The influence of citrus orchard age on the ecology of entomopathogenic fungi and nematodes

A three-year survey of the ecology of entomopathogenic nematodes (EPN) and entomopathogenic fungi (EPF) was undertaken on soils from citrus orchards of different ages to determine the influence of orchard age on the ecology of entomopathogenic fungi and nematodes. The influence of mulch and irrigation method on the occurrence of EPN and EPF was also determined. Most of the isolates recovered (n = 810) were Beauveria sp. (87.88% of all isolates), followed by Metarhizium sp. (11.87% of all isolates). Only 0.24% of soil samples collected during this study tested positive for EPN. All EPN isolates recovered were Heterorhabditis bacteriophora. No significant differences in EPF occurrence were recorded between orchards under drip and micro-sprinkler irrigation. EPF occurrence was significantly lower (P = 0.016) in orchards covered by mulch (31.85% +/- 2.07% occurrence) than in orchards with no covering (38.57% +/- 1.57% occurrence). EPF occurrence of 40.33 +/- 2.13% was highest in non-bearing orchards, followed by mature orchards (nine years or older) (36.76 +/- 2.05% of samples) with the lowest EPF occurrence of 25.30 +/- 2.02% reported in juvenile orchards (four to eight years old). Juvenile orchards sustain significantly less EPF than mature and non-bearing orchards because of the combined negative impact of less favourable environmental conditions (lower shade density) and fungicide applications.