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Cocoa is prone to plant pathogens, especially Phytophthora which thrives on high humidity and surface wetness. Phytophthora causes two-dimensional disease in cocoa as Phytophthora pod rot and Phytophthora stem canker. Phytophthora pod rot is traditionally controlled by sprays of fixed copper fungicide including cuprous oxide.
Some aspects of Asian agronomy may pre-dispose cocoa to Phytophthora in both its pod rot and stem canker phases. For instance, in parts of Vietnam, where the cocoa industry is relatively new, durian trees are also infected by the same Phytophthora palmivora pathogen. These trees are shade species of choice for cocoa due to the quick and high financial rewards from durian fruit. A big question is whether the P.palmivora pathogen can move between these two susceptible tree species and mutually aggravate disease levels on both. Similarly, coconut, long established as a shade tree for cocoa and especially in Asia, is also susceptible to P. palmivora which causes bud rot disease in the palms.
Copper as a traditional ‘medicine’
Copper fungicides, which kill germinating spores on the pod surface through contact action, have been used for 100 plus years, while systemic fungicide, which enter plant tissue to suppress and stop further growth of established infections, have only a 40-year pedigree. Systemic fungicides are widely used on other crops but the economics of cocoa cultivation means relatively little progress has been made with these generally more expensive products on this commodity crop. Most progress with systemic fungicides has occurred in Asian cocoa due to several factors, including more modern and superior cocoa production economics, compared with Africa and South America, and the inherent inventiveness and innovation of Asian cocoa farmers and industries.
Copper is a broad spectrum fungicide which hits and disables the microbial metabolism at a number of different points and as such is called a multi-site action fungicide. Systemic fungicides are more specific, targeting just one site in the microbe’s metabolism. They are correspondingly called single-site mode of action fungicides and this delivers a much narrower activity spectrum compared with contact multi-site action fungicides like copper. For instance, copper’s broad activity spectrum covers all true fungi as well as many other microbes and epiphytic plant pests. Phytophthora is a member of the Oomycetes. It was traditionally regarded as a true fungus and originally placed in the ‘Fungi’ Kingdom. Phytophthora was later reassigned, together with the ‘water moulds’, to the Plant Kingdom ‘Chromista’ (the algae) which has since been renamed the ‘Chromalveolata’.
The phenylamide group of systemic ‘fungicides’ of which metalaxyl is best known are specifically active against the Oomycetes including Phytophthora. Phenylamide specificity against the Oomycetes is due to the unique ability to disrupt the correspondingly unique nuclear RNA synthesis pathways of these ‘Chromalveolata’ microbes.
Downside of such specificity is fungicide resistance, two words that still haunt agrochemical manufacturers who saw many new and costly-to-develop systemic products reduced to a fraction of their commercial usefulness and worth in a short space of time. Fungicide resistance is the development and eventual dominance of pathogen populations no longer susceptible to the action of the active ingredient used.
Resistance strikes single-site action fungicides because they present a much higher risk of single gene mutations arising by chance and capable of blocking the fungicide’s mode of action. The fungicide essentially selects out the resistant (non-susceptible) mutation which remains unscathed while rest of the pathogen population is destroyed. The resistant strain eventually predominates.
Phytophthora species resistant to fungicide actives of the phenylamide grouping began to appear in other crop situations before scientists got around to evaluating the products in earnest on cocoa. Occurrence of metalaxyl-resistant Phytophthora in other crops raised awareness and caution and tailored their use in cocoa accordingly.
Phenylamide fungicides were still used but in restricted ways that minimised risk of resistance. Much of the early work was carried out in Papua New Guinea using a range of application strategies for metalaxyl that minimised selection pressure on the Phytophthora pathogen population. These included the use of systemic fungicides in stem injections and canker paint but not as sprays targeted at pods or sprayed over the whole canopy. Sprays were used only when alternated with contact multi-site action fungicides like cuprous oxide or even in spray mixtures with cuprous oxide, the latter providing cover and protection against resistance development to the systemic fungicide. Reasoning behind such strategies is that if resistant populations did start to evolve then they would be destroyed by the broad spectrum multi-site action fungicide in the mixture. This was the rationale.
Tree injection damage
The phosphonates are another group of systemic chemicals evaluated for control of Phytophthora in cocoa. They do work but sometimes cause considerable confusion in more ways than one. Phophonates is a general term that covers phosphorous acids, neutralised metal alkali salts of the acid (e.g. potassium phosphonate) and phosphite which is the form in which these chemicals are absorbed by plant tissue.
All three names are found when referring to this group of chemicals which are claimed to demonstrate some classical direct fungicidal action. They control plant pathogens and specifically Oomycetes, to which Phytophthora belongs, but to call them fungicides is an enigma and not only because Phytophthora is no longer regarded as a true fungus. This is because at least part of their action is through indirect effect of phosphite ions which stimulate the cocoa plant’s metabolism to generate ‘anti-Phytophthora’ mechanisms to suppress the pathogen.
Efficacy of phophonates through direct and indirect action varies widely depending on the particular Phytophthora species and the plant host as there is some direct fungicidal action on the pathogen but how much is not altogether clear. Since they do not act directly on the pathogen, the risk of resistance developing is considered correspondingly low.
Phosphonates have been widely evaluated in cocoa and shown to work especially when applied as trunk injections. Some estates have used these chemicals with great success but reports from Indonesia and elsewhere indicate many farmers and estate managers are reluctant to subject their trees to the collateral damage caused by trunk injection. Persistent damage can reduce tree vigour and open the way for other bark infecting pathogens not affected by phosphonates.
Copper left standing
More than one hundred years after their discovery, copper fungicides continue to monopolise the market mainly because there is nothing that combines their efficacy and general safety, and because their relative low cost is compatible with the inherently ‘tight’ economics of cocoa cultivation. Basic chemistry indicates cuprous oxide as the most active of the fixed (sparingly soluble) copper fungicides. It is easy to use and apply as a spray to control Phytophthora pod rot, or in canker paint to control Phytophthora stem canker.
Nordox Ago Grade cuprous oxide (Nordox AS, Oslo Norway) is micronized to mean particle size of 1.2mµ (micron) that provides highly tenacious deposits resistant to the intense weathering pressure from heavy tropical rainfall falling on cocoa wherever the crop is grown. It is formulated as wettable powder (WP) and wettable granule WG) in a range of products including ‘high copper’ products containing 75 per cent copper (Cu).
No phtyotoxicity, operator hazard or any effect on microbial dynamics of cocoa bean fermentation or the development of flavour during curing (fermentation and drying) and roasting of the cocoa beans, have been reported in the use of copper fungicide so far. The countries and trading blocs (European Union, United States and Japan) which import the largest proportion of the world’s cured cocoa beans have placed strict rules on the usage of pesticides in cocoa destined for their markets.
Cocoa producers are aware of the rules framed for the usage of particular pesticides in different countries. For instance there is a long list of pesticides currently used in cocoa around the world which are not approved for use within the EU. Cuprous oxide fungicide is approved for use in the EU which stipulates an MRL of 50 mg/kg in consignments of cured (fermented and dried) cocoa beans entering the EU. Cuprous oxide can be used in organic cocoa production within the limits laid down by the designated organic cocoa certification authority.