ENZYMATIC-ACTIVITY OF THE MYCELIUM COMPARED WITH OOSPORE DEVELOPMENT DURING INFECTION OF PEA ROOTS BY APHANOMYCES-EUTEICHES

To describe the disease cycle of the root pathogen Aphanomyces euteich es, enzymatic activity in the mycelium was compared with the developme nt of oospores in pea roots. Plants were inoculated with two zoospore concentrations to achieve different disease levels. Hyphae were staine d for fungal alkaline phosphatase activity in the roots. Additionally, enzyme activity was measured after electrophoresis of an A. euteiches -specific glucose-6-phosphate isozyme. Development of oospores in the roots was measured after staining the oospores with trypan blue. In pl ants inoculated with the higher zoospore concentration, the enzymatic activity of the pathogen mycelium peaked 10 to 14 days after inoculati on, when oospore formation was initiated. Oospore formation was associ ated with a gradual increase in disease symptoms. At the last harvest, plants inoculated with the higher zoospore concentration had died. In these plants, oospores were found in 90% of the root length, while th e enzymatic activity of the mycelium was low. This suggests that the p athogen mycelium is only active on living plants and does not grow sap rophytically on dead plant material.