Delineation of the three brown rot fungi of fruit crops (Monilinia spp.) on the basis of quantitative characteristics

The three Monilinia spp., known as the brown rot fungi of fruit crops, are usually distinguished from each other on the basis of (qualitative) colony characteristics. We linked these qualitative features to unambiguously defi ned, quantitative colony and germ tube characteristics. A wide collection o f isolates of Monilinia fructicola (Winter) Honey, Monilinia laxa (Aderhold & Ruhland) Honey, and Monilinia fructigena (Aderhold & Ruhland) Honey was used to determine growth rate and sporulation intensity on potato dextrose agar (PDA) at 22 degrees C under two light regimes (darkness, 12 h light : 12 h dark). The following germ tube characteristics were determined on wate r agar after incubation for 18 h at 22 degrees C in darkness: length of the (leading) germ tube, distance to the first branch, and the number of germ tubes per conidium. Increase in colony diameter from day 3 to day 5 and spo rulation intensity measured after 14 days was the highest in M. fructicola, whilst M. laxa and M. fructigena showed considerable overlap in these feat ures. The length of the germ tube after 18 h incubation was shortest in M. laxa, ranging from 161 to 466 mu m, fn M. fructicola and M. fructigena thes e ranges were 465-851 and 307-806 mu m, respectively. The occurrence of mor e than one germ tube per conidium was most prominent in M. fructigena. Disc riminant analysis on the basis of different combinations of the quantitativ e characteristics measured, showed that the combination of growth rate on P DA and length of the germ tube was sufficient to delineate the three brown rot fungi. One of 11 M. fructicola isolates was misclassified, the same hel d for M. fructigena (one misclassification of nine isolates). No misclassif ications occurred in M. laxa.