Impact of Preconditioning Temperature and Duration Period on Carpogenic Germination of Diverse Sclerotinia sclerotiorum Populations in Southwestern Australia

The soilborne pathogen Sclerotinia sclerotiorum (Lib.) de Bary is the causal agent of Sclerotinia stem rot, a severe disease of broad-leaf crops including canola/rapeseed (Brassica napus) that can result in significant yield losses. Sclerotia, the hard melanized resting structure of the pathogen, requires preconditioning before carpogenic germination can occur. We investigated the effect of preconditioning temperature (4, 20, 35, 50°C, and field conditions) and duration (0, 30, 60, 120, 179, 240, and 301 days) on germination of S. sclerotiorum sclerotia collected from five canola fields in the southwestern Australia grain belt. The ecological diversity of each population was characterized using mycelial compatibility group (MCG) typing. No response was observed for isolates conditioned at 4°C at any time period, indicating that chilling is not a preconditioning requirement for these isolates. Sclerotia required preconditioning for a minimum of 60 days before any significant increase in germination occurred, with no further increases in germination recorded in response to longer conditioning after 60 days. The highest germination was observed in sclerotia conditioned at 50°C. The MCG results indicated significant diversity within and between populations, suggesting local adaptation to different environments as well as ensuring the ability to respond to seasonal variation between years.