Proc. of Second World Avocado Congress 1992 p. 143

Use of Worm-Composted Sludge as a Soil Amendment for Avocados in Phytophthora-infested Soil

Gary S. Bender

Cooperative Extension, Building 4, 5555 Overland Avenue, San Diego, CA 92123, USA

 

William L. Casale and Mohamad Rahimian

Department of Plant Pathology, University of California, Riverside, CA 92521, USA

Abstract. Avocado root rot (caused by Phytophthora cinnamomi) is a devastating disease to avocados in San Diego County. Avocados replanted into infested soils have often been unsuccessful. An experiment was conducted to determine if organic-amended mounds could improve survival of replant trees in infested soil. A new product, known as vermi-compost (VC), was tested. VC is an earthworm casting product produced from a mixture of composted sewage sludge and straw.

VC or composted chicken manure was mixed into the top 30 cm of mounds at a rate of 20 L compost per 20 L soil. 'Hass' on G755 were planted into the mounds one week after mounds were amended. Survival rate of trees after one year was: 5/6 (VC, mound), 4/6 (Aliette foliar spray, mound), 2/6 (mound alone), 1/6 (composted chicken manure, mound), 2/6 (flat ground, Aliette foliar spray), and 2/6 (flat ground). Information on growth rates and tree health will be presented.

Greenhouse trials were conducted to examine the protective effect of VC in nursery mixes. Potting mix (UC-5) was amended with VC to a final volume of 0, 12.5%, 25%, and 50% VC, infested with P. cinnamomi on ground millet (0.1 g/L soil) and transplanted with 8-week-old Persea indica seedlings (16 trees per treatment). The effect of VC was most clearly observed at 6 weeks after transplant; only 2 plants in the 50% VC treatment were dead, whereas all 12 plants in the treatment without VC were dead. After 24 weeks, all plants were dead.

A total of 14 different bacteria and 16 different fungi were isolated from the rhizosphere of surviving plants. Trichoderma was the most common fungus found in the 50% VC treatment with an average population of 7 X 10⁵ colony-forming units per root.