phosphonates and its use for managing phytophthora root

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INTRODUCTION Phosphonate fungicides are alkali metal salts of phosphorous acid. In plants, phosphonates dissociate into several anions, hereafter referred to as phos-phite. Phosphite anions are involved in the control of Oomycete plant patho-gens such as Phytophthora cinnamomi.

In South Africa, three phosphonate fungicide formulations are widely used, including ammonium phosphonate, potassium phosphonate and alkyl phos-phonate. In avocado, mainly potassium phosphonates (Fighter®, Avoguard® and Rootmaster 98®) are used and are registered for application as trunk injec-tions to manage root rot caused by P. cinnamomi. Based on tree health, two different dosages are applied; 0.3 g a.i./m2 canopy diameter for the treatment of healthy looking trees (preventative treatment) or mildly diseased trees, and 0.5 g a.i./m2 canopy diameter for severely diseased trees.

An alkyl phosphonate, also known as fosetyl-Al (Aliette®) is registered as a foliar spray on avocado on established trees, and on nursery trees as a root drench and foliar spray. However, Aliette foliar sprays are not used on established trees commercially, due to the high cost of the product. In South Africa, Brilliant® is the only ammonium phosphonate that is registered for use as a fungicide.

Trunk injection timing and resulting phosphite residues in fruit It has long been known that the time of application of phosphonates is very im-portant for the effective management of Phytophthora root rot on avocado. This is due to the source-sink transloca-tion of phosphite.

Phosphonates and its use for managing Phytophthora

root rot on avocadoAdéle McLeod STELLENBOSCH UNIVERSITY

Two application windows are recom-mended for the application of phospho-nates on avocado, which coincide with the two root flushes. The two applica-tion windows consist of the periods after the summer- and spring foliar flushes have hardened off. Applications made after the summer foliar flush has hardened off (usually from February to April, depending on the cultivar and production region) is deemed the most effective for phosphonate applications. This is most likely due to the fact that there are no fruit on trees that will also act as a strong sink for phosphite.

The application window follow-ing the hardening off of the spring foliar flush, usually around November/December, is a less effective applica-tion window, likely due to small fruit on trees being a strong competitive sink for phosphite in addition to roots. Products registered as trunk injections in South Africa recommend application during both application windows. The latter approach is also supported by published research from Darvas et al. (1984). However, due to the enforce-ment of maximum residue limits (MRL) for fosetyl-Al in 2013, some growers have started avoiding applications after the spring flush has hardened off. This is due to the fact that this application window contributes to high fruit resi-dues with small fruit being a strong sink for phosphite.

The long-term effect of injecting phosphonates only after the summer flush has hardened off is still unknown, but it is likely to result in a reduced ef-ficacy in Phytophthora root rot control.

Problems with MRLs result-ing from stem injection.Information on strategies which will limit MRLs at or below the EU restric-tion, but still provide effective control, is

limited. This is partly due to our lim-ited understanding of the temporal nature of phosphite translocation in avocado trees.

It is known that phosphite is trans-located in a source-sink nature at the time of application (Whiley et al., 1995). However, what is unknown is whether re-mobilization of phosphite can take place in trees. In apple trees, phosphite residues were still present in buds and fruits two years after phosphonate application, suggesting a remobilization of phosphite in plant storage organs to fruits (Malusa and Tosi, 2005).

Evidence for the re-mobilization of phosphite to fruits following initial application has also been evident in avocado trials conducted by Stel-lenbosch University and ZZ2. In trials conducted over two seasons, application of phosphonates only after harvest in fall when no fruits were present on trees, resulted in fruit residues that were significantly higher than the untreated control treatment (Fig. 1). Another aspect which is poorly understood regarding phosphite translocation to fruits, is whether translocation of phosphite and re-mobilization of phosphite to fruit differs due to the method of application (foliar sprays versus injections).

Potential of foliar sprays to reduce the risk of exceed-ing EU MRL limitationsThere is currently no data to indicate whether application method has an influence on fruit residues. However, trials in South Africa have shown that when phosphonates are applied as 1% a.i. foliar sprays only after har-vest, fruit residues can be lower than when a trunk injection (1 g a.i./m2)


root rot on avocado


root rot on avocado


root rot on avocado


root rot on avocado


root rot on avocado


root rot on avocado


root rot on avocado


root rot on avocado


root rot on avocado

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Figure 1: Fosetyl-Al fruit residues (mg/kg) for avocado fruits obtained from avo-cado trees receiving different phosphonate treatments in trials conducted in the (A) 2016/17 (‘Maluma Hass’ on ‘Duke7’) and (B) 2017/18 seasons (‘Maluma Hass’ on ‘Dusa’ [Mookeetsi] and ‘Carmen’ on ‘Dusa’ [Letaba]). Phosphonate products applied as foliar sprays included a potassium phosphonate (Fighter®) and ammonium phos-phonate (Brilliant®). Following each treatment name, in brackets, are the number of applications that were made in fall (after harvest) and summer (after the spring foliar flush hardened off). The red dotted line indicates the European Union residue limit. Histogram bars of the same colour that have the same letters do not differ signifi-cantly from each other (P > 0.05) based on Fisher’s least significance test.

Several new trials are in process of being established for the 2018/19 season in order to try and finalise the registration data for registering foliar sprays on avocado in South Africa.

It is difficult to estimate the cost of foliar phosphonate applications in comparison to trunk injections due to variables associated with low-density orchards (low number of trees/ha) versus the newer high-density orchards (higher number of trees/ha). For the high density orchards, four or five foliar potassium- or ammonium phosphonate sprays will be slightly less expensive than two trunk injections, considering product and application costs. However, with increasing labour costs, in future,

foliar sprays are likely to become even more cost-effective in comparison to trunk injections.

