Australia's pistachio farmers expected a bumper crop this year. Instead they had a harvest of horrors, with nuts blackened by a fungus that had never before caused an outbreak in pistachios.

The culprit was anthracnose, a fungal disease best known for infecting mangoes. It raced through the industry, resulting in a harvest some 50 percent smaller than expected – and half of that was inedible.

What the disease means for the future of Australia's pistachio farmers remains to be seen, but for the rest of the world it's yet another cautionary example of fragility in modern agriculture.

"The wide cultivation of genetically uniform plant populations fosters rapid evolution among the pathogens," said Scot Nelson, a plant pathologist at the University of Hawaii. "Because of this greed, new pathogens or newly reported host-pathogen combinations arise almost daily around the world."

Anthracnose, usually caused by the fungus Colletotrichum gloeosporiodes, is the most important and destructive of all known mango diseases. Affected plants develop blackened lesions on flowers, leaves and fruit, the skins of which become crusty and split.

Until recently, anthracnose was considered to be mango-specific, showing little inclination to infect other members of the mango family, including cashews and pistachios. In 2001, when Australian researchers noted a case of anthracnose in pistachios in New South Wales, it was the first report of its kind.

A year later, Themis Michailides, a plant pathologist at the University of California, Davis, Kearney Agricultural Center, also noticed anthracnose in a commercial pistachio orchard. Like the Australian case, it was caused by a Colletotrichum species closely related to C. gloeosporiodes, but neither of those infections led to outbreaks, and the disease seemed consigned to background levels of harm.

However, in December 2010 and January of this year – Australia's summertime – the country experienced torrential rainfalls unprecedented in the nation's modern record-keeping history. Conditions were prime for an outbreak of moisture-loving pests, and anthracnose grows best in precisely those conditions.

Seemingly out of nowhere, it exploded. Farmer losses ranged from 40 to 100 percent.

>'Pathogens will adapt and evolve in relation to our unbalanced systems. In this sense, the evolution of plant pathogens is largely human-directed.'

According to Nelson, the outbreak isn't just a function of weather. It's likely a result of monoculture crop practices, in which just one or a few varieties of a crop are planted. Australia's pistachios are descended almost entirely from a single cultivar developed in the early 1980s; selected for the nuts' flavor, aesthetically pleasing color and easy-splitting shells, the variety was an easy choice for farmers – but with that choice, the seeds of an epidemic may have been planted.

"Those are the varieties that are best for them. That's why we have monocultures: They can't plant something that isn't profitable. But monocultures create these problems," said Michailides.

Monocultures are the rule in modern agriculture, and are especially vulnerable to disease, as a pathogen that can infect one plant will likely be infectious to the rest. That's happening now in Australia with Tropical Race Four, a fungus that threatens to destroy the country's banana industry, and may eventually doom modern bananas altogether.

But increased vulnerability isn't the only problem. Monocultures also function as evolutionary crucibles for pests, exerting selective pressure in a single direction: toward any mutation that helps the pest spread.

That seems to be happening in the resurgence of wheat rusts – the defeat of which decades ago was the Green Revolution's founding achievement. And that may have happened, at a smaller scale, with the emergence of anthracnose in pistachios.

"Evolution lies in the probabilities and numbers. In a larger pathogen population there is a higher probability of fit mutants developing," said Nelson. "Not only do the genetically uniform crops provide no physical barrier to between-plant dispersal of pathogens, they provide a large population of plants upon which enormous populations of the pathogen develop."

Fortunately for Australia's pistachio growers, fungicides do exist that may contain anthracnose for a while. But eventually the fungi may adapt to those, and it's unlikely to disappear. The spore load from this year's outbreak will be so massive that ordinary, less-than-record rainfall may be able to trigger future outbreaks.

As for whether the disease might spread to California, Themis voiced concern. More generally, the combination of monoculture practices and increasingly extreme weather will likely generate even more such outbreaks, as pests that might otherwise have stayed dormant are released.

"Pathogens will adapt and evolve in relation to our unbalanced systems," said Nelson. "In this sense, the evolution of plant pathogens is largely human-directed."

Image: Anthracnose-infected pistachio. (Themis Michailides)

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