Monthly Archives: Nov 2014

The evil chocolate sisters – Frosty Pod and Witches Broom

Cacao pod hanging on the Theobroma cacao tree in Indonesia.

Cacao pod hanging on a cacao tree in Indonesia.

Cacao Frosty Pod! Sounds like the name of a new breakfast cereal. Hidden behind this misleading, cheerful name is a nasty disease of cacao plants that – together with its sister species Witches Broom – is destroying cacao plantations in South America and seriously threatening our demand for the most delicious chocolate.

How to make chocolate from cacao trees

Let’s first have a look at the origin of our chocolate. A few years ago, I have been working as a guide in the Palmengarten – Germany’s biggest botanical garden – in Frankfurt/Main. One of my favorite plants to show was Theobroma cacao – the cacao tree. It is surprising how few school kids know that chocolate is not growing as a chocolate bar on a tree.

Chocolate is made from cacao beans that are produced inside massive fleshy pods that stick out from the stem and branches of the evergreen cacao tree. After harvest, the beans are removed from the pods and set in a wooden box for fermentation. During the fermentation process, a soup of yeast and bacteria enhances the chocolate flavor and reduces the bitterness. After drying the fermented beans, grinding and adding milk and sugar, the cacao beans have been promoted to a bitter sweet symphony of chocolate flavors.


Cacao beans in a chocolate shop in Mexico.

Where is cacao grown?

Cacao is grown in the hot and humid tropical regions around the Equator. Two-third of the world’s cacao is produced in only four West-African countries: Ivory Coast, Ghana, Nigeria and Cameroon. Ivory Coast and Ghana dominate cacao production: together they cultivate half of the world’s cacao. Now you might wonder: If most of the cacao is produced in West-Africa, why should we worry about a fungus attacking the cacao trees in South America?


South America is home to the finest chocolate variety

The reason is the following: The cacao grown in West-Africa is mainly the Forastero variety – also called bulk cacao – that is used for manufacturing mass-market chocolate. In South America, the Criollo and Trinitario varieties provide the delicious and delicate “fine grade” cacao beans. Only 5% of the world’s cacao beans are considered “fine grade. Ecuador is the world’s largest producer of “fine grade” beans, followed by Venezuela, Panama and Mexico.

Evil chocolate sister – Witches Broom

In the early 19th century, Ecuador was one of the biggest chocolate suppliers. Over 30% of the world’s cacao was produced in Ecuador allowing the “Cacao kings” living north of the capital of Guayaquil extravagant lifestyles. Their excesses were abruptly ended in 1921 when Witches Broom eradicated the Ecuadorian cacao crop.

Witches Broom refers to a deformity of the leaf-bearing branches and stem-borne cacao flowers, which result in a dense mass of swollen branches tipped with bunches of stunted leaves – the brooms. The stalks that support the cacao pods become thickened and the entire fruit is distorted. Witches Broom is caused by the fungus Moniliophthora perniciosa – a mushroom-forming fungus. Mushrooms are the sexual fruiting bodies of this class of fungi. The mushrooms appear on the cacao broom when it has dried out and they produce millions of basidiospores that are blown by the wind to neighboring cacao trees.

Frosty pod rot of cacao caused by M. roreri with whitish to creamy-colored spores on the pod surface. Image from Plant Health Progress article: The Impact of Plant Diseases on World Chocolate Production

Frosty pod rot of cacao caused by M. roreri with whitish to creamy-colored spores on the pod surface.
Image from Plant Health Progress article:
The Impact of Plant Diseases on World Chocolate Production

Frosty Pod – the really evil chocolate sister

Frosty Pod Disease is caused by the Witches Broom’s sister species – Moniliophthora roreri. Frosty Pod is like a crippled, but more aggressive version of Witches Broom. No mushroom formation or any other sexual fruiting body has ever been observed for the Frosty Pod fungus.

(Maybe it’s the lack of sex that explains its crave for chocolate.). It is named after a layer of white mycelium that develops on dark, chocolate-colored spots on the cacao pods. These dark spots appear 40-80 days after the fungal spores has germinated on the pod and penetrated the pod epidermis. During this asymptomatic stage, Frosty Pod causes internal damage to the pod and beans. One week after the appearance of the dark spots, the characteristic white powder appears on the pod surface. After ca. three month, the fruits become dry and remain mummified on the cacao trees trunk, where they serve as mass producers of spores (over 7 billion per fruit!) that cause waves of infection over a long period of time.

Frosty Pod Rot is found in all north-western countries in South America. It is more destructive than black pod (a cacao disease in West Africa) and more dangerous and difficult to control than witches broom. In affected countries from Panama to Mexico the yield losses can be higher than 80% and Frosty Pod is the main yield-limiting factor.

Sources: Aime et al. , Mycologica, 2005,  International Cacao Industry (,, “Cacao Diseases: Important Threats to Chocolate Production Worldwide” Myths and Misnomers, Harry C. Evans, CAB International, Egham, Surrey, UK


Coffee rust or Why the British drink Tea instead of Coffee.

© Sarah Maria Schmidt

© Sarah Maria Schmidt

There is not a single morning that I do not start with a cup of coffee. Without caffeine, my brain and body refuse to function. To ensure a thorough supply with coffee throughout the day, I have one espresso machine at home, another at work and a filter coffee machine for guests. I am sharing my passion (and dependency) for coffee with the coffee rust fungus Hemileia vastatrix – an iconic pathogen that made the British drink tea instead of coffee.

