Nineteen L-amino acids, the corresponding D-amino acids and 10 sugars were tested as elicitors of zoospore taxis, encystment and cyst germination in Pythium aphanidermatum (Edson) Fitzp., P. catenulatum Matthews and P. dissotocum Drechs. Selected compounds were also tested for promotion of germ-tube growth and competition in zoospore taxis. Each fungus responded to a different spectrum of compounds at different stages of development; for example, in P. aphanidermatum L-glutamine elicited taxis but not germination, whereas glucose elicited germination but not taxis. At each stage of development the fungi showed common responses to some compounds but different responses to others. For example, all fungi showed positive taxis to L-aspartic acid, L-asparagine, L-glutamic acid, L-glutamine and L-alanine, but differed in taxis to sugars. Nevertheless, all three fungi responded to the same broad range of compounds at one stage or another. The compounds that elicited germination did not always promote growth, and D-amino acids had no effect at any stage, indicating that taxis, encystment and germination were receptor-mediated. However, some non-attractant amino acids interfered with chemotaxis, to different degrees in the three fungi.
The findings are discussed in relation to host-location mechanisms, including host-differential taxis to roots. Variations in chemoresponses of Pythium are suggested to involve receptors for a common range of compounds, differentially active at different stages of the pre-infection sequence.