Validation of the Lipbalm Tube Assay for Evaluation of Fungicide Sensitivity in Field Isolates of Monilinia fructicola

Abstract

Simple and rapid techniques for monitoring fungicide sensitivity of pathogen populations can be helpful to optimize spray programs. In this study, a lipbalm tube assay developed previously in our laboratory was investigated for shelf life duration, ability to differentiate M. fructicola from fungal contaminants, and ability to accurately determine the sensitivity offield isolates of Monilinia fructicola to sterol demethylation inhibitor (DMI), benzimidazole (BZI), and quinone outside inhibitor (QoI) fungicides. The sensitivity of isolates with different DMI, BZI, and QoI sensitivity phenotypes to the above-mentioned fungicide classes was not altered on agar disks sliced from 30-day-old, fungicide-amended potato dextrose agar (PDA) tubes stored at 4°C compared to disks from freshly prepared fungicide-amended PDA. At the storage temperature of 22°C, however, the sensitivity started to decrease after 10 days. Colony growth of M. fructicola on PDA disks was visually distinguishable from Alternaria alternata, Aspergillus niger, Cladosporium herbarum, Gilbertella persicaria, Penicillium expansum, and Rhizopus stolonifer but not Colletotrichum acutatum after 72 h of incubation at 22°C. The sensitivity of 40 M. fructicola field isolates to DMI, BZI, and QoI fungicides was determined by direct inoculation of lipbalm tube disks with conidia from field-derived peach fruit with toothpicks under semi-sterile conditions and by the traditional Petri dish assay. A strong correlation was observed between the two assays in determining fungicide sensitivity of M. fructicola field isolates. Because of its simplicity and reliability, the lipbalm tube assay will be a useful tool for trained crop consultants to determine orchard-specific resistance profiles outside a research laboratory for precision brown rot management. Accepted for publication 11 September 2009. Published 18 November 2009.