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Calcium Interference with Zoospore Biology and Infectivity of Phytophthora parasitica in Nutrient Irrigation Solutions

First author: Department of Plant Pathology, Oklahoma State University, 110 Noble Research Center, Stillwater 74078-3032; and second author: Institute of Cell and Molecular Biology, University of Edinburgh, Daniel Rutherford Building, Mayfield Road, Edinburgh EH9 3JH, United Kingdom

Calcium, applied as either CaCl2 or Ca(NO3)2 to water or calcium-free soluble fertilizer solution (Peters 20-10-20 Peat 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 meq. These concentrations also curtailed zoospore motility; 20 meq 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 meq 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.

Additional keywords: disease management, flood irrigation, greenhouse recirculating systems, oomycota.