“Dear Jerry, my cocoa plants are infected with black pod disease. They are cropping OK, but can I do anything to improve their health?” asks Peter in Townsville (Australia).
Hi Peter, By coincidence I have just answered a similar enquiry from Sierra Leone. Phytophthora is primarily a root rot disease and it can spread throughout a plant using the vascular system. Some plants are more susceptible than others and there may be multiple host plant species in your garden.
Certain types of Phytophthora can devastate entire ecosystems and ruin orchards. Jarrah dieback, P. cinnamomi disease, is recognised by conservationists as a key threatening process.
Visitors to my annual open day are often bemused when they are told that unless they first disinfect their footwear they cannot enter. I’m not keen on having Phytophthora because it cannot be eradicated. Once your garden is infected you have manage it.
Cocoa, Theobroma cacao (Malvaceae) aka cacao tree is a compact tropical evergreen tree native to Central America. The seed, aka cocoa beans are used to produce chocolate. Ivory Coast is the leading grower, producing more than two million tonnes of cocoa beans a year.
A range of different Phytophthora species can cause black pod disease symptoms in cocoa but without laboratory verification it’s not possible to guess which one it is or how virulent the strain you are dealing with is. Nor does a negative result from a single laboratory test mean a plants is uninfected.
Phytophthora is a root rot disease that can spread throughout a plant. It cannot be eradicated so you must manage it.
Some strains can be highly host specific. Black pod of cocoa has several hosts. Identify how many other host plant species you are growing in your garden. If cocoa is the most important, cull the others. If it is the least important, cull the cocoa instead and cut your losses.
This ineradicable microscopic disease is spread in soil, stormwater runoff, subsurface drainage, mulches, compost, tools, machinery (like mowers, leaf blowers and pruning equipment) and also on shoes. Not only your shoes can spread disease spores, animals ranging from green tree ants nesting in cocoa trees to birds, reptiles, pets and livestock can spread them.
The area where you grow cocoa is essentially a contaminated zone, so use a disinfectant foot bath and disinfectant in a spray bottle to decontaminate footwear and equipment each time you have worked there and apply before you leave.
Focus on horticultural hygiene: ensure no waste like weeds, cocoa prunings, cocoa leaf litter, fallen fruit or mulch leave the contaminated zone unless they are being incinerated or buried a minimum of 0.5m deep within the contaminated zone.
Conventional growers may use foliar sprays of sodium phosphite, a systemic fungicide to suppress Phytophthora disease. Effectiveness varies according to how well it is applied, how sick the plants are, and how aggressive the specific strain of Phytophthora you are dealing with is.
Organic growers focus their attention on cultural controls. There’s a practical reason why citrus growers apply a fresh layer of mulch before the wet season. Phytophthora citrophthora is one species that attacks both citrus and causes black pod of cocoa.
* Before mulching, prune the trees so no branches hang lower than 1m above the ground.
* Mulch caps the soil, reducing the risk of rain splashing fungal spores up into the foliage.
* Lifting the crown reduces the likelihood that rain splash carrying spores will hit and reinfect the foliage.
Phytophthora is more aggressive where trees have too much shade and they are grown too closely together. Prune to encourage air movement between trees and to encourage the penetration of sunshine. Foliage and fruit that remain wet after rain are more susceptible to attack.
All kinds of Phytophthora thrive in wet, poorly drained, compacted soil that is depleted of organic matter and oxygen, so deal with that.
* Adjust the soil pH, if necessary, to suit the crop;
* Provide water and trace elements as or if required.
* Seasonal mulching with disease-free compost encourages earthworms that help aerate and drain soil.
* Planting in mounds enhances drainage around the root system.
When I managed Sydney Botanic Gardens, the former rose garden was infected with a mild strain of Phytophthora which seriously sickened some rose beds. We replaced the roses with non-host ornamentals.
Other roses just looked permanently sad. We sustained them by routinely foliar feeding in the morning in dry weather with a strong solution of seaweed. We nourished them through their leaves rather than relying on their infected, compromised root systems to provide that function. This worked very well for the three years it took to fund the design and construction of a new rose garden with stock grafted on more resilient Rosa x fortuniana root stock.
Director, Seed Savers’ Network Inc.
17th November 2019