Monitoring Fungicide Sensitivity Levels and Mycelial Compatibility Groupings of Sclerotium rolfsii Isolates from Florida Peanut Fields

Abstract

ABSTRACT Sclerotium rolfsii, the causal agent of peanut stem rot, is a diverse pathogen that has exhibited decreases in sensitivity to fungicides in areas where they are frequently applied. To better understand this pathogen's diversity and its response to various fungicides in Florida, a monitoring survey was done to examine isolates from several peanut producing areas using morphological characteristics, mycelial compatibility groupings and fungicide sensitivity profiles. A high level of morphological diversity was observed among a small number (N = 15) of isolates which was affirmed by both Shannon-Weiner (E = 0.812) and Simpson's (D = 0.280) indices. However, despite this high level of diversity, fungicide sensitivity of these isolates to flutolanil (EC50 = 0.031 ppm) and tebuconazole (EC50 = 0.008 ppm) appears to remain relatively unchanged when compared to a previous baseline study. Utilizing a small number of isolates, this monitoring survey indicated the EC50 values for the products azoxystrobin (EC50 = 0.050 ppm), prothioconazole (EC50 = 0.213 ppm), penthiopyrad (EC50 = 0.016 ppm) and solatenol (EC50 = 0.005 ppm). A trend for hormesis was also noted in this survey (e.g. flutolanil), but further research is necessary to better understand sub-lethal fungicide dose effects on increasing mycelial growth. It is apparent from these results that despite the high levels of phenotypic diversity in S. rolfsii populations, current fungicide management practices should remain effective for disease control.