COS 121-6 - Inferring the transmission dynamics of the coffee leaf rust (Hemeleia vastatrix) from spatiotemporal infection patterns in a long-term monitoring project

Background/Question/Methods

Coffee leaf rust (Hemeleia vastatrix) is an economically significant agricultural fungal plant pathogen that has had outbreaks worldwide. Transmission of coffee leaf rust urediniospores can occur over regional and local scales through wind, water splash, and direct contact. Changing environmental and management conditions influence these transmission modes and could lead to surprising shifts in longstanding dynamics, or critical transitions. Understanding the predominant transmission processes within a farm can therefore help predict outbreak dynamics and develop management interventions. The environmental requirements for dispersal and infection suggest that different transmission modes could manifest as characteristic spatial patterns on a plant and within a farm. We examined the spatiotemporal trends of the initiation of seasonal infections of 640 coffee plants that were monitored monthly in a 45-ha plot in southern Mexico over a 36-month period following a regionally devastating outbreak in 2013. We hypothesized that long-distance wind transmission would be evident as infections starting in the upper part of the plant and distributed throughout the farm as a random spatial pattern. In contrast, we expected locally transmitted infections from rain splash or contact would initiate lower on the plant and would be spatially clustered and associated with existing infections on the farm scale.

Results/Conclusions

We find that infection rates were higher in the bottom and middle thirds of coffee plants, suggesting that water splash plays an important role in germination. Rainfall in the previous month increased infection rates in all parts of the plant; but the middle stratum was the most positively affected by high rainfall. Infection probability in the middle of the plant exceeded that of the bottom and top strata following heavy rainfall, perhaps reflecting the effects of higher splash reach under these conditions. To contextualize our results, we present a conceptual model of how increasing rainfall redistributes fungal spores on the coffee plant through wash-off and splash, and initiates spore germination leading to infection. At a farm scale, local transmission appears to play an important role, while evidence for long-distance transmission is less clear. Spatial patterns appear to differ seasonally, which may suggest different modes of transmission that reflected seasonal weather patterns and cycles of local infection intensity. These dynamics suggest that distributed replanting of susceptible coffee plants with resistant varieties, an intervention already underway, may be critical to interrupting seasonal outbreaks.