Peruvian cacao collection trip yields treasures

A stand of very old trees, in an unexpected location, has yielded a coveted type of cacao tree. Usually, cacao trees are found along rivers, but these gems were found at a higher altitude than normal, and in Peru instead of Ecuador or Venezuela.

Collection expeditions in 2008 and 2009 through the Amazon Basin of Peru uncovered the exceptional find, along with other distinctive new populations of cacao.

Agricultural Research Service researchers at the Sustainable Perennial Crops Laboratory (SPCL) and the Systematic Mycology and Microbiology Laboratory (SMML) in Beltsville, Maryland, and Peruvian collaborators came away with hundreds of new cacao tree samples from these trips. One of these, discovered by additional collaborators from Marañon Chocolate, was Pure Nacional - an old, very rare and highly coveted variety that has garnered a great deal of interest from makers of fine-flavour chocolates.

SPCL research leader Lyndel Meinhardt, SPCL geneticist Dapeng Zhang and SMML mycologist Gary Samuels (now retired) collaborated with the Instituto de Cultivos Tropicales (ICT), a research centre in San Martin, Peru, to identify the new varieties of cacao. The researchers are studying 342 cacao specimens collected from 12 watersheds and categorising the DNA of the specimens. The group has identified new cacao types with unique flavors that are distinctly Peruvian, which may one day be marketed in the same way as wine - by geographical provenance.

The cacao tree, Theobroma cacao, produces beans that provide the raw material to make chocolate. The tree is cultivated in some tropical countries with the right environmental conditions. The origin of cacao is the Amazon region of South America. Like many tropical tree crops, seeds of this plant lose their viability quickly after being harvested. For this reason, varieties or types of cacao must be maintained in living germplasm banks.

“The majority of the material in cacao germplasm banks was collected prior to the 1940s. There are more than 5000 different varieties of cacao currently in collections around the world,” says Meinhardt.

“While this sounds like a large amount, most are breeding lines derived from a small number of types, so it actually represents a small fraction of the genetic diversity that still exists in the wild, especially in the centre of origin of this species.”

To address this limitation, expeditions were begun in 2008 to explore the upper Amazon River area in Peru. The purpose of these trips was to find and collect wild cacao trees and attempt to establish them in a living germplasm bank in Tarapoto, Peru, Meinhardt says. These trips were jointly funded by the US Department of Agriculture and INCARGO, Peru’s Ministry of Agriculture.

“In 2008, seven river systems were explored and 190 cacao trees were sampled. Of the initial 190 trees collected, 128 were successfully reestablished in the germplasm bank. In 2009, five more river systems were explored in two expeditions and a total of 152 trees were collected and reestablished in the germplasm bank in Tarapoto,” explains Meinhardt. “From the 12 river systems explored, we have identified three completely new populations of cacao that were not previously known to science.”

These expeditions collectively represent one of the largest efforts ever conducted to search for wild cacao. The new populations could be sources of disease resistance or could have potential new flavour traits. The fine-flavour chocolate industry is keenly interested in obtaining new and unique flavour sources.

“Our results combined with the needs of the fine-flavour chocolate industry have led to new collaborations that will look at the primary gene pool of cacao. Together with industry we will attempt to gather information on the genetic diversity of wild cacao from all of the countries in South America within the centre of diversity,” says Meinhardt.

Niche chocolates from South America are not new. The varieties Arriba, from Ecuador, and Porcelana, from Venezuela, are two of the most famous. Arriba has a strong, complex taste that stays on your tongue for a long time, while Porcelana features a unique light fruit flavour. ARS and ICT are helping Peru create its own niche in the chocolate industry by working with San Martin’s Oro Verde cooperative and Marañón Chocolate. Peru’s tropical conditions - 60% of the country is covered in tropical forest - make it suitable for producing exceptional chocolates.

During the 2008 collection trip to Peru, Meinhardt also recorded the incidence of the devastating witches broom disease (WBD) in wild cacao trees in the upper Amazon region. The scientists studied areas along the Aypena, Charupa, Nucuray, Pastaza, Ungumayo, Ungurahui and Urituyacu Rivers and determined the overall severity of WBD infection based on the percentage of symptoms on flower cushions, flushes (new stem growth) and fruits.

A team of scientists, including ARS’s Zhang, Meinhardt, and Samuels and ICT plant pathologist Enrique Arevalo, found that 14.7% of flower cushions and 13.7% of trunks were infected, and 9.1% of the trees along the Aypena River were infected. The other river areas had similar results.

The incidence of WBD observed during the survey suggests that there is a high level of WBD resistance in these wild Peruvian cacao populations. The scientists are now studying the samples to determine which are best suited for both unique flavour and WBD resistance. WBD can cause yield losses of 75% in susceptible varieties.

In addition to collecting cacao germplasm, the team isolated other fungi from disease-free leaves and trunks of the wild cacao trees. This large collection of ‘endophytic’ fungi - fungi that occur in disease-free tissues of all plants - may provide protection against diseases such as WBD either by stimulating the immune system of the plants or through direct parasitism or antibiotic effects against pathogens. Samuels found that several fungal species previously unknown to science were found in the cacao tissues. The potential for biological control using these endophytic fungi is being evaluated at Beltsville by SPCL scientists Bryan Bailey and Ron Collins.

The genome for WBD has been sequenced, and it may hold clues for developing control measures to reduce its impact in the future. This was reported in the journal Biomed Central Genomics in 2008.

While scientists have the genomes of some cacao populations in hand and are working diligently to improve production and disease resistance, improvements can sometimes lead to unintended consequences, like a change in flavour, according to Meinhardt.

“There are a lot of great chocolate sources; the task is to find them and preserve them before they are lost,” says Meinhardt. “Mother Nature has done a great job of creating these exceptional cacao trees.”