Effects of acidic fog on productivity of celery and lettuce and impact on incidence and severity of diseases

The effects of acidic fog on productivity of celery and lettuce were examined using a portable fogging apparatus to expose field plots to simulated fog episodes. Acidic solutions were formulated to simulate fog reported for southern California. Celery (Apium graveolens var. dulec cv. Bishop) and lettuce (Lactuca sativa cv. Mesa 659) were exposed in separate experiments once or twice each week during crop growth and maturation to 1 - to 2-h of fog at specific acidity levels between pH 1.6 and 3.8. Necrosis occurred as small, irregularly shaped white (celery) or brown (lettuce) lesions on the most exposed portions of the plants. Fog necrosis sufficient to reduce marketability of celery was evident after single exposures at pH 2.4. Although injury occurred on wrapper leaves of lettuce at acidity levels as high as pH 3.4, marketability was reduced from foliar necrosis only after exposure to acidity levels of pH 2.6 or lower. Repeated exposure of lettuce to acidity below pH 2.0 was necessary to reduce yield. The field experiments demonstrated that acidic fog influenced natural incidence of disease. At low pH fog exposure, celery had increased occurrence of basal stalk rot and lettuce exhibited increased bacterial soft rot. Indirect effects of acidic fog on host-pathogen relationships and subsequent crop marketability may be more important than the direct effects of acidic fog exposure on plant tissue.