Thermal sensitivity of Phytophthora cinnamomi and long‐term effectiveness of soil solarisation to control avocado root rot

Root rot caused by the fungus Phytophthora cinnamomi is a major disease of avocados worldwide. Heat sensitivity of a collection of P. cinnamomi isolates was determined by exposing agar discs containing mycelium or mycelium plus chlamydospores at various temperatures for different periods. Long-term effectiveness of soil solarisation to control Phytophthora root rot was evalu- ated in two field trials. In the first, soil disinfestation by solarisation was applied in 1990 to a naturally infested plot before planting avocado (Persea americana) and vinatigo (Persea indica) seedlings. In the second trial, estab- lished avocado trees were solarised for four consecutive summers (1996- 1999). Results for heat sensitivity showed that fungal mycelium was inactivated after 1-2 h at 38� C. However, 1-2 h at 40� C was needed to kill all propagules when chlamydospores were present. Fungal growth inhibition after thermal treatments was related to levels of time and temperature, and detrimental effects occurred as consequence of sublethal thermal doses. Soil solarisation presented long-term positive effects when applied as a pre- planting treatment. Five years after solarisation, disease severity (0-5 scale where 0 = healthy and 5 = dead plant) of avocado and vinatigo planted in solarised soil was 2.03 and 0.71, respectively, compared with 4.65 and 4.84 in controls. Eleven years after solarisation, the percentage of dead plants in so- larised soil was 73% for avocado and 43% for vinatigo but 100% in controls. In contrast, an insufficient level of control was observed in established or- chards, probably because of the lower temperature reached during solar- isation under the shade of tree canopy. In this situation, maximum temperatures at 5-cm depth were 10-13.7� C lower than under solar-heated mulch, only exceeding 40� C in 1997.