EFFECT OF TEMPERATURE ON CORKY ROOT DISEASE OF LETTUCE AND GROWTH OF THE PATHOGEN RHIZOMONAS-SUBERIFACIENS

The effect of temperature on in vitro growth of Rhizomonas suberifacie ns, the causal agent of corky root of lettuce (Lactuca sativa), and on disease severity was assessed in temperature controlled root-zone cab inets and in microplot field experiments. Maximal growth of R. suberif aciens in vitro occurred between 29 and 33-degrees-C. In root-zone cab inets, disease severity and the resulting reduction in plant growth we re lowest at 10-degrees-C and greatest at 31-degrees-C. In field exper iments, daily increases in disease severity increased exponentially wi th minimum, mean, and maximum soil temperatures averaged over 8, 16, o r 24 days prior to disease measurement. Maximum soil temperature measu red at a depth of 10 cm and averaged over 24 days, gave the best corre lation with daily increases in disease severity (R2 = 0.82). A simple, significant relationship was identified correlating yield loss to deg ree-days between the second and seventh true leaf stages of lettuce.