Dr Terry Mabbett describes the theory and practice of successfully treating black pod and stem canker in West Africa with the application of cuprous oxide.
THE GROUP KNOWN as Phytophthora species is a ubiquitous and versatile series of plant pathogens causing black pod and stem canker in cocoa (Theobroma cacao) with the capacity to destroy yield and kill trees. Phytophthora and other downy mildews in the class Oomycetes were regarded as true fungi but are now re-classified with ‘water moulds’ in a group called the Chromista.
Broad host range including many forest trees means they are ideally placed to move into and exploit cocoa that was first planted on partially cleared forest land with some wild trees left in situ as upper storey shade. Phytophthora megakarya in West Africa probably established on cocoa in this way. Phytophthora is controlled by copper-based fungicides first used on cocoa 100 years ago.
Without copper as a fungicide this crop could not be produced at current levels. Protectant copper fungicides kill spores as they germinate and before germ tubes penetrate the pod. As such they must be in place before Phytophthora spores alight.
Specific host and pathogen factors combine to make Phytophthora the most important disease of cocoa worldwide. Host pathogen relationship Cocoa is an understorey tree in the Amazonian rain-forest thriving in heavy rainfall, high humidity and shade.
Early mycologists called the Oomycetes (downy mildews) the ‘water fungi’ because they rely on free water and high relative humidity (RH) for spore germination, infection, spore production and dissemination. Where cocoa thrives, so does Phytophthora. The disease was called ‘black pod’ but now Phytophthora pod rot to avoid confusion with other diseases such as brown pod rot caused by Lasiodiplodia (Botryodiplodia) theobromae. Phytophthora occurs in almost every climate and ecosystem. Four species (Phytophthora palmivora, P. megakarya, P. capsici and P. citrophthora) are established pathogens of cocoa.
Flower cushions, pods at all stages of development (including cherelles), foliage, bark and roots are prone to infection. Phytophthora persists in old pods on trees or ground. P. megakarya grows and sporulates on cocoa debris in the soil. Evergreen cocoa trees produce pods continually for 50 years or more with individual pods taking 5 months to mature. This together with fallen pods and bark infections (stem cankers) means trees are vulnerable year-round infection from sporeproducing infections.
High frequency and heavy pod losses characterise Phytophthora on cocoa. Crop losses average 50 per cent worldwide rising to 100 in some parts of West Africa where excessively wet conditions allow. Phytophthora megakarya in West Africa where over half the world’s cocoa is grown is the most aggressive, with severe infection resulting in loss of virtually all harvestable pods.
Phytophthora palmivora is less pathogenic but has a wider host range covering some 200 different plant species including tree crops and wild forest trees used for cocoa shade. Pods at all positions from high branches to lower trunk and all stages from tiny cherelles (new pods) to full-size mature pods are vulnerable. Phytophthora pod rot is a dark necrosis expanding rapidly to leave a completely black and dried out mummified pod. Infection of full grown pods causes most loss because Phytophthora induced ‘cherelle wilt’ is compensated for by pollination and fertilisation of later formed flowers.
Phytophthora cherelle wilt can be confused with natural thinning called physiological cherelle wilt. Phytophthora causes stem cankers on the trunk and branches and necrosis of the chupons (vertical shoots), leaves, roots and nursery seedlings. Phytophthora megakarya in West Africa persists on cocoa debris from where spores are splashed onto the trunk and lowpositioned pods to play a key role in disease epidemiology. Phytophthora’s strength lies in its versatile growth and reproduction
including mycelium and several spore types – asexual spores as sporangia or resting chlamydospores and sexually produced oospores. Main spread and dissemination is via sporangia produced most quickly and profusely at 80 per cent RH and temperature range of 25-30 °C. Sporangia appear like a fine white powder on the pods some 4-6 days after infection. Sporangial infection progresses in three ways, by producing mycelium, more sporangia or motile zoospores in presence of free water required for movement.
