Masters Theses

Date of Award

5-2005

Degree Type

Thesis

Degree Name

Master of Science

Major

Entomology and Plant Pathology

Major Professor

John Sorocham

Committee Members

Alan Windham, Thomas Samples, Mark Windham, Robert Trigiano

Abstract

Dollar spot, caused by Sclerotinia homeocarpa, is the most common turf disease golf course that superintendents deal with on a regular basis. Chemical control has been the front line of defense against this disease; however, most of the fungicides that formerly controlled dollar spot have lost efficacy over time due to development of resistance by the fungus. The objectives of part one of this thesis were to determine the following: (i) if resistance to the fungicides thiophanate-methyl, iprodione, and propiconazole exists in Tennessee, and northern Mississippi isolates of field-collected S. homeocarpa; (ii) if isolates collected in Michigan differ in levels of resistance to isolates collected in Tennessee and northern Mississippi; (iii) if multi-resistance to different fungicide classes occurs in these isolates; and (iv) if chemical practices of golf course superintendents have an effect on the occurrence of fungicide resistance in the southern-collected isolates.

Fungicide resistance was observed in nine of the ten southern locations tested. Additionally, three locations from Michigan also exhibited resistance. One location from Michigan known to be sensitive to all fungicides used in this study did not exhibit any resistance. Of the 14 total locations tested, seven exhibited resistance to iprodione and thiophanate-methyl, and two exhibited resistance to propiconazole. One location exhibited multi-resistance to thiophanate-methyl and iprodione, another exhibited multi-resistance to thiophanate-methyl and propiconazole, and another location exhibited multi-resistance to all fungicides tested. The highest fungicide active ingredient concentration at which resistance occurred to iprodione and thiophanate-methyl was 1000 µg ml-1, and the highest concentration to which resistance occurred to propiconazole was ten µg ml-1. No significant difference was detected among levels of resistance in isolates from the Michigan and southern locations. For all locations, fungicide resistance correlated with fungicide use at that location.

The purpose of the second investigation was to determine the following: (i) if resistance to the fungicide chlorothalonil exists in Tennessee and northern Mississippi by testing isolates of S. homeocarpa field-collected in this area; (ii) if the ‘wild type’ isolate from Michigan is resistant to chlorothalonil, and if so, if it has differing sensitivity from the southern-collected isolates; and (iii) if chemical practices of golf course superintendents have an effect on the occurrence of resistance to chlorothalonil.

All nine isolates tested grew in chlorothalonil amended PDB at rates up to 5 µg active ingredient ml-1 PDB. Two isolates grew in 10 µg ml-1 PDB, and none grew in 20 µg ml-1­­ PDB. Variation was observed in the relative growths of all isolates at all concentrations, and no correlation was observed between geographic region of collection and fungicide sensitivity. Additionally, no correlation was observed between chlorothalonil use in situ and chlorothalonil sensitivity in vitro.

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