Avocado trees are highly susceptible to attack from the water mould Phytophthora cinnamomi that causes phytophthora root rot. The fungus like organism attacks the roots of the avocado tree and leaves the tree with a poorly functioning root system. The tree wilts, causing leaf drop, branch dieback and fruit drop, and slowly dies. A comprehensive management method has been developed for managing phytophthora root rot called the Pegg Wheel. However, the most significant part of this management remains the use of the chemical phosphite, which was developed during the 1970s and 1980s.
Phosphite is produced by the dissociation of phosphonate ions. The phosphonate is supplied agronomically either with an injection or by foliar spray. The first chemical used to supply the phosphonate ions into the tree was Fosetyl-Al. It is an organophosphate composed of ethyl phosphonate anions and aluminium cations. Later developments saw Fosetyl-Al replaced by potassium phosphonate solutions (usually called phosphorous acid or phosphonic acid). It has been suggested that the phosphite itself can act directly on the Phytophthora organism by causing accumulation of excessive amounts of polyphosphate and pyrophosphate. The accumulation of these compounds causes inhibition of important cellular processes that leads to the death of the organism (McDonald, Grant and Plaxton, 2001). Other hypotheses suggest that there are a few sites of control in addition to the one describe above. There are interactions with the plants own defences and the phosphite and phytophthora organism as well. The phosphite ion (H2PO3-) is not useable by the plant as a replacement for phosphate (H2PO4-, HPO4) despite repeated research efforts trying to determine if it does, but is probably also useful as a bio stimulant (Achary et al., 2017). It can suppress normal phosphate deficiency responses in plants such as increased root growth (Gómez-Merino and Trejo-Téllez, 2015).
Early research showed the benefits of ‘suppressive soils’ in management of phytophthora in 1974 and had produced some root rot resistant rootstocks too. The research into chemical control began in the late 1970’s, with the use of a soil drench of metalaxyl (Broadbent and Baker, 1974; Allen et al., 1980; Ben-Ya’acov and Michelson, 1995). Metalaxyl is a broad spectrum systemic fungicide commonly used on oomycete organisms (which is what phytophthora is). Metalaxyl applied to both seedlings and established 7-year trees was able to encourage increased growth of feeder roots and was able to briefly suppress chlamydospore (a reproductive structure of phytophthora) production. In 1983, research in South Africa compared Fosetyl-Al and metalaxyl, applied as tree injections, and found that Fosetyl-Al resulted in a greater reduction in phytophthora symptoms than metalaxyl (Darvas, Torien and Milne, 1983). Root growth itself was not measured in this instance. Five years of foliar Fosetyl-Al application, starting in 1978 in South Africa, was found to be less effective at reducing phytophthora inoculum than metalaxyl (applied as a drench), but produced healthier trees (Darvas, 1983), hinting at the interaction with the plants own defences from the Fosetyl-Al.
Further research into the use of metalaxyl (soil drench), Fosetyl-Al and phosphorous acid (both injected) for the management of phytophthora occurred in Queensland, Australia. Similar to the earlier South African research, metalaxyl was less able to manage phytophthora root rot symptoms than Fosetyl-Al and phosphorous acid. In very real terms, fruit yields from the metalaxyl treatments was 4.3kg per tree, as compared to 53.7kg per tree in the Fosetyl-Al treatment, and 55.4kg per tree with the 10% phosphorous acid injection. When 20% of phosphorous acid was injected, yield increased further to 67.5kg per tree (Pegg et al., 1987). A residual effect in the roots was found to last up to three years, however, the authors were cautious to note that reduced sensitivity to the phosphite had already been noticed in France and therefore advised caution with the use of these chemicals. It was for this reason that the integrated management method called the ‘Pegg Wheel’ was developed; to combine chemical use with benefits provided by the ‘suppressive soils’ discovered in 1974 in northern Australia.
Further work with phosphorous acid followed in the 1990’s to improve the technique, particularly with regard to the application timing in relationship to vegetative and root flushes. Application of phosphorous acid as an injection led to the greatest root phosphite levels when applied to trees that had only mature leaves and no active growth flush (Pegg and Whiley, 1990). The difference in timing is substantial, with root phosphite levels reaching only 6mg kg-1 when vegetative flush was occurring, but reached 30 mg kg-1 when applied when leaves were mature. A foliar method of application was also evaluated from 1998 to 2000 by Whiley et al. (2001). While the foliar spray was as effective as the trunk injection in some circumstances, it did not appear to be quite as effective when the tree was carrying a heavy crop load. Staghorning (heavy pruning down to main trunk structure), combined with follow-up foliar sprays were very effective at improving the health of trees showing advanced phytophthora root rot symptoms.
