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cacao black pod disease

Last Updated on January 8, 2021 by monica chan

Researchers at the Penn State College of Agricultural Sciences, USA, conducted a study that discovered a chemical that could protect cacao trees from black pod disease. The researchers found high levels of the antimicrobial compound clovamide in the leaves of a disease-resistant strain of cacao, which could help in breeding black pod rot resistant trees.

Black pod disease is a pathogen that presents a threat to cocoa trees. Annually the pathogen can cause a yield loss of up to 30% and up to 10% of total trees can be lost completely, but it has the potential to completely destroy a crop if left untreated.

This pathogen can manifest anywhere on the cocoa tree, but it will mostly show on the fruit, turning it into a brown / black colour. The best means of control is to identify the disease early before it is able to spread. One current method of control is to ensure leaf litter is not left for long on the ground, but if the pathogen still spreads, then a chemical control can be introduced.

The disease is mostly found in tropical areas where the cocoa trees are located and need rainfall in order to spread its spores.

cacao trees
IMAGE: Ben Knollenberg/Penn State

The discovery by the researchers of the new chemical is significant because the fungal disease poses a serious problem across the world where cacao is grown, noted researcher Mark Guiltinan, J. Franklin Styer Professor of Horticultural Botany and professor of plant molecular biology, College of Agricultural Sciences.

He said,

This is the first time that clovamide has been implicated in cacao resistance to pathogens, and the innovative method we used to measure the compound in the leaves could have a major impact in the quest to develop highly productive, disease-resistant varieties of cacao.

But these results may have consequences for advancing disease resistance in other plant species, as well.

In the study, leaves of a cacao variety known as “Scavina 6,”, which is tolerant of the fungus that causes black pod rot. The leaves of this particular cacao strain were found to accumulate high levels of clovamide and several other metabolites that are known to be involved in responses to stress.

Compared to a susceptible cacao strain, Imperial College Selection 1, concentrations of clovamide were up to 58 times higher than in Scavina 6 leaf extracts.

Researchers discovered this dramatic difference in clovamide accumulation by comparing the abundance of hundreds of specialised metabolites in the two cacao strains using liquid chromatography and tandem mass spectrometry at Penn State’s Metabolomics Facility, part of the Huck Institutes of the Life Sciences.

Professor Guiltinan said,

These results pointed to clovamide as a potential contributor to disease resistance, and laboratory tests supported this hunch.

Lab tests confirmed that the compound inhibits the growth of three pathogens of cacao in the Phytophthora genus. In one experiment, clovamide inhibited the activity of the enzymes proteinase and pectinase, which are known to break down plant cell walls. 

According to the researchers, their study indicates that clovamide prevents the pathogen’s ability to break down cell walls while killing and digesting plant tissue.

with cacao trees
IMAGE: Ben Knollenberg/Penn State

Working with collaborators in Brazil’s cacao research center CEPLAC (Comissão Executiva do Plano da Lavoura Cacaueira), the researchers are measuring clovamide content in existing cacao tree populations. The goal of this work is to see how well clovamide content correlates with disease resistance in a field setting.

Lead researcher Ben Knollenberg, a postdoctoral scholar in plant science at Penn State suggested, the method researchers used to measure clovamide levels in leaves could greatly streamline and condense the breeding process.

He said,

The Scavina 6 strain has been utilised as a parent in cacao-breeding programs for decades, but it hasn’t been clear why it’s resistant to black pod rot, which makes breeding for resistance difficult.

Breeders cross Scavina 6 with higher-yielding varieties, for example, and they get a bunch of seeds or offspring, which may or may not have inherited the disease resistance. Evaluating the disease resistance of this new generation of trees requires years of growth and measurements in field trials, which requires land, labor and money.

If breeders can screen for leaf clovamide content when the trees are just seedlings in the greenhouse, they can eliminate trees that lack the trait before transplanting the seedlings to the field. This will increase the chances of obtaining disease-resistant varieties and will reduce the resources required for cacao breeding. 

The researchers hope that using clovamide as a resistance marker becomes a useful tool in cacao breeding programs, which would be especially welcome since much of the cacao tree crossing is done in countries that don’t have well-funded breeding programs.

Knollenberg added,

We think this will accelerate breeding and make it more efficient so that breeders can develop resistant varieties more quickly.

For example, instead of growing 1,000 trees to evaluate resistance, with the ability to measure the relative abundance of clovamide in the leaf tissues, they could grow just the 100 highest-clovamide trees instead.

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