Effects of boron and mefenoxam on growth of the isolates of Phytophthora cinnamomi obtained from avocado orchards

Avocado root rot caused by Phytophthora cinnamomi is a major disease of the crop, management of which can be accomplished, partially, by reducing the amount of the pathogen inoculum in the soil. Boron is known to have fungicidal properties and has been used in controlling plant diseases. Although boron at a concentration of 1 μg mL −1 led to increased mycelial growth of P. cinnamomi , increasing the dose rate to 10 μg mL −1 decreased colony diameter; no mycelial growth occurred at 1000 μg mL −1 . In the absence of inoculation with P. cinnamomi or treatment with boron (negative controls), avocado seeds germinated and emerged healthy, whereas no germination occurred in seeds of the positive control (inoculated with P. cinnamomi ; no boron) treatment, or seeds inoculated with P. cinnamomi and treated with different concentrations of boron. Seeds treated with 500 μg mL −1 boron were more severely infected with P. cinnamomi than the positive controls, but seed infectivity of the pathogen decreased with increasing dose rates of boron. Although little fresh roots growth was observed even in the roots of the positive control seedlings (inoculated with P. cinnamomi and not treated with boron), almost no fresh roots formed on boron-treated plants, while negative control seedlings (non-inoculated with P. cinnamomi , not treated with boron) formed abundant fresh roots. However, as the applied boron dose increased, there was a decrease in the severity of root crown rot of the seedlings. All P. cinnamomi isolates were sensitive to mefenoxam with EC 50 values between 0.002 and 0.3 μg mL −1 .