I could barely contain my excitement. What was this fluffy white stuff that had taken down large fleabane rosettes? Had I accidentally stumbled upon an effective biocontrol option for glyphosate-resistant Canada fleabane?
I wondered, what would it take to reproduce this potent material so we could inoculate plants throughout Ontario?
I quickly took a sample to the pest diagnostic clinic to get a positive identification and begin my plans of Canada fleabane eradication and cult hero status in Ontario.
A little context. Late last summer, my friend and colleague Pete Smith (University of Guelph) collected a bunch of Canada fleabane plants for me because I wanted to test out some newer herbicides to see what might be worth looking at in the field during the 2020 season. I got busy doing other things and Pete decided to put the plants in a vernalization chamber to slow their growth so that I could revisit them once I had more time.
The cool (5 C) and moist conditions of the vernalization chamber resulted in about 25 per cent of the plants having this white fluffy growth on the rosettes (Figure 1 at top).
Within days of being transported to the warmer growth rooms, the infected rosettes had completely died (Figure 2 below), and I was giddy.
The excitement quickly ended days later when the results came back from the pest diagnostic clinic. The pathogen was… Sclerotinia sclerotiorum, more commonly known as white mould and one of the more significant pest problems in soybeans.
I doubt soybean producers would appreciate a civil servant spreading white mould around the countryside, even if it was effective at killing Canada fleabane.
So, does the identification of white mould growing on Canada fleabane provide any value?
At the very least, it may explain one of the causes of overwintering mortality that many have observed with fall-established Canada fleabane rosettes. Often people have speculated that cold temperatures are the culprit for plant death over winter, but perhaps white mould is another contributing factor. More significantly, if Canada fleabane is a host for white mould, then effective control of this weed will be one of many important management practices to reduce white mould growth in soybean.
(For more information on disease and insect management, check out the Crop Protection Network. If looking for resources on white mould on this site, you will have to use the U.S. spelling of “mold.”)