Abstract
Organic avocado production in California has been increasing in recent years; approximately 5% of acreage is now organic or has the potential to be organic. Avocado root rot caused by Phytophthora cinnamomi is still the most important disease, and an organically approved substitute for phosphonate treatments for root rot control would be highly desirable. Work done in South Africa indicated that potassium silicate (a naturally mined substance) appeared to induce at least a temporary tolerance to root rot in mature avocado trees. Some of the avocado growers in California also believe that saponin derived from yucca juice or extract from Quillaja saponaria may also have the ability to control root rot. This potassium silicate work was repeated on replant trees in California using ‘Hass’ grafted on clonal ‘Duke 7’ rootstocks planted in a grove where mature avocado trees had died from root rot. Trees in the fi eld trial were treated with root drenches just before planting in July 2007 and thereafter three times in 2007 and fi ve times in 2008. The drenches included low and high rates of potassium silicate (1% and 2% Silmatrix, 2 L/tree), low and high rates of Quillaja extract (2% and 4% QL Agri, 2 L/tree), a buffered phosphorous acid drench (30 mL/L of 0–28–25, 2 L/tree), and a water control. There were 20 replications for each treatment. Results for the potassium silicate and the Quillaja extract were disappointing compared to the buffered phosphorous acid with fi nal tree health ratings (0 = healthy, 5 = dead) as follows: four trees at stage 4 and 5 low rate potassium silicate, seven trees at stage 4 and 5 high rate potassium silicate, four trees at stage 4 and 5 low rate Quillaja extract, fi ve trees at stage 4 and 5 high rate Quillaja extract, two trees at stage 4 and 5 phosphorous acid, and nine trees at stage 4 and 5 water control. Tree growth (as measured by tree height) was also disappointing for the potential organic treatments.
