Summary: Swimming zoospores of the fungus exposed to a range of ions were much more sensitive to cations than anions. One cation, Ca, induced zoospores to encyst and subsequently germinate, but most cations induced encystment only and were toxic at higher concentrations. In general, the sensitivity of zoospores to a cation increased with its charge density. At 0.3 μm, Lareduced the viability of a zoospore population to 50%, while Fe(20 μm) and Mn(50 μm) induced encystment with only a slight decrease in viability. The other divalent and monovalent cations tested (Mg, Ca, Ba, Li, Na, K, Cs, NH ) were effective in inducing encystment or reducing viability at higher concentrations. Each ion showed a distinctive concentration–response curve. Only Fand CHCOOamong the anions tested (Cl, NO , F, HPO , SO , CHCOO) had any effect on zoospores, and at 20 mM (pH 6.0) they reduced viability.

The cysts (cystospores) of the fungus were generally less sensitive to cations than were swimming zoospores, and only Csand K(50 mM) reduced viability to the same extent in each population. Both zoospores and cysts of this fungus had a broad tolerance to pH and temperature, but cysts were more resistant to low temperatures than were motile zoospores.


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