Quantitative detection of Colletotrichum godetiae and C. acutatum sensu stricto in the phyllosphere and carposphere of olive during four phenological phases

A duplex qPCR detection method was developed to detect and quantify Colletotrichum godetiae and C. acutatum sensu stricto (s.s.) in olive tissues. The method proved highly specific and sensitive with a detection limit of 10 pg for each pathogen. The analysis of green and senescent leaves, fertilized fruitlets with floral residues, green fruit and symptomatic and asymptomatic fruit collected in May, June, October and December revealed a high incidence of both C. godetiae and C. acutatum s.s. in Calabria, southern Italy. In comparison with previous reports, these results highlighted an ongoing population shift from C. godetiae to C. acutatum s.s. Interestingly, C. godetiae was slightly more abundant in terms of number of infected samples, yet the quantity of C. acutatum in infected samples was always higher, suggesting greater aggressiveness and/or sporulation ability of the latter pathogen. The populations of both C. godetiae and C. acutatum s.s. increased sharply in December even though both pathogens were detected widely in asymptomatic samples in May, June and October, confirming an important role of latent infections in the disease cycle. A large quantity of both C. godetiae (1.7 × 108 cells/mg of tissue) and C. acutatum s.s. (7.5 × 108 cells/mg of tissue) was estimated in symptomatic fruit, presenting an enormous inoculum potential for secondary infections. Two other important observations were a high incidence and quantity of both pathogens in senescent leaves and in fertilized fruitlets with floral residues as compared to green leaves.