Control of Phytophthora cinnamomi by soil application of calcium fertilizers in a Dehesa ecosystem in Spain

Abstract: Dehesa Los Bueyes is an open woodland located at Huelva province (southern Spain), mainly composed by mature Quercus ilex suffering the root rot disease caused by the soilborne pathogen Phytophthora cinnamomi with high levels of tree mortality recorded. The pathogen was firstly isolated from soil and root samples from symptomatic oaks analyzed 12 years ago. Field experiments to prevent the spreading of the disease were conducted at Los Bueyes by soil application of calcium fertilizers. Three plots (200×50 m) were marked and treated with CaCO3 (3000 kg/ha) or CaSO4 (3500 kg/ha), remaining unfertilized the third plot (control). Soil samples were taken in three different points per plot in autumn 2011, just before fertilizations, and in late spring and autumn 2012. The evolution of the inoculum density of P. cinnamomi in the soil was assessed by extension of soil suspensions on NARPH selective medium and counting of P. cinnamomi colonies obtained, but significant differences were not achieved. Inoculum infectivity was evaluated by baiting (eucalyptus leaves) on soil-water suspensions (1:40). Samples taken in spring 2012 showed significantly lower levels of infectivity in fertilized plots compared with control samples, but this reduction remained only for CaSO4 in samples taken in autumn 2012. Consequently, fertilization with CaSO4 appears as the best option to minimize oak root infections.