EFFECT OF THE MATRIC COMPONENT OF SOIL-WATER POTENTIAL ON INFECTION OF PEPPER SEEDLINGS IN SOIL INFESTED WITH OOSPORES OF PHYTOPHTHORA-CAPSICI

Pepper seedlings (Capsicum annuum 'Keystone Resistant Giant') were grown in soil infested with 50 oospores of Phytophthora capsici per gram of soil. The matric component of soil water potential (psi(m)) was adjusted to 0, -2.5, -5.0, or -10.0 J/kg (1 J/kg = 10 millibars). Plants grown in soil at psi(m) values less than 0 J/kg were either not saturated or exposed to a single 2- or 24-h saturation period after 10 days. Plants were not infected without a period of soil saturation, but 33-83% of the plants grown at psi(m) values of -2.5, -5.0, or -10.0 J/kg before saturation showed symptoms of disease. Absence of disease in plants held in soil at constant psi(m) values of -2.5, -5.0, or -10.0 J/kg without a saturation period suggests that either oospore germination did not occur or oospores that germinated did not cause infection. In additional experiments, the incidence of foliar symptoms of disease increased over time after a single saturation period, and the rate of disease increase was similar after a saturation period of either 2 or 24 h. Incidence of disease 8 days after the saturation period was higher in plants grown for 10 days (65.3, 83.7, and 86.7%) than in plants grown for 5 days (27.8, 34.7, and 40.2%) at psi(m) values of -2.5, -5.0, or -10.0 J/kg before saturation. Incidence of foliar symptoms, the rate of disease increase, and final incidence of root infection were lower in plants grown in soil at a constant psi(m) of 0 J/kg than in plants grown in soil for 5 or 10 days at psi(m) values of -2.5, -5.0, or -10.0 J/kg before saturation. Less than 10% of the plants grown in soil maintained under constant saturation became infected, thus suggesting that continuous periods of soil saturation were not conductive to disease development. Total areas under the disease progress curve (AUDPC) increased with decreasing psi(m) and were higher in plants held at psi(m) values of -2.5, -5.0, or -10.0 J/kg for 10 days than 5 days before saturation. Leaf disks placed on free water in funnels during the saturation period became infected within 2 h, indicating that zoospores were present in the soil water. Germination of oospores in soil extract was predominantly by production of sporangia and increased from 8 to 47% between 5 and 10 days. Oospores of P. capsici may have germinated and formed sporangia in soil held at psi(m) values of -2.5, -5.0, or -10.0 J/kg, and zoospores released during brief periods of soil saturation probably acted as primary infective propagules.