The infection capacity of P. expansum and P. digitatum on apples and histochemical analysis of host response

Fruit ripening is a complex process that involves a variety of biochemical changes and is also associated with increased susceptibility to pathogens. The present study determined the effects of fruit maturity and storage conditions on the infection capacity of a host (P. expansum) and non-host (P. digitatum) pathogen on apple. A range of inoculum concentrations and two different storage temperatures were utilized. Exposure to P. expansum at 20 degrees C resulted in significant differences in rot dynamics in apples collected at the earliest harvest date compared to all later harvest dates and inoculum concentrations assayed. Greater differences in infection capacity between harvests were obtained when fruit was stored at low temperature (0 degrees C). In contrast, P. digitatum was able to infect apples only under specific conditions and disease symptoms were limited to the initial wound inoculation site. When apples were resistant to P. digitatum, a visible browning reaction around the infection site was observed. Histochemical analyses of tissues surrounding the wound site were conducted. A positive reaction for lignin was observed in immature apples as early as 1 day after inoculation with either pathogen. Experiments conducted with the non-host pathogen indicated that lignification was an essential component of resistance in apples harvested prior to maturity or at commercial maturity. Apples harvested at an over-mature stage and inoculated with P. digitatum did not show evidence of staining for lignin until 7 days post-inoculation. Control samples only showed positive reaction in immature harvest. Results demonstrated that the maturity stage of fruit is an important factor in apple resistance to both P. expansum and P. digitatum and that lignin accumulation seems to play an important role when resistance is observed. Moreover, this is the first report demonstrating that P. digitatum, a non-host pathogen, has a limited capacity to infect apples. (c) 2012 Elsevier B.V. All rights reserved.