SUPPRESSED GERMINATION AND EARLY DEATH OF PHYTOPHTHORA-INFESTANS SPORANGIA CAUSED BY PECTIN, INORGANIC-PHOSPHATE, ION CHELATORS AND CALCIUM-MODULATING TREATMENTS

In laboratory experiments, chemical treatments were applied to sporang ia of Phytophthora infestans incubated at 12 degrees C, conducive to c ytoplasmic cleavage and release of zoospores (indirect germination), a nd at 20 degrees C, conducive to germination by hyphal outgrowth (dire ct germination). Both types of germination were suppressed by applying increasing concentrations (1-5 mM) of CaCl2 or MgCl2, or by low conce ntrations of pectin, inorganic phosphate, chelators (EGTA, BAPTA), cal cium channel-blockers (lanthanum, gadolinium, verapamil) or compounds that interfere with intracellular calcium-mediated processes (trifluop erazine, caffeine). The suppression by some treatments was partly over come by adding Ca2+ or Mg2+ in the early stages of incubation, but Ca2 + was usually more effective than Mg2+, and suppression at 20 degrees C was more easily overcome than at 12 degrees C. Pectin (0.1%) or BAPT A (5 mM) caused rapid death of sporangia at both 12 degrees and 20 deg rees, whereas EGTA (5 mM) or Na2HPO4 (5 mM) caused rapid death only at 12 degrees. The findings indicate that germinability and viability of P. infestans sporangia are strongly affected by the external availabi lity of Ca2+ or other divalent cations, especially during zoosporogene sis.