Survival of Mucor piriformis in Soil of Apple Orchards in California

Four apple orchards were sampled periodically in 1995 and 1996 to determine the population levels of Mucor piriformis, the causal agent of Mucor rot. The highest population, 119 propagules of M. piriformis per g of dry soil, occurred during winter. Populations declined during summer and fall and increased again in winter and early spring of the following year. Isolates from soil and apple fruit of these orchards contained +, -, and neuter mating types, with the + being the predominant mating type. Depending on the orchard, up to 27.7% of apples infected by M. piriformis had zygospores developed on them. The time of increase in M. piriformis populations corresponded with postharvest drop and decay of apples on the orchard floor. Experiments were carried out to compare changes in M. piriformis populations in soil with intact fruit left on the orchard floor, sliced fruit, or after removal of fruit from the orchard. The greatest numbers of M. piriformis occurred in soil with apple pieces, followed by soil with intact apples, and the lowest in soil without apples. Results suggest that the best time to sample soils to determine the occurrence of M. piriformis in California apple orchards is from January to March, and practices of flail-mowing the orchard floor after harvest may increase population levels of M. piriformis in soil.