Ascospore liberation, germination and attachment to host surface by Pseudopeziza medicaginis

Apothecia require a period (16–20 hr) of free water or a relatively longer period (at least 24 hr) in conditions of high relative humidity (RH) before ascospore discharge occurs. Provided the RH remains high (97–99%) ascospore discharge continues from individual apothecia for 7–14 days. The process of discharge of spores from individual asci in water is described. Germination of ascospores took 24–36 hr and infection of the host a further 24 hr. Thus 3½ days of high RH were required before infection occurred. This is suggested as the reason why lucerne rarely becomes infected with P. medicaginis in southern Australia during the summer months. Ascospores of P. medicaginis discharged naturally were liberated in pairs. Each spore pair was enveloped in a common membrane. It is believed that single spores are liberated only when the spore pair membrane ruptures during discharge. Single spores lack the membrane. The spore pair membrane is believed to serve two functions. Firstly, it adheres to and becomes more or less contiguous with the host cuticle. This attachment appears necessary for penetration of the host. Secondly, spores in pairs germinated more readily on host surfaces than single spores, and infections were noted only in association with spore pairs.