the International Tropical Research Center (CATIE) in Costa Rica. The plots differ by type of shade (Gravilea, Inga, and diversified), the intensity of cropping practices and levels of fertilization (high, low), and by altitude (lower: 1250-1500 m, higher: 1500-1750 m).
A systemic approach to fighting disease—involving environmental, agronomic, and chemical controls—will almost always be more effective and more sustainable than single, silver-bullet solutions. The results of this research will assist in decreasing the substantial confusion caused by the leaf rust epidemic in Central America and lead to improved training for farmers and help refine best practices for managing rust.