OBSERVATIONS ON THE BIOLOGY OF CLOVER ROT

The field behaviour of clover rot in eastern England indicates that infection takes place by ascospores, and not by soil‐borne mycelium developing directly from the sclerotia. To support this view, experimental evidence is presented to show that cutting of red clover at the time when the ascospores are being shed, reduces the percentage infection in the following spring.Ascospore infection of clover causes a ‘non‐aggressive’ spotting which may develop into an ‘aggressive’ infection. Sclerotinia trifoliorum has been shown to remain viable in non‐aggressive spots for at least 16 weeks. Similar symptoms occur when beans are infected with S. trifoliorum var. fabae. Non‐aggressive and aggressive infections are two phases of a sequence in time, and not two alternative types of infection dependent on the initial spore load. The occurrence of non‐aggressive and aggressive phases in nature explains (1) the observed delay between ascospore discharge and the appearance of obvious disease symptoms, and (2) the seasonal nature of the disease.Aggressive infection develops only in a saturated atmosphere and within a temperature range of 5–20° C., and its establishment is favoured by a heavy dosag