CALCIUM INTERFERENCE WITH ZOOSPORE BIOLOGY AND INFECTIVITY OF PHYTOPHTHORA-PARASITICA IN NUTRIENT IRRIGATION SOLUTIONS

Calcium, applied as either CaCl2 or Ca(NO3)(2) to water or calcium-fre e soluble fertilizer solution (Peters 20-10-20 Feat Lite Special), aff ected several important stages of Phytophthora parasitica zoospore beh avior relevant to infection and disease spread. Release of zoospores f rom sporangia was suppressed by Ca2+ concentrations in the range of 10 to 50 meg. These concentrations also curtailed zoospore motility; 20 meg of Ca2+ in fertilizer solution caused all zoospores to encyst with in 4 h, whereas 94% of zoospores remained motile in unamended solution . In addition, Ca2+ in the range of 10 to 30 meg stimulated zoospore c ysts to germinate in the absence of an organic nutrient trigger, while suppressing the release of a single zoospore (diplanetism) from cysts that did not germinate. In growth chamber experiments, the amendment of the fertilizer solution with 10 or 20 mM Ca(NO3)(2) greatly suppres sed infection of flood-irrigated, containerized vinca seedlings in a p eat-based mix by motile or encysted zoospores of P. parasitica. These results demonstrate that Ca2+ amendments interfere with P. parasitica zoospore biology at multiple stages, with compounding effects on epide miology, and suggest that manipulation of Ca2+ levels in irrigation wa ter or fertilizer solutions could contribute to management of Phytopht hora in recirculating irrigation systems.