THE EFFECT OF AERATION ON COLONY DIAMETER, SPORANGIUM PRODUCTION AND ZOOSPORE GERMINATION OF <i>PHYTOPHTHORA CINNAMOMI</i>

Abstract

S ummary The reduced aeration which occurs in waterlogged soil was simulated in the laboratory using O 2 /N 2 gas mixtures, and its effects were studied on six isolates of Phytophthora cinnamomi Rands. Mycelial growth of five isolates grown on agar under a flowing atmosphere, and sporangium production by all of the isolates grown in bubbled liquid culture, were severely reduced under anaerobic conditions. Three of the isolates showed an increase in the number and rate of development of sporangia with increasing O 2 concentration of the gas stream. The other isolates showed no increase in the number of sporangia formed at dissolved O., concentrations above 1.3 mgl −1 . There was no effect of O 2 concentration, either during growth (13.5 or 21% O 2 ) or germination, on the proportion of zoospores that germinated on agar or on initial growth of germ tubes. However, after 19 h, the total length of germ tubes including branches increased with increasing O 2 concentration during germination. The number of branches in 11 of the 12 combinations examined was also positively correlated with O 2 concentration.