Effects of Gypsum Soil Amendments on Avocado Growth, Soil Drainage, and Resistance to Phytophthora cinnamomi

Phytophthora root rot of avocado, a soilborne disease caused by Phytophthora cinnamomi Rands, continues to be devastating to the cultivation of avocados despite more than 70 years of research on control methods. Phytophthora root rot has been estimated to be present in two-thirds of the avocado groves in California, causing losses of almost 30% of the total value of the crop (5). P. cinnamomi most commonly moves into a grove on tools, with infected nursery stock, and with water runoff from nearby sites infested with the pathogen. The pathogen destroys the small, absorbing roots of the avocado, leaving the affected tree with little water uptake ability. The aboveground symptoms of the disease are not obvious until the roots are heavily infected, and death of the tree usually follows within 1 to 2 years (17). Successful control of Phytophthora root rot has been difficult to achieve, due in part to the poor soil conditions in many California avocado groves. These soils are typically low in organic matter and drain poorly, providing an environment conducive to growth and spread of the pathogen. The use of rootstocks tolerant to Phytophthora root rot, cultural practices that increase the infiltration of water into the soil, and fungicides can be part of an integrated approach to lower disease incidence and severity. More recently, mulches have been shown to be efficacious in decreasing the level of avocado root rot in field trials (11), using a combination of organic materials similar to the successful Ashburner system used in some Australian avocado groves. The Ashburner system of soil amendments and cover cropping has been shown to create a soil environment suppressive to Phytophthora root rot (4). The factors contributing to this suppressiveness included high levels of exchangeable calcium, high organic matter, and high levels of exchangeable nitrogen. These conditions are not present in most California avocado groves. Amendments that increase the infiltration of water into the soil should decrease disease incidence. Calcareous soil amendments, such as CaSO4·2H2O, have been shown to decrease avocado root rot in some studies (6,16), although the mode of action has not been determined. It has been proposed that calcareous amendments could decrease disease by increasing water infiltration into the soil. In this study, gypsum was incorporated into California avocado soil in order to determine whether this calcareous amendment would decrease incidence and severity of root rot in avocado seedlings in a highly controlled greenhouse situation. The effects of gypsum amendments on infiltration of water into the soil, on avocado growth, and on disease resistance were examined in an attempt to explain the disease suppressiveness.