Abstract
Zoospores of Pythium aphanidermatum were strongly attracted by crude exudates from cucumber roots and then encysted. When the root exudates were fractionated, chemotactic substances were found to be unadsorbed by 0.5% activated charcoal and soluble in 80% hot ethyl alcohol. These substances were also ethyl ether insoluble and cationic. By paper chromatographic bioassay and amino acid analysis for this cationic fraction, serine and glutamic acid showed zoospore attraction and encystment. Serine was more chemotactic agent than glutamic acid, but it was impossible to clarify the mechanism of attraction of P. aphanidermatum zoospores toward cucumber roots within seconds in terms of serine and glutamic acid. Chemotactic response was also found by the extracts of rhizosphere soil of cucumber. This may indicates that another chemotactic substances may exist in the extracts of rhizosphere soil besides amino acids found in cucumber root exudates.