Optimal liberation of zoospores from the sporangium occurs at a water film temperature of 15-18 ºC and ambient R.H. of 70-80 per cent. Dynamic disease reservoir Diverse infection foci generate a self-perpetuating reservoir of inoculums continually circulating in through rainfall and other water movements. Inherently high humidity within cocoa canopies regularly drenched with rainfall creates an explosive disease situation. Spores spread down the canopy in drips, drops and rivulets and upwards from the ground in soil splashes and aerosol droplets with opportunities for spore entrapment at various points.
These may be ‘natural’ sites like flower cushions and axils of pod peduncles (stalks) and chupons (vertical leaf-bearing shoots) or exogenous sites like bromeliad epiphytes and termite nests. Insects like small black ants (Crematogaster striatula) spread spores when using dead plant tissue to construct their tents around peduncles of developing pods. Spore dissemination is assisted by rats and squirrels feeding on sugar-rich pods with pod wounds providing points for pathogen entry.
Cultural control Phytophthora can be managed by good cultural practice to promote tree health and vigour, canopy aeration and crop hygiene. Pruning for good air circulation and humidity control is essential. Trees should be kept below 4m so all pods can be sprayed using a lever-operated knapsack sprayer without extension lance. Cocoa flowers are borne in cushions directly on the trunk and branches to leave many pods at ground level. Stem canker occurs on lower trunks especially when P. megakarya is involved.
Weed growth may actually reduce amount of inoculums splashed up from soil but on balance weeds around trees encourage Phytophthora through increased humidity. Regular and complete removal of diseased, damaged or unwanted pods from trees and ground with disposal away from cocoa is the other core requirement for good cultural control. Phytophthora’s wide host range must be considered when selecting shade trees. Avoid species within same plant family as cocoa (Sterculiaceae), including Theobroma sp, Cola sp and gum producing Sterculia sp, as well as trees from related plant families including Bombacaceae and Malvaceae.
Invingia gabonensis (elephant mango – family Irvingiaceae) is an important wild tree host of P. megakara in West Africa and the species from which Phytophthora most likely moved into cocoa. Copper fungicides Many cocoa farmers in West Africa are forced to spray routinely against Phytophthora. Deposits of protectant fungicide must withstand intense rainfall to ensure long term pod protection.
A sparingly soluble fungicide profile with gradual redistribution of copper ions to protect hitherto unsprayed areas is required. Fungicide formulations must be physically compatible with the spray machines used in cocoa and economically compatible with long term use by generally resource poor cocoa farmers. Fungicides are sprayed frequently over sustained periods to manage high disease pressure especially during wet seasons. As such deposits and residues must be free of any phytotoxic effect, not jeopardize bean fermentation or allow evolution of fungicide-resistant strains of pathogen.
Copper-based fungicides were first used on cocoa over 100 years ago and monopolise the market today because they still fulfil these requirements. The first copper fungicide was ‘Bordeaux’, a mixture of blue copper sulphate and strong alkali (slaked lime – calcium hydroxide). Copper sulphate is too soluble for use as a stand-alone agricultural fungicide, washed off easily and causing phytotoxic damage to many green plants. Mixing with lime forms a less soluble complex. Bordeaux mixture is still used today but is largely superseded by particulate and fixed (sparingly soluble) copper compounds such as copper oxychloride, cupric hydroxide and cuprous oxide.
Underpinned by basic chemistry and supported by field trials in Brazil cuprous oxide is considered to be the most active. Cuprous oxide Cuprous oxide is a protectant fungicide so spray timing is critical with decisions on which part(s) of the canopy to target clearly important. Deposit tenacity is another key consideration. Due consideration must also be given to any interaction between application frequency and dosage level, so farmers achieve most costeffective deposition and control from the minimum required total weight of product applied.
farmers are financially strapped so choice may come down to what sprayers they already have or what they can afford. Most efficient and cost effective application is achieved by targeting pods by spraying with lever-operated knapsack sprayers. Pods at all stages of development including cherelles on flower cushions must be sprayed. Research by IPARC (International Pesticide Application Research Consortium) showed most efficient and effective spray deposition on pods is achieved using narrow cone nozzles instead of the variable cone nozzles invariably supplied with lever-operated knapsack sprayers.