(Very) Brief history of coffee culture and trade

Let’s go back in time.

In the early days, coffee was mainly used as medicine and food additive by North African tribes. Around the early 1500s, the Turkish and Arabians had got the hang of it and enjoyed their coffee in socially amiable coffeehouses. By the early 17th century, Europeans got hooked on coffee. Coffeehouses had sprung up in all major cities of Europe and had become popular places to meet, enjoy coffee and discuss philosophy, religion, and politics.

Some smart Dutch businessmen saw their opportunity and began to invest in and trade coffee. They grew coffee in their colonies beginning in Ceylon in 1658. In 1796, control of Ceylon was handed over to the British and British colonials began to clear the Ceylon rain forest to establish coffee plantations. By the 1870s, Ceylon’s plantations were exporting nearly 100 million pounds of coffee a year, most of it to England. For a few decades, Ceylon was the world’s top coffee producer.

Coffee Berries and Flower. © Sarah Maria Schmidt

Coffee Berries and Flower. © Sarah Maria Schmidt

The coffee rust fungus – a perfect parasite

In 1869, the coffee rust fungus Hemileia vastatrix (named for the unusual shape of its spores: smooth on one side, roughened on the other) entered the stage in Ceylon. Coffee rust infections occur on the coffee leaves. The first symptoms are small, pale yellow spots on the upper leaf surface. As these spots grow, masses of orange uredospores (~400.000 per spot) appear on the leave undersurface. The spores germinate on the humid leaf surface and the fungus enters the plant tissue through natural openings. Inside the leaf, the rust fungus forms an intimate connection with the coffee plant by growing a feeding structure – called haustorium – within the living plant cell. Via the haustorium, the rust fungus absorbs its food from the coffee plant – without killing its host. When all the nutrients are sucked out, the infected leaves prematurely fall off and eventually the tree dies, often before it can produce coffee berries. The fallen leaves are still full of fungal spores.Rust fungi produce spores in huge amounts. One orange spot contains more than 400.000 uredospores. This means that a single, heavily infected coffee plant can produce millions of infectious spores, which are blown by the wind to other coffee plants.

Coffee Rust. © Sarah Maria Schmidt

Coffee Rust. © Sarah Maria Schmidt

Coffee rust turned Ceylon into a country of tea growers

After Hemileia’s arrival on Ceylon, annual coffee harvests in Ceylon plummeted. Many coffee growers were ruined. Former coffee plantations were left to rot. A few far-sighted growers recognized that the plantations could be turned over to tea-growing. One of them was the Scotsman Thomas Lipton, who purchased five ex-coffee plantations in 1890 to grow tea, which he sold in his grocery chains. He was the first to sell tea in boxed small quantities to make it available for everyone. Ever since the raging of coffee rust, Ceylon is known as the exporter of the world’s finest teas.

Tea that the British like to enjoy in the morning, afternoon and evening. Instead of a delicious cup of coffee! 1


1 Some people claim that it was not coffee rust, but the bad quality of the Ceylon coffee that turned the English into tea drinker.

Sources:; The Triumph of the Fungi. A rotten history. Nicholas P. Money. Oxford University Press. 2007.


My parents have a glass of freshly squeezed orange juice every morning. They believe in its health benefits. My mom claims that she did not have a cold ever since they started their day with a glass of fresh orange juice. They buy huge boxes of oranges every week at the local market.

A few weeks ago they stopped. The oranges were too expensive.

A disease called Citrus Greening – also known as Huanglongbing (HLB) or yellow dragon disease – next to environmental effects (drought in Brazil, hurricanes in Florida, dry summer in Spain) – diminishes our oranges and spurs the prices. Named for its small, partially green fruits, Citrus Greening has ravaged orange fields in Florida and the US East Coast. Infected trees produce misshapen fruits with bitter juice and dark aborted seeds that drop prematurely. Eventually the tree stops bearing fruits and dies.

Misshapen, green orange fruits from a Citrus Greening – infected tree.

Citrus Greening is transmitted by a tiny mottled brown insect – the Asian Citrus Psyllid (Diaphorina citri). The psyllid feeds on the stems and leaves of citrus plants. Not limited to orange trees, it also feasts on grapefruits, pomelos, mandarins, lemons and lime. While it feeds, it damages the young citrus leaves. But that is not what it makes it so nasty.

The nasty thing about the Asian Citrus Psyllid is that it can take up a bacterium in its body and transmit it to the next citrus plant it feeds upon. This Huanglongbing (HLB) bacterium can kill a citrus tree in as little as five years and there is no known cure and no resistant citrus plant.

Asian Citrus Psyllid.

Both the Asian Citrus Psyllid and the HLB bacterium originate from Asia or India and then spread to other parts of the world where citrus is grown and where they have no natural enemies. The Asian Citrus Psyllid – and with it the HLB bacterium – can easily move by wind from one citrus-growing area to another.

Researchers in Florida, Brazil and California are working hard on breeding HLB-resistant citrus trees and developing chemical controls with insecticides. Insecticides however have the disadvantage that they are also toxic to honey bees.

The most promising weapon against the Asian Citrus Psyllid is its natural enemy – the parasitic wasp Tamarixia radiata. Females of this tiny wasp – not bigger than the dot at the end of this sentence – lay their eggs underneath the psyllid nymphs, and after hatching, the parasitoid larvae attack and kill the psyllid.

Sounds brutal? Might save our orange juice!



“Asian Citrus Pyllid and Hanglongbing Disease”. UC Davis. August 2013

Pictures: Wikipedia