Compost suppressiveness against Phytophthora spp. on Skimmia japonica and azalea

Suppression of soil-borne plant diseases with composts has been widely studied. Composts have been found to be suppressive against several soil-borne pathogens in various cropping systems. Ornamental plants are generally cultivated in pots, allowing the use of suppressive substrates to control zoospore-producing pathogens, like Phytophthora sp. The objective of the present work was to assess compost suppressiveness against Phytophthora cinnamomi on Rhododendron spp., and against Phytophthora nicotianae, an emerging pathogen on Skimmia japonica. A municipal compost that showed a good suppressive activity in previous trials on vegetable crops was used. Compost was mixed at 10, 20 e 40% (v/v) with a commercial peat substrate, used as control. Substrates have been inoculated at 1g/l dosage of wheat and hemp kernels of Phytophthora spp. and after one week 15-20 plants were transplanted for each treatment in 2 liters volume pots and placed in greenhouse at 20 degrees C. A chemical control (Metalaxil-M) was also used. Diseased plants were assessed weekly after transplanting and above-ground biomass of plants was assessed at the end of the trials. Results showed a significant disease control when compost was used at 20-40% on S. japonica, without showing any phytotoxic effect. Disease suppression was shown at 40% on azalea, but compost was slightly phytotoxic on plants. The use of compost based substrates can be a suitable strategy for controlling soil-borne diseases on ornamentals, but results depend also on alkalinity tolerance of plants.