Masters Theses

Date of Award

5-2009

Degree Type

Thesis

Degree Name

Master of Science

Major

Entomology and Plant Pathology

Major Professor

Mark Windham

Abstract

Corynespora cassiicola, causal agent of Corynespora leaf spot, causes severe epidemics in African violet production facilities. Recent concerns over loss of fungicide efficacy has led to this investigation of baseline sensitivity distributions to thiophanate-methyl, iprodione and fludioxonil fungicides and temperature sensitivity study to evaluate fitness of high and low sensitivity groups. During a disease outbreak, 325 single lesion isolates were collected and 40 isolates were selected randomly for an in vitro assay to determine EC[Subscript50] (the point at which 50% of mycelial growth was inhibited) values on potato dextrose agar (PDA) or PDA amended with thiophanate-methyl, iprodione and fludioxonil. EC[Subscript50] values for iprodione and thiophanate-methyl ranged from 0.0833 to 0.6478 [Mu]g/ml and 0.0157 to 0.1539 [Mu]g/ml with mean values of 0.2828 [Mu]g/ml (Figure 1-1, see Chapter 2 appendix) and 0.0553 [Mu]g/ml (Figure 1-2), respectively.Fludioxonil EC[Subscript50] values ranged from 0.0013 to 0.0103 [Mu]g/ml and the mean value was 0.0075 [Mu]g/ml (Figure 1-3). A resistance factor for each fungicide was calculated by dividing the least sensitive isolate's EC[Subscript50] value to the mean EC[Subscript50] for that fungicide. A lower resistance factor for fludioxoni-amended plates (1.37) than for iprodione (2.39) or thiophanate-methyl (2.78) (Table 1-1) indicates a tendency toward less insensitivity to this fungicide in this population. Correlation coefficients were calculated to determine cross-sensitivity between fungicides. Of the three fungicides, iprodione and fludioxonil had a moderately significant correlation (r = 0.38686) (P = 0.0125) (Table 1-2), indicating moderate cross-resistance.To determine fitness ranges of isolates to each fungicide, three isolates least sensitive to each fungicide along with three isolates most sensitive to each fungicide were used with mean radial growth area recorded every three days for twelve days across 10, 15, 20, 25, 30 and 35° C temperatures. Across all fungicide sensitivity groups, growth did not occur at 10° C and was very limited at 35° C. Optimum growth for all isolates within sensitivity groups was 25° C (Figure 2-1 to Figure 2-3, see Chapter 3 appendix) across all fungicide and observation times. Differences in growth area (mm²) between the least and most sensitive isolate groups did not differ for the thiophanate-methyl isolate group (P = 0.2246), the iprodione isolate group (P = 0.0512), or the fludioxonil isolate group (P = 0.6070) based on linear mixed model analysis.Even though significant differences did not exist in this analysis, temperature sensitivity information is an important part of fungicide resistance management. Developing baseline and temperature sensitivity data is the first step in determining fungal population sensitivity shifts for better resistance management strategies.

Download
Files over 3MB may be slow to open. For best results, right-click and select "save as..."

Share

COinS