SUMMARYIn glasshouse experiments, isolates ofFusarium nivale, applied as mycelial macerates, caused marked reductions in the number and length of roots, and in the height and fresh weight of S. 321 perennial ryegrass seedlings. Spore inocula failed to cause infection.Damage byF. nivalewas increased when inoculation was immediately followed by four alternate 12 h periods in a growth cabinet at 0°C and in a warm glasshouse atc.17°C, succeeded by a further 11 days in the same glasshouse.Although inoculated seedlings receiving no cold treatments initially showed damage, regrowth of roots and some recovery of the shoots occurred in a fortnight. Exposure of the plants after that time to four intermittent 12 h periods at 0°C, however, arrested their recovery and caused further root damage.The implication of these results in the importance of low temperature in the winterkill syndrome is discuss