Evidence of cork barrier formation as a resistance mechanism to berry disease (Colletotrichum coffeanum) in arabica coffee

The histology of resistance to coffee berry disease (CBD) was studied in artificially inoculated berries and hypocotyls of 6-week old seedlings of a number of varieties ofCoffea arabica L. In resistant varieties a phellogen was quickly formed some cell layers below the site of infection and progress of the fungal invasion was effectively blocked by a complete barrier of suberized cells. Such barriers were absent or incompletely developed in CBD susceptible varieties. A highly significant correlation (r=0.87) was found between the frequencies of complete barrier formation in berries and in hypocotyls of young seedlings, while both were also highly correlated to observed mature plant resistance (r>-0.93). Resistance to CBD in arabica coffee may to an important extent be based on the formation of cork barriers. These cork barriers confine the pathogen to the small volume of tissue external to the barrier so that its growth is severely restricted. Such a resistance mechanism is likely to be stable (race-nonspecific). The almost identical response to infection observed in berries and hypocotyls provides further evidence that plants with resistance to CBD can be reliably preselected by the hypocotyl inoculation test.