Response of <i>Ophiosphaerella herpotricha</i> and <i>O. korrae</i> , causal agents of spring dead spot, to fungicides <i>in vitro</i> and <i>in situ</i>

Ophiosphaerella herpotricha and O. korrae are the two most common fungal species that cause the bermudagrass disease spring dead spot (SDS) in North America. Previous research shows they differ in aggressiveness, response to fertilizer type, ability to grow at certain pH ranges, and sensitivity to certain fungicides. The purpose of this project was to determine how O. herpotricha and O. korrae respond in vitro and in situ to various fungicides and fungicide groups. An in vitro fungicide sensitivity assay was conducted on four O. herpotricha isolates and four O. korrae isolates testing 13 different fungicides belonging to three different fungicide groups: (DMIs: fenarimol, mefentrifluconazole, myclobutanil, propiconazole, tebuconazole; QoIs: azoxystrobin, fluoxastrobin, pyraclostrobin; SDHIs: fluopyram, fluxapyroxad, isofetamid, penthiopyrad, and pydiflumetofen) at six different concentrations (0, 0.001, 0.01, 0.1, 1, or 10 μg mL-1). An in situ fungicide screening tested the same fungicides as the in vitro study with the exception of fenarimol and fluopyram. This trial was conducted at locations containing either a predominant O. herpotricha, O. korrae, or mixed Ophiosphaerella population. Most fungicides were generally less effective against O. korrae than O. herpotricha in vitro and in situ. Moreover, the SDHIs were the most efficacious group of fungicides against SDS, regardless of Ophiosphaerella species, with isofetamid, penthiopyrad, and pydiflumetofen all producing low EC50 values in vitro and generally excellent SDS suppression in situ. This research demonstrates that fungicide selection should be based on which Ophiosphaerella species is predominant at a certain location or that a highly effective fungicide option for both species be selected.