Penicillium expansum is by far the most important decay pathogen on apples and has developed resistance to several postharvest fungicides. In the absence of effective controls, spore loads for this pathogen can gradually increase in apple packinghouses and on apple bins. High inoculum densities result in increased losses to decay both during storage and in packed fruit that is sent to retail markets. In most packinghouses, postharvest treatments containing diphenylamine plus thiabendazole (Mertect) still control Botrytis, but these recycling drenches redistribute spores of P. expansum. As a result, postharvest drenching may cause increased problems with Penicillium decay whereas omission of drenching sometimes results in losses to Botrytis and/or losses to carbon dioxide injury which is also controlled by DPA treatment.
Until the 1990’s, P. expansum was considered primarily a wound pathogen and inoculum was assumed to originate in the orchard each fall. During the late 1990’s, however, we discovered that P. expansum can invade non-wounded Empire apples through the fruit stem during CA storage. Tests conducted in autumn of both 1998 and 1999 using fruit from six different orchards in western NY showed that differences in boron content of fruit and leaves were significantly correlated with susceptibility to decay. Fruit with high boron concentrations (greater than 35-40 ppm) developed more decay than fruit with lower boron levels when stems were uniformly inoculated with conidia of P. expansum immediately after harvest and were then held in long-term CA storage.
Several sources of inoculum for postharvest decays have been recognized, but the relative importance of each source presumably varies with geographic location, time of year, and management factors unique to each farm and packinghouse operation. Spores can originate in the orchard, from storage bins, from contaminated storage rooms, and in the packinghouse. Research in New York has shown that Penicillium spores that recycle from year to year on storage bins and in storage rooms are far more important to the disease cycle than are spores that originate in the field.
Penicillium spores on dry surfaces can remain viable for several years, so P. expansum can recycle from year to year in contaminated bins, storages, and packinghouses. Recycling of Penicillium in packinghouses contributes to fungicide resistance problems because strains that recirculate from year to year within storages are repeatedly exposed to postharvest treatments and to selection pressure for resistance to the fungicides.
The diagram below illustrates the various inoculum sources and cycling that can occur with P. expansum in apple storages.
Links to more information in Scaffolds Fruit Journal
2005 New Fungicide Options for Postharvest Decay Control (download) /(web)
Last updated 31 August 2014
