THE CONFIRMATION OF THE VARIETY FABAE KEAY OF SCLEROTINIA TRIFOLIORUM ERIKSSON

There is confusion as to whether the Sclerotinia on Vicia faba is a normal strain of Sclerotinia trifoliorum as found on clover and related legumes or a distinct variety (var. fabae ). Pathologically the two fungi appear to be the same, and field observations indicate that beans in the rotation may aggravate the incidence of clover rot. A re-investigation of the morphology of the two fungi is made to assess the status of var. fabae.
The two fungi could not be distinguished by apothecial characters or cultural behaviour. Ascospore measurements confirm the existence of S. trifoliorum var. fabae , although 5. trifoliorum proper also occurs on beans. Var. fabae has not been found on any legume besides beans. Isolates from trefoil and sainfoin agree in spore size with S. trifoliorum on clover.
When isolates of S. trifoliorum are co-inoculated on the same Petri plate, they either do not react, or react to form a 'demarcation line'. Although a demarcation line is assumed to imply a genetic difference, non-reaction does not necessarily imply genetic identity. Non-reaction and the formation of a demarcation line also occur when var. fabae strains are co-inoculated together. When an isolate of S. trifoliorum is co-inoculated with an isolate of S. trifoliorum var. fabae an 'incompatibility line' is formed. Use can be made of this fact in identification.
On the basis of co-inoculations 5. trifoliorum and S. trifoliorum var. fabae are both shown to constitute a diverse range of biotypes. Segregation of biotypes in the ascus as a result of heterokaryosis in the ascus-initial is demonstrated. The view is expressed that heterokaryosis is the mechanism underlying the production of new biotypes.