Abstract: A study on the decline of China fir was made during 1985 —1988. This is a popular disease now occurred in Fujian province of China. The enlarged roots of the trees became as cock-foot caused by the damage of nematode Tylenchulus semipenetrans Cobb,a new host just discovered in addition to citrus. The nematode has 10—12 generations per year. Their generation is metagenesis. Individual development is uncoreplete. Its sex ratio is 56:1 between female and male. Rate of egg hatch in root soaking water is 90%. Its maximum temperatuee for growch is 35℃. Death point of eggs is 40℃. Density of nematodes are increased with the ages of trees and are highest in south slope and lowerest in a mixed stand with Choerospondias axillaris. Distribution of nematodes in soil is coincided with the nutrition roots. Their amounts are mostly in 30—40cm deepth in the middle and lower slope. The aspiration strength and the amounts of N, P, K contained in diseased roots are larger than thoes in healthy ones as well as the activities of multiphenol oxidase, hydrogen peroxidase and iso-peroxidase. Invasion of nematodes induced the parasite of Fusaria in soil; thus caused the decay of roots. Hot water treatment in 40—50℃ soaking the diseased roots had prominently effect to kill the nematodes. Using methamidophos,Carbofuran and ALdicarb in an area of about 24000 acres an effective results of control is 70 —80%. Mixed stands with Paulownia spp.,Choerospondias axillaris, or Sassafras tzumu had less population of nematodes. Cryptomeria fortunei and wild persimmon are also descovered as new hosts.

Key words: China fir, Tylenchulus semipenetrans, Life-history, Biochemistry and physiology, Control