Development of a semi-selective medium for improved isolation of the turfgrass dollar spot pathogenSclerotinia homoeocarpafrom host tissues

Abstract

Dollar spot, caused by Sclerotinia homoeocarpa, is one of the most economically devastating diseases of amenity turfgrasses worldwide. The pathogen is readily isolated from active lesions, but detection from seed, dormant host tissue and other plant debris that may serve as a source of primary inoculum is difficult. A semi-selective medium was developed to enhance isolation of S. homoeocarpa. Various fungicides used on turfgrass, the pH indicator dye bromophenol blue, and two pH levels, were assessed for their effects on, and in the case of bromophenol blue, response to growth of S. homoeocarpa and contaminant fungi frequently isolated from field and seed samples. Amendment of the medium to pH 4 promoted growth of S. homoeocarpa in the absence of fungicides and enhanced pathogen growth relative to contaminant isolates on medium amended with 0.1 μg mL −1 triticonazole or 5 μg mL −1 azoxystrobin. The growth rate of S. homoeocarpa on these three media, as determined by in vitro radial growth assays, was consistent for many isolates representing the majority of S. homoeocarpa vegetative compatibility groups. Isolation efficiency on the pH-amended media was tested against antibiotic-amended potato dextrose agar from field samples and artificially inoculated turfgrass seed. In both cases, the medium amended to pH 4 and containing 5 μg mL −1 azoxystrobin significantly decreased contaminant growth with no adverse effects on recovery of S. homoeocarpa in comparison with antibiotic-amended PDA. The enhanced specificity of this medium will be a useful tool for selective detection of S. homoeocarpa and identification of sources of initial inoculum for dollar spot development.Résumé: La brûlure en plaques, causée par Sclerotinia homoeocarpa, est une des maladies du gazon d'agrément dont les répercussions économiques sont des plus importantes à l'échelle mondiale. L'agent pathogène est facilement isolé à partir de lésions actives, mais la détection à partir de semences, de tissu dormant et d'autres débris végétaux qui peuvent servir de source primaire d'inoculum n'est pas aussi simple. On a développé un milieu semi-sélectif pour mettre en relief l'isolement de S. homoeocarpa. Divers fongicides utilisés sur le gazon, l'indicateur de pH bleu de bromophénol et deux niveaux de pH ont été évalués quant à leurs effets sur S. homoeocarpa et, dans le cas du bleu de bromophénol, quant à la réaction par rapport à sa croissance ainsi qu'aux champignons contaminants fréquemment isolés d'échantillons de sol et de semences. L'amendement du milieu pour obtenir un pH de 4 a stimulé la croissance de S. homoeocarpa en l'absence de fongicides et a favorisé la croissance de l'agent pathogène par rapport aux isolats contaminants se développant sur le milieu amendé avec 0.1 µg mL −1 de triticonazole ou 5 µg mL −1 d'azoxystrobine. Le taux de croissance de S. homoeocarpa sur ces trois milieux, comme défini pas les biotests de croissance radiale in vitro, était uniforme pour plusieurs isolats représentant la majorité des groupes de compatibilité végét...

Sclerotinia homoeocarpa Overwinters in Turfgrass and Is Present in Commercial Seed

“…The isolate was collected from Lot 1 of cv. ‘Shark’ on the triticonazole–amended semi-selective medium [27] . Colony morphology ( Fig.…”

“…In an experimental replicate, a total of 1000 seeds from each seed lot were plated on two fungicide-amended media semi-selective for S. homoeocarpa [27] , one with 5 ppm azoxystrobin and the other with 0.1 ppm triticonazole, in 100 mm diameter petri plates (Fisher Scientific, Hanover Park, IL). Twenty-five seeds were placed on each petri dish, resulting in a total of 20 plates of each medium and 40 plates total per seed lot in each experimental replicate.…”