Influence of Simulated Acidic Rain on Bacterial Speck of Tomato

Abstract

Ambient rain in southern Ontario has a volume‐weighted average pH of approximately 4.2. Tomato (Lycopersicon esculentum Mill var. ‘Chico III’) seedlings were exposed to simulated acidic rain in specially designed chambers. The inoculum of Pseudomonas tomato (Okabe) Alstatt, causal agent of bacterial speck, was sprayed on plants before or after exposure to acidic rain of pH 2.5, 3.5, and 4.5, as well as on plants not exposed to the simulated acidic rain. Speck symptoms (small, dark, brown spots with yellow halos) were found on all inoculated plants. Exposure of plants to simulted acidic rain inhibited speck development, but the inhibition was greater on plants exposed to acidic rain after inoculation than on those exposed to acidic rain before inoculation. Spot necrosis, a typical response to acidic rain, occurred on up to 15 to 20% of the leaf area on all tomato plants treated with acidic rain at pH 2.5. Plants also showed a decrease in growth (height and fresh and dry weights) with an increase in rain acidity. Leaves injured by simulated acidic rain and examined histopathologically displayed cellular malformations including hyperplasia and hypertrophy. Pseudomonas tomato failed to grow on acidified King B medium or Difco nutrient broth adjusted to pH 3.5 or lower.