Evaluation of surfactants and new oomycete fungicides for the control of Phytophthora root rot of citrus, caused by Phytophthora parasitica

The toxicity and efficacy of a commercial soil surfactant (Naiad ®) and new fungicides were evaluated against Phytophthora parasitica and Phytophthora root rot of citrus. The effect of Naiad ® on Phytophthora root rot of navel orange trees on sweet orange rootstocks, total microbial soil populations, citrus tree health, and soil properties was also evaluated in a multi-year field trial. Mycelial growth was insensitive to Tween 20, Tergitol NP-10, and Naiad® at concentrations as high as 200 μg a.i./ml. The duration of zoospore motility and the production of sporangia were significantly reduced in the presence of these surfactants. Naiad® at 20, 100, and 200-μg a.i./ml penetrated columns of soil 5.1, 10.2, and 20.3 cm in length and effluents reduced the duration of zoospore motility to less than 30 min.; In greenhouse experiments, Naiad® treatments reduced the incidence and severity of Phytophthora root rot of citrus, regardless of soil inoculation method (zoospores or chlamydospores). Application of 200 μg/ml Naiad® with each watering was the only treatment comparable to the standard fungicide treatment (Ridomil 2E or Ridomil Gold EC) in all trials. Except root flush applications, all Naiad® treatments reduced Phytophthora root rot. Naiad® applied during every irrigation at 100 μg a.i./ml and Ridomil 2E applications reduced soil populations of P. parasitica and increased rootstock diameter and fresh root weight of trees as compared to the control. Surfactant and fungicide treatments had no apparent negative effect on total soil bacterial or fungal populations, plant health, or soil properties. Application of Naiad ® in the first 12 h of a wetting cycle reduced colonization of leaf baits for up to 108 h.; Mefenoxam, with EC₅₀ values ranging from 0.02 to 0.81 and 0.01 to 0.04 μg/ml, was the most inhibitory fungicide to P. parasitica mycelial growth and sporangia production of the fungicide evaluated. Fenamidone was the least effective with EC₅₀ values ranging from 0.08 to 1887 and 1.44 to 50.10 μg/ml, respectively. Zoospore motility was most inhibited by fenamidone (EC₅₀ ≤ 0.08 μg/ml). Single applications of these fungicides at 2 μg/ml significantly reduced Phytophthora root rot, as compared to water controls, in greenhouse trials.