Annual cycle of root nematodes on white clover in pasture

Fortnightly sampling of pasture for H. trifolii cysts in soil, larvae in soil, and larvae and females in roots was carried out from November 1971 to December 1972. Cyst numbers were stable at 600‐800 per 350 ml core except for periods in autumn and spring when they fell to 200‐300 per core. The decreases were correlated with the appearance of larvae in the roots which developed into females and subsequently returned to the soil as cysts. In spring the invading larvae died in the roots, but a reinvasion led to larval counts twice those in autumn. The greater spring invasions appear due to more favourable soil moisture levels; soil moisture was 10% in autumn and 40% in spring. The occurrence of larvae in the soil generally preceded root invasion. The times of invasion correspond with the autumn and spring periods of pasture growth. The combination of nematode invasion and low soil moisture in autumn appears to be important in the poor autumn growth of white clover and its consequent lack of vigour in areas of yellow‐grey earths.