Effect of calcium on the growth and virulence of Phytophthora clandestina

Calcium was found to significantly stimulate both the mycelial growth and v irulence of Phytophthora clandestina, the major soilborne fungal pathogen o f subterranean clover (Trifolium subterraneum L.). On average, the addition of calcium as either CaCO3 or CaCl2 to 3 artificial media increased the in vitro growth of 15 isolates from 20 to 135%. Calcium was also shown to inc rease the severity of root disease caused by 6 isolates of P. clandestina b y up to 100% in a glasshouse experiment. The addition of Ca2+ as CaCl2 had a greater stimulatory effect on the virulence of P. clandestina towards cv. Woogenellup seedlings (up to 57% reduction in seedling biomass) than suppl ementing with CaCO3 (maximum 36% decrease). The virulence of 6 isolates from 3 different pathogenic races of P. clandes tina were examined in the glasshouse experiment. Differences in the virulen ce of FI clandestina towards cv. Woogenellup seedlings was shown to be depe ndent on the individual isolate rather than the pathogenic race to which it is assigned.