Study uncovers weak point of disease that decimated cocoa production in Brazil

Pest caused Brazil, formerly one of the top cocoa producers, to become an importer of cocoa products.

By Karina Toledo

Agência FAPESP – Universidade Estadual de Campinas (Unicamp) researchers have uncovered how Witch’s Broom  – the main disease affecting cocoa production in the country –resists the defense mechanisms of the host plant and the most potent fungicides on the market.

The discovery, published in the New Phytologist magazine, paves the way for the development of new drugs capable of combating the disease effectively.

Caused by the fungus Moniliophthora perniciosa, Witch’s Broom is known by this name because it dries the leaves of the cocoa tree and makes the tree branch resemble an old broom. The affected areas can no longer perform photosynthesis. To make matters worse, these areas release toxic substances that reduce fruit production. The few fruit produced are useless for chocolate production.

“In the initial infection phase, the plant detects the actions of the invader and releases large quantities of nitric oxide (NO), a substance capable of blocking the fungus’ respiratory chain. But we discovered that Witch’s Broom has an alternative breathing mechanism that allows it to resist the attack,” explains Gonçalo Amarante Guimarães Pereira, coordinator of Unicamp’s Genomic and Expression Laboratory, where the study was conducted.

Ironically, the plant itself suffers from the toxic effects of the NO. The infected limbs die and become a banquet for the M. perniciosa fungus. “The fungus has a biotrophic phase, in which it lives within the still-living plant, and a necrotrophic phase, in which it feeds off dead cells,” says Pereira.

The team coordinated by Pereira discovered that during the biotrophic phase, when the fungus is being attacked by the plant, Witch’s Broom produces an enzyme called alternative oxidase. This protein allows it to generate a minimal quantity of energy and guarantees its survival.

“In this period, the fungus almost stops growing because the energy available is small, but it manages to resist the attack,” explains Pereira.

When the plant finally determines that the battle has been won, it sends signals to the cells of the infected branches to commit suicide – a process known as apoptosis. As the NO is no longer released, the fungus turns off the alternative respiratory mechanism, enters the necrotrophic phase and begins to feed voraciously on the dead cells.

On two fronts

The scientists then had the idea of testing a combination of the main fungicides – azoxystrobin and salicilhydroxamic acid – capable of simultaneously inhibiting the respiratory mechanisms of the fungus.

“We observed that when the main type of respiration is blocked by azoxyistrobin and the alternative type of respiration is maintained, the fungus remains in the biotrophic phase. However, the fungus completely ceases its growth if we combine this fungicide with an inhibitor of alternative oxidase,” explains Pereira.

Although it has proven promising in laboratory tests, this combination of compounds cannot be used as a commercial fungicide. “The formula is highly unstable and degrades easily. For this reason, its use in the field is infeasible,” says Pereira.

The team is now working in partnership with a manufacturer of agricultural products to find a molecule that acts similarly to the combination used in the laboratory but can remain stable for long periods on the shelf.

“We have the opportunity to create a really effective product for the control of tropical fungi. There is not a fungicide of this type anywhere in the world. It is something of great technological importance,” says Pereira.
 
The researchers are conducting additional tests with the same combination of molecules capable of blocking the alternative oxidase enzyme. This combination is being tested against other species of tropical fungus, such as soybean rust and coffee rust. “The lab results indicate that we are on the right track,” Pereira says.

Conducted with FAPESP funding, the study is part of the doctoral theses of Daniela Thomazella and Paulo Teixeira and falls under the auspices of a Thematic Project coordinated by Pereira.

Brazilian efforts to determine the mechanism of action of a disease that has drastically affected the production of cocoa in Brazil began around the year 2000 with the mapping of the M. perniciosa genome.

When it reached southern Bahia in 1989, most likely from the Amazon region, Witch’s Broom dropped national production from approximately 400,000 tons to a little over 120,000. Brazil, previously one of the largest exporters of cocoa, began importing seeds of inferior quality from countries such as Indonesia.

Currently, the disease is present in all cocoa-producing countries in Central and South America.