Fruit maturity and post-harvest environmental conditions influence the pre-penetration stages of Monilinia infections in peaches

Brown rot caused by the fungi Monilinia laxa (Aderhold and Ruhland) Honey, M. fructicola (Winter) Honey, or M. fructigena (Aderhold and Ruhland) is a serious fungal disease of peaches. The fungal infection process begins when fungal conidia germinate on the fruit surface to produce germ tubes and/or appressoria, and the incidence of brown rot increases as fruit approaches maturity. The interaction between the fungal infection process, peach maturity, and the environmental conditions is not well understood. Accordingly, the objectives of this investigation were to investigate germ tube and appressorial formation by M. laxa and M. fructicola when they were exposed to peach skin from mature and immature fruit at various temperatures and relative humidities (RHs). The greatest number of germ tubes was found when M. laxa or M. fructicola was incubated in culture medium which contained a skin extract of mature peaches. In contrast, the greatest number of appressoria was found when M. laxa or M. fructicola was incubated in culture medium which contained a skin extract of immature peaches. Although M. fructicola produced the same number of germ tubes and appressoria at 4 °C, M. fructicola produced more germ tubes than appressoria at temperatures higher than 10 °C. M. laxa produced more germ tubes than appressoria at any temperature, except when it was incubated for 48 h on culture medium which contained a skin extract of immature peaches at 10 °C at 80% or 100% RH, or at 25 °C at 60% RH. M. laxa conidia germinated better than M. fructicola conidia at low temperatures. Germ tube and appressorial formation by Monilinia spp. were influenced by fruit postharvest handling. The number of germ tubes that were formed by M. laxa conidia was significantly greater than that for M. fructicola when the conidia were incubated at 100% RH, and this number increased after 3 days of refrigeration. The number of appressoria that were formed by both Monilinia spp. also increased after 3 consecutive days of refrigeration. Negligible or no germination of M. fructicola and M. laxa conidia occurred when the RH was 60%. We concluded that the dissimilar abilities of M. laxa and M. fructicola to germinate and form appressoria at low temperatures conferred a competitive advantage to M. laxa to survive during fruit postharvest refrigeration and cold storage at 4 °C.