BIOLOGY OF THE HOP CYST-NEMATODE HETERODERA-HUMULI .1. LIFE-CYCLE ANDSURVIVAL

In an intensively sampled English hop garden, cysts of Heterodera humu li were found in every sample taken. Most occurred in the top 15 cm of soil, decreasing in number by half with every 15 cm in depth. Numbers of second-stage juveniles fluctuated during the vegetative period in a pattern which indicated that more than one generation, possibly two to three, were produced. The generation time was about 1.5 months. In pot experiments under controlled conditions (20 degrees C; 16 h light/ day) the life cycle lasted 40 days. The optimal temperature was 20 deg rees C for the development of H. humuli in the roots of its host Humul us lupulus. Most H. humuli juveniles invaded hop roots at 15 degrees C , but egg hatch was greatest at 20 degrees C. In moist soil, second-st age juveniles survived for at least 54 days and thereafter invaded roo ts and reproduced.