Quality Control Guidelines for the Preparation and Maintenance of Inoculum of Sclerotinia sclerotiorum

Submitted by Linda Kull, University of Illinois

Mycelium of S. sclerotiorum on a soybean stem. Photo credit: Craig Grau.

Avoid serial subculturing when conducting long-term screening projects. Screening isolate(s) should be maintained as a single stock culture plate (or maintained as sclerotia) from which all inoculum plates for repeated experiments are produced.

To produce inoculum for each experiment, take a plug from the stock culture plate and transfer onto a new potato dextrose agar (PDA) plate. Incubate at 20°C until colony diameter is about 30-40mm, then set up inoculation plates by taking plugs from the leading edge of growth.

Incubate the inoculation plates for 24 hrs. at 20°C and inoculate using plugs from the leading edge of the colony. These two transfers from the stock culture plates tend to 'equilibrate' the mycelial growth to reduce the influence (on mycelial growth rates) of maintainance or length of time the fungus has spent in storage.

Hyphal tipping, although important to produce pure cultures, may influence or reduce aggressiveness.

Light intensity during incubation after inoculation appears to affect aggressivness. (See Barbara Pennypacker Environmental sensitivity of soybean cultivar response to Sclerotinia sclerotiorum). Lower light intensity increased disease severity/aggressivness. So the suggestion here is to ensure that the light intensity is uniform across the experimental design, such as the greenhouse benches or multiple growth chambers.

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