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-free soluble fertilizer solution (Peters 20-10-20 Feat Lite Special), affected several important stages of Phytophthora parasitica zoospore behavior relevant to infection and disease spread. Release of zoospores from 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 within 4 h, whereas 94% of zoospores remained motile in unamended solution. In addition, Ca2+ in the range of 10 to 30 meg stimulated zoospore cysts 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 suppressed infection of flood-irrigated, containerized vinca seedlings in a peat-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 epidemiology, and suggest that manipulation of Ca2+ levels in irrigation water or fertilizer solutions could contribute to management of Phytophthora in recirculating irrigation systems.