Chemical and biological control of Phytophthora root rot of container-grown citrus plants

The production of potted ornamental citrus plants is a very important sector of the citrus industry in Sicily. A major disease of container-grown citrus is Phytophthora root rot caused by Phytophthora nicotianae and P. citrophthora. The effectiveness against this disease of the phenylamide fungicide kiralaxyl (ISAGRO, Italy), an isomer of benalaxyl, was evaluated in comparison with mefenoxam and Remedier, a biological product based on Trichoderma harzianum and T. viride (10 8 CFU/g, corresponding to 2% of each a.i.), developed by ISAGRO for the control of soil-borne fungal diseases of ornamental and horticultural crops. Potted sour orange (Citrus aurantium) seedlings were used as test plants and soil of pots was infested with Phytophthora citrophthora grown on autoclaved wheat kernels plus V8juice-CaCO3. Products were applied as soil drenches 3 months after the infestation of the soil. The treatment was repeated one week later. Mefenoxam was applied at a concentration corresponding to 476 g a.i./ha. Kiralaxyl was appled at two different concentrations, corresponding to 120 (low dosage) and 476 g a.i./ha (standard dosage), respectively. Remedier (50 g of each a.i. /ha) was applied either alone or in combination with kiralaxyl at low dosage. The effectiveness of treatments was evaluated in terms of both percentage of infected roots, determined by isolation on a selective medium, and inoculum density in the soil determined by a real-time PCR assay. The most effective treatments were mefenoxam, kiralaxyl at the standard dose and the combination of Remedier and kiralaxyl. Both kiralaxyl at low dosage and Remedier alone reduced the amount of P. citrophthora inoculum in the soil and the percentage of infected roots compared to the untreated control but were less effective than the other treatments.