The motorised knapsack mistblower is the only mainstream portable option for whole canopy spraying. Mistblowers were developed to reduce spray volume and provide quicker, easier and more effective whole canopy application. Spray volume can be reduced to even lower levels by fitting ULV (ultra low volume) jets. Hand-held air-assisted ULV sprayers (atomisers) powered by two-stroke petrol engines have been used to control foliar disease in other evergreen tropical tree crops like citrus, and there is no reason why they could not be used in cocoa. Operators should mix and suspend cuprous oxide wettable powder or wettable granule in water before adding an appropriate oil to make a water:oil emulsion sprayable mixture.
Concentration of cuprous oxide and ratio of oil:water used requires prior evaluation with users mindful of potential blockage of the fine (low flow rate) nozzles used in these sprayers. Spraying: Dosage x Frequency Cocoa in Nigeria, Cameroon and the wetter parts of Côte d’Ivoire traditionally requires 10-12 applications per year. Brazilian scientists concerned over logistics and cost of frequent spraying consolidated control into fewer sprays of higher dosage so amount of cuprous oxide applied per year stayed the same.
Success was ascribed to high redistributive capacity of cuprous oxide deposits over extended periods of time. These high-concentration applications produced correspondingly thicker deposits and ‘laminal’ liberation where the inner mass of cuprous oxide deposit is protected against weathering, thus providing extended fungicidal activity and protection. Recent development of ‘high copper’ formulations (Nordox 75WP and Nordox 75WG) improved practicalities of using higher doses in fewer sprays. Farmers can double dose of active copper using just 50 per cent more of the high copper formulation instead of twice the amount required when using a standard 50 per cent WP.
Farmers apply same amount of active copper in less product with more cost effective transit and storage from factory gate to farm gate. Stem canker control Phytophthora also causes stem canker which kills tress by girdling the main stem (trunk). Stem canker in West Africa is caused by soil-based inoculums of P. megakarya infecting at low positions on the trunk and P. palmivora generally higher up the tree. Cuprous oxide is widely used in stem canker control. Cankers are cleaned off by cutting and scraping, making sure all diseased tissue is removed together with 1cm of clean bark around the canker.
The cleaned area is treated by brushing with cuprous oxide fungicide paint and the whole area sealed with a waterproof and insect proof layer of grease. Early-stage Phytophthora stem canker has few if any external symptoms and is therefore difficult to detect, but careful bark scraping will reveal a pink-red discolouration of the cambial tissue. Any scraping and wound cleaning must be conducted with care. Large treated areas may fail to heal sufficiently quickly. Extra bonus for copper No fungicide should aggravate existing disease problems or cause new ones, while simultaneous control of other pests is a bonus.
Cuprous oxide scores high on all counts. It is a broad spectrum fungicide and bactericidal too and also controlling a wide range of ‘green’ epiphytic plants that grow on cocoa trees. These include algae, lichen, mosses, ferns and bromeliads. Molluscs (snails and slugs) that frequently damage nursery cocoa seedlings are susceptible to sprays of cuprous oxide fungicide. Copper ions play a vital role in the nutrition of green plants as a micronutrient (trace element). There are no recorded cases of Phytophthora resistance to any copper fungicide.
Systemic single-site action Phytophthora-specific fungicides showed early promise for Phytophthora pod rot control, but development of resistant strains from use on other crops caused more careful and restricted use in cocoa. This included alternation of systemic sprays with copper sprays and use in mixtures with copper fungicide to provide protection against resistance development to the systemic fungicide.