Studies on virulence of the take‐all fungus, Gaeumannomyces graminis, with reference to methology

SUMMARY Current methods for take‐all assessment in laboratory experiment were examined; it was shown that the extent of vascular discoloration may not reflect virulence of a fungal isolate or host resistance to the pathogen under some experimental conditions. A new assessment method for take‐all is described, based on the ability of transport eosin past infection sites. It enables hosts or isolates to be compared by ET 50 values, the times from inoculation when 50% of plants fail in eosin‐uptake through the three oldest seminal roots. Use of this technique suggested that barley roots were less affected than were wheat roots by Gaeumannomyces graminis var. tritici . Further experimental results showed that an isolate of G. graminis that had lost part of its virulence in culture yielded some single‐conidium progeny more virulent than itself. When single‐condium isolates or a mycelial isolate and its single‐conidium progeny were jointly inoculated on wheat, the amount of disease was less than that caused by the more virulent isolate alone.