Effect of soil solarization on the control of Phytophthora root rot in avocado

Abstract

Phytophthora root rot is of paramount importance in avocado orchards of southern Spain. Soil solarization has been demonstrated to control the pathogen in infested areas from which infected trees had been removed. We aimed to determine whether soil solarization in established avocado orchards controls the disease. Soil solarization increased average maximum hourly soil temperatures by 6.5 6-9°C in unshaded areas of avocado orchards in coastal areas of southern Spain, depending on depth and year. The corresponding temperatures in shaded areas were c. 2-3°C lower. P. cinnamomi in soil, on infected avocado rootlets, and in a nutrient substrate buried at 30-60cm depth was reduced to negligible amounts after 6-8 weeks of solarization in both unshaded and shaded locations of avocado orchards. P. cinnamomi could not be detected in avocado rootlets up to 14 months later, suggesting a long-term effect. Soil solarization did not affect growth of the trees, and fruit yields were increased as compared with control plots. Following soil solarization for 3 weeks from mid-July 1994, when maximum hourly temperatures reached 33-36°C, P. cinnamomi could not be recovered from a depth of up to 45cm in unshaded areas or from a depth of up to 30cm in shaded areas after the initial 10-day period. The viability of inoculum of the pathogen buried at depths between 15 and 60cm in bare soil was determined by sequential sampling in two solarization experiments starling 12 June and 4 July 1995, respectively. In the first experiment, P. cinnamomi could not be detected at any depth after 4-8 weeks of solarization in unshaded areas but could be recovered at all depths except 15cm in shaded areas. In the second experiment, where temperatures were higher and the soil surface not shaded, P. cinnamomi could not be recovered after 2 weeks at 15 and 30cm.