Conclusion In conclusion, phosphonates remain an important component of managing Phytopthora root rot on avocado. The products are, however, problematic with regard to market access due to fruit residue problems.

When using phosphonates in a preventative management strategy, foliar applications have the potential for avoiding exceedances of the EU MRL. However, maintaining residues below that required by certain German supermarkets (1/3 of EU MRL) will be problematic based on trials conducted thus far. For severely diseased avocado

TO PAGE 32Spray volume =

tree height × tree canopy diameter × 900

row width

was applied only after harvest (Fig. 1A). The phosphonate dosages used for the aforementioned trunk and foliar sprays yielded similar Phy-tophthora root rot control; the treat-ments resulted in comparable suppres-sion of the pathogen in the roots (Fig. 2) and root phosphite concen-trations (data not shown).

It was also evident from trials that the fruit residues from foliar sprays were somewhat more ‘predictable’ than those from trunk injections. At some trial sites, the registered trunk injection dosage resulted in residues that exceeded the MRL, whereas at other sites it did not (Fig. 1).

Foliar sprays have the potential to reduce fruit residues and have the ad-vantage of not being as labour inten-sive as is trunk injections. Preventative foliar potassium phosphonate sprays (0.5% a.i.) have been used success-fully for several years in the Australian avocado industry for the management of Phytophthora root rot.

Foliar sprays are only effective as a preventative treatment before trees show symptoms and on mildly affected trees. This is due to the fact that severely diseased trees do not have enough foliage to take up phos-phonate foliar sprays.

In South Africa, registration trials have been conducted for the past two seasons (2015/16 and 2017/18) in or-der to register potassium phosphonate (Fighter®) and ammonium phospho-nate (Brilliant®) as foliar sprays. The trials conducted by Stellenbosch Uni-versity and ZZ2 in the Mooketsi and Letaba regions have shown that four or five 0.5% a.i. foliar sprays applied after harvest at weekly intervals will be effective in suppressing Phytophthora root rot in a preventative management strategy.

When applying this high dosage fo-liar sprays, it is important to adjust the pH of the foliar sprays to a pH of 7.2, in order to avoid phytotoxicity. The vol-ume of spray applied is also important, since it not only affects costs but also efficacy. In trials conducted in South Africa, a spray volume calculated using the following Unrath formula have been found effective:

A

B

2016/17 trials

2017/18 trials

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Figure 2: Effect of different phosphonate treatments applied to avocado orchards on the amount of Phytophthora cinnamomi produced by avocado roots sampled in the orchards from each treatment. The data are for (A) a trial conducted in the 2016/17 season in Letaba and (B) two trials conducted in the 2017/18 season. In the latter two trials, there were no significant trial x treatment interaction (P = 0.722 ), and there-fore the data of the two trials were combined. The potassium phosphonate used was Fighter® and the ammonium phosphonate was Brilliant®. For each histogram, bars followed by the same letter do not differ significantly from each other (P > 0.05) according to Fisher’s least significance difference test.

trees suffering from Phytophthora root rot, trunk injections twice or three times a year remains the only option when using phosphonates. In these situations, it seems that there is as yet no solution to the balance between ef-fective disease management and fruit residues.

It is important to use an integrated management strategy for managing root rot, which will assist in a faster and a more sustained recovery of diseased trees. The most important management strategies include proper irrigation scheduling, planting on ridges, using tolerant rootstocks and the regular application of mulches and organic material to orchards.

It is also of utmost importance that, when new orchards are established, young trees should be protected against Phytophthora root rot for the first three years from planting. The ap-plication of a phenylamide soil drench at planting is important, followed by regular applications of phosphonates according to label recommendations.

AcknowledgementsThe information and data provided within this article, would not have been possible without the support of the following institutes and people: • The South African Avocado Grow-

ers' Association, the main funder of phosphonate research projects, which first started in 2013;

• Rolfes Agri and Arysta for financial support of the phosphonate re-search, and their interest in register-ing foliar phosphonates;

• Siyethemba Masikane, a Stellenbosch University honours and MSc student involved in the research trials;

• Precious Novela and Phillimon Mo-hale from ZZ2 for conducting the spray trials and research inputs;

• ZZ2, including Pieter Pieterse, for initiating the research of finding alternatives to phosphonate trunk injections and partial funding of the initial research;

• Bekker Wessels (ProCrop) for critical

reading and editing of the article and useful discussions;

• Australian colleagues Graeme Tho-mas, Liz Dann and Tony Whiley for useful discussions.

REFERENCESDARVAS, J.M., TOERIEN, J.C. and MILNE,

D.L. 1984. Control of avocado root rot by trunk injection with phosetyl-Al. Plant Disease 68: 691-693.

MALUSA, E. and TOSI, L. 2005. Phos-

phorous acid residues in apples after foliar fertilization: Results of field trials. Food additives and Contami-nants 22: 541-548.

WHILEY, A.W., HARGREAVES, P.A., PEGG, K.G, DOOGAN, V.J., RUDDLE, L.J., SRANAH, J.B. and LANGDON, P.W. 1995.Changing sink strengths influence translocation of phos-phonate in avocado trees (Persea Americana Mill.) trees. Aust. J. Agric. Res. 46: 1079-1090.

Phosphonates and its use for managing Phytophthora root rot on avocadoPhosphonates and its use for managing Phytophthora root rot on avocadoPhosphonates and its use for managing Phytophthora root rot on avocadoPhosphonates and its use for managing Phytophthora root rot on avocadoPhosphonates and its use for managing Phytophthora root rot on avocado

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