Suppressed germination and early death of Phytophthora infestans sporangia caused by pectin, inorganic phosphate, ion chelators and calcium-modulating treatments

Abstract

In laboratory experiments, chemical treatments were applied to sporangia of Phytophthora infestans incubated at 12 °C, conducive to cytoplasmic cleavage and release of zoospores (indirect germination), and at 20 °C, conducive to germination by hyphal outgrowth (direct germination). Both types of germination were suppressed by applying increasing concentrations (1–5 mM) of CaCl2 or MgCl2, or by low concentrations of pectin, inorganic phosphate, chelators (EGTA, BAPTA), calcium channel-blockers (lanthanum, gadolinium, verapamil) or compounds that interfere with intracellular calcium-mediated processes (trifluoperazine, caffeine). The suppression by some treatments was partly overcome by adding Ca2+ or Mg2+ in the early stages of incubation, but Ca2+ was usually more effective than Mg2+, and suppression at 20 °C was more easily overcome than at 12 °C. Pectin (0.1%) or BAPTA (5 mM) caused rapid death of sporangia at both 12 ° and 20 °, whereas EGTA (5 mM) or Na2HPO4 (5 mM) caused rapid death only at 12 °. The findings indicate that germinability and viability of P. infestans sporangia are strongly affected by the external availability of Ca2+ or other divalent cations, especially during zoosporogenesis.