The research demonstrated that the movement of phosphonate through the tree was influenced by changing sink strength. This knowledge has now led to the current recommendation to apply potassium phosphonate (phosphorous acid) during the autumn/winter period when the roots are actively growing and when the leaves are not. While the phosphonate does still move into the leaves immediately after application in the first 24 hours, after a few days it moves down to the roots (Whiley et al., 1995). Whiley recommended a phosphite level in the roots of 20-40mg kg-1 for adequate control. This has led to the current recommendation of a minimum root phosphite content of 25mg kg-1. Take note that this is the minimum targeted level. Phosphite levels do slowly drop, so to achieve this level during the spring and summer requires a level of up to 150mg kg-1 during autumn and winter.
That recommendation may need to change due to decreasing sensitivity of phytophthora to phosphite. In orchards in both South Africa and Australia the use of Phosphorous acid sprays has led to decreased sensitivity to phosphite (Duvenhage, 1994; Dobrowolski et al., 2008; Dann et al., 2017). Data does show that sensitivity is retained at higher root levels above 80 mg kg-1 so this has been suggested as a new minimum level needed for adequate control (Graeme Thomas, Pers. Comm. 2019). Verifying the levels is done by the testing of feeder roots in a qualified laboratory. When levels are low, more applications are required. While it is concerning that there is decreasing sensitivity, it has been suggested that the effectivity of phosphite is due to a relationship between the plant, phytophthora, and the phosphite; when a root with high phosphite levels is attacked by the phytophthora it undergoes a hypersensitive response and produces new feeder roots to replace those attacked, thus ensuring the maintenance of the root system (Ken Pegg, Pers.comm. 2019). Other research has also suggested that phosphite upregulates plant defences to the pathogen.
While more than one publication has suggested upregulation of plant host defences occurs, the exact reason for this effect is not clearly explained. The most direct mention of the effect is in research into the use of phosphite in jarrah seedlings. It was found that at low root phosphite levels, the plant defences were activated (quantified by measurement of plant defence enzymes), while at higher phosphite levels the plant defences were not upregulated (Jackson et al., 2000). Another interesting interaction has been observed between phosphate and phosphite concentrations. When higher concentrations of phosphate are present, the efficacy of phosphite is reduced (Ma and Mcleod, 2014). The implication of that research is that some studies on phosphite’s effect on phytophthora may have overestimated the efficacy of the phosphite as they used low concentrations of phosphate in their experimental designs. Even higher phosphate levels may be found in roots in the real world suggesting there is more work to be done exploring the relationship.
Years of research into applications of phosphorous acid has resulted in very good management of phytophthora root rot. The best control is currently known to be achievable with applications during the period when the roots are growing and the leaves are not—namely autumn/winter. Root testing has been useful in Australia and may be replicated in South Africa as well (Ma and Mcleod, 2014). Root testing is well established and used in Australia and can allow effective monitoring of phosphite levels. The minimum phosphite level in roots needs to be maintained at least above 25 mg kg-1 but better control may be achieved with a higher minimum level of 80-100 mg kg-1. While the suggested change to higher values is due to the appearance of less sensitive phytophthora isolates, it would be premature to say that the life span of phosphite is limited. Phosphite has been used for a very long time and will likely remain useful for many more years, especially when paired with the integrated management procedures described in the Pegg wheel.
For more information on phosphite use please visit the Avocados Australia website.
References
Achary, V. M. M. et al. (2017) ‘Phosphite: a novel P fertilizer for weed management and pathogen control’, Plant Biotechnology Journal, 15(12), pp. 1493–1508. doi: 10.1111/pbi.12803.
Allen, R. N. et al. (1980) ‘Fungicidal control in pineapple and avocado of diseases caused by phytophthora cinnamomi’, Australian Journal of Experimental Agriculture, 20(102), pp. 119–124. doi: 10.1071/EA9800119.
Ben-Ya’acov, A. and Michelson, E. (1995) ‘Avocado rootstocks’, Horticultural Reviews, 17(1383), pp. 381–429. doi: 10.1002/9780470650585.ch11.
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Darvas, J. M. (1983) ‘Five years of continued chemical control of Phytophthora root rot of avocados’, pp. 72–73.
Darvas, J. M., Torien, J. C. and Milne, D. L. (1983) ‘Injection of Established Avocado Trees for the Effective Control of Phytophthora Root Rot’, California Avocado Society, 67(1955), pp. 141–146.
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Gómez-Merino, F. C. and Trejo-Téllez, L. I. (2015) ‘Biostimulant activity of phosphite in horticulture’, Scientia Horticulturae. Elsevier B.V., 196, pp. 82–90. doi: 10.1016/j.scienta.2015.09.035.
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McDonald, A. E., Grant, B. R. and Plaxton, W. C. (2001) ‘Phosphite: its relevance in agriculture and influence on the plant phosphate starvation response’, Journal of Plant Nutrition, 24(10), pp. 1505–1520. doi: 10.1081/PLN-100106017.
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Whiley, A. W. et al. (1995) ‘Changing sink strengths influence translocation of phosphonate in avocado (Persea americana Mill.) trees’, Australian Journal of Agricultural Research. doi: 10.1071/AR9951079.