A Definitive Test to Determine Whether Phosphite Fertilization Can Replace Phosphate Fertilization to Supply P in the Metabolism of 'Hass' on 'Duke 7' (A Preliminary Report)

Phytophthora root rot causes extensive damage to feeder roots of avocado. If uncontrolled, it results in a progressive decline in tree health and the eventual death of the tree (12, 14, 15). Phytophthora root rot is a major factor limiting avocado production world wide. Effective control of Phytophthora root rot of avocado has been achieved with fosetyl-al (Aliette® Rhone-Poulenc, aluminum tris-o-ethyl phosphonate) through either foliar application (1) or trunk injection (5). Coffey and Bower (3) suggested that phosphorous acid was the toxophore responsible for inhibition of Phytophthora spp. when host plants were treated with fosetyl-al. Degradation of fosetyl-al to phosphorous acid within plant tissues was confirmed by Saindrenan et al. (11). Subsequently, Darvas and Bezuidenhout (4) reported that phosphorous acid was more effective in controlling avocado root rot than fosetyl-al. For avocado growers, potassium phosphite (the salt which results from the neutralization of phosphorous acid with KOH) used as a trunk injection is an inexpensive and effective product for Phytophthora root rot control (2). Trunk-injected potassium phosphite initially moves to the leaves via the xylem and then to the roots with the photosynthate via the phloem (7). Thus far, it appears nontoxic at fungicidal dosages to either plants or animals. Potassium phosphite has been used in extensive injection programs with citrus and avocados in South Africa and Australia with no apparent toxicity, to date. In California, neither phosphorous acid, nor its salt, has been registered for use as a fungicide. To my knowledge, no commercial sponsors are pursuing registration. Demonstration that phosphite can replace phosphate fertilizer as a source of P in avocado metabolism may provide a scientific basis for avocado growers to use phosphite.