Shade tree Chloroleucon eurycyclum promotes coffee leaf rust by reducing uredospore wash-off by rain

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anglais. Shade trees stimulate many pathways that influence disease development in opposite directions, and that, inaddition, may interact with environment. To better understand shade trees effects on coffee leaf rust (CLR), westudied three disease stages separately: sporulation, uredospore wash-off by rain, and uredospore deposition onleaves. The study was conducted during almost one year in the long-term trial on coffee-based agroforestrysystems established by CATIE in 2000, in Turrialba, a low altitude area of Costa Rica. We only used the Full Sunand Shade provided by Chloroleucon eurycyclum treatments. For studying sporulation, we harvested diseasedleaves every three weeks and collected the uredospores present on the lesions. For assessments of uredosporewash-off, we located containers at ground level below the coffee bushes and in the interval between rows ofcoffee bushes, and removed them after rainfall events (43 rainfall events studied) to count the number of uredosporescollected. For uredospore deposition, we used varnish to capture deposited uredospores on apparentlyhealthy coffee bush leaves (55 dates). We also studied the raindrop kinetic energy by using splashcups, on 19rainy days. The number of uredospores produced and preserved was 2.22 times higher below shade trees than infull sun, whereas the number of uredospores lost by wash-off, below the coffee bush, was 1.62 times lower.Reduced wash-off was probably due to raindrop interception by shade trees and stemflow and to the increasedkinetic energy of the raindrops in the understory of the Shade treatment (twice as high as that measured in fullsun), which reduced the capacity of coffee leaves to intercept raindrops. In addition, we found 1.43 times moreuredospores deposited on apparently healthy leaves below shade trees than in full sun, partly due to the highernumber of uredospores produced and preserved below shade trees. Increasing throughfall and reducing raindropkinetic energy below shade trees seem crucial to improved CLR regulation. This can be achieved by selectingspecific shade tree functional traits and by implementing shade pruning during the rainy season.