EFFECT OF ROOT TEMPERATURE, PLANT-AGE, FREQUENCY AND DURATION OF FLOODING AND INOCULUM PLACEMENT AND CONCENTRATION ON SUSCEPTIBILITY OF ASPARAGUS TO PHYTOPHTHORA ROT

The conditions and recommended procedure to maximise disease severity for greenhouse screening of asparagus seedlings for resistance to Phytophthora rot are described. The optimum temperature for disease development on asparagus seedlings inoculated with Phytophthora megasperma var. sojae was 15-degrees-C and 18-24-degrees-C for P. cryptogea. The same temperatures were also optimal for indirect germination of sporangia of the two species. Young plants (20-29 weeks old) were more susceptible to Phytophthora root and crown rot than older plants (37-54 weeks old). V-8 vermiculite inoculum at a concentration of 1 : 100 (v:v) resulted in a minimum number of seedlings escaping infection and, when placed 60-80 mm below the seed also minimised pre-flooding root rot and damping off. Maximum disease severity was attained in the shortest time when seedlings were flooded three times for 48 h each time, but comparable disease severity resulted from flooding four times for 8, 24, or 48 h.