Influence of Simulated Acidic Rain onPhytophthora cinnamomiand Phytophthora Root Rot of Blue Lupine

Abstract

Effects of acid deposition on P. cinnamomi were examined with simulated rain solutions (pH 5.6, 4.0, 3.2, or 2.4). Lakeland sand, and blue lupine seedlings. Infected radicles were buried in soil that was subsequently saturated for 15 min with solutions and then drained to -3.0 to -2.3 kPa. The number of sporangia formed on radicles decreased linearly with increasing solution acidity (47% fewer sporangia formed in soil treated with pH 2.4 solution than with pH 5.6 solution). Release of zoospores from sporangia incubated in soil extracts was unaffected by the acidity of solutions used to prepare extracts. Seedlings grown in soil, inoculated with zoospores, and exposed to simulated rainfall (2.4 cm. 1 hr) at pH 2.4 had 44% fewer infection sites on roots than did seedlings exposed to rain at pH 5.6. Effects of rain acidity on onset of disease symptoms and rate of disease increase were not consistent among seedlings maintained for 28 days in infested soil and repeatedly exposed to simulated rains. Although simulated acidic rain significantly affects epidemiologically important steps in the life cycle of P. cinnamomi, gradual deposition of H+ in rain probably has little short-term effect on Phytophthora root rot if plants remain exposed to inoculum.