2014 ASHS Annual Conference

Efficacy of chlorine to control Phytophthora nicotianae in the presence of nitrogen salts or increased concentrations of peat was evaluated under laboratory conditions.  Solutions with peat at 0, 10, 20, 40, or 80 mg·L^-1 were inoculated with P. nicotianae at 10,000 zoospores·mL^-1 and were then combined with 0, 2, or 4 mg·L^-1 of chlorine with 10 minutes contact time.  In a separate experiment, ammonium sulfate ((NH₄)₂SO₄) or potassium nitrate (KNO₃) at 50 mg·L^-1 nitrogen were combined with 0, 2, or 4 mg·L^-1 of chlorine and inoculated with P. nicotianae with 10 minutes and 24 hours contact time. In the absence of chlorine, peat, or nitrogen, 92% or 81% of tomato leaves were infected after 10 min or 24 h, respectively. Increasing chlorine concentration decreased the infection of tomato leaves exposed to the solutions to ≤12.5% and to 0% infection after 10 minutes and 24 hours, respectively.  Peat at all concentrations levels did not affect the efficacy of chlorine in controlling tomato leaf infection.  Nitrogen did not affect tomato leaf infection. Nitrogen decreased the density of viable zoospores by more than 58% in a zero chlorine solution compared with the no nitrogen control, but had no effect on zoospore density at 2 or 4 mg·L^-1 applied Cl.  Ten minutes contact time was therefore adequate for both free chlorine (hypochlorous acid) and total chlorine (nitrogenated chlorine compounds chlorine) to control zoospores.  Despite the positive results of chlorination under these controlled conditions, filtration and extended contact time of chlorine are recommended to reduce chlorine demand in commercial applications until more organisms are tested.