Unexpected fungus decimates Australia's pistachio crop

Australia's pistachio farmers were expecting a bumper crop this year, but a fungus has decimated the harvest.

Curiously, it had infected the nuts only rarely until now. Is a genetic mutation or a spate of bad weather to blame?

Chris Joyce, a pistachio grower and research chair of Australia's Pistachio Growers Association says that the country's pistachio trees produced a lush flowering display last October and because of this growers expected a large crop. But after record rainfall during the Australian summer, over half the nuts – more than 1000 tonnes – were killed by anthracnose, a fungal infection that can be caused by different infective agents. Joyce says many of the surviving nuts are severely infected and unsellable.

Anthracnose infects the skin of ripe fruit, leaving black spots and causing the fruit to quickly rot. Barbara Hall, a plant pathologist with the South Australian government who tested the spoiled goods, says the causative agent here is Colletotrichum acutatum, a widespread fungus that commonly infects almonds, olives and strawberries. But reports of pistachio infections are rare, and outbreaks unheard of. So what went wrong this year?

Scot Nelson at the University of Hawaii at Manoa in Honolulu suspects a spontaneous genetic mutation allowed the fungus to infiltrate pistachios and rapidly spread. "Because pathogens are able to adapt so quickly, there are new ones developing all the time, so it's not surprising," he says.

According to Nelson, because pistachio trees – like most farmed crops – have a similar or identical genetic make-up to each other, they were sitting ducks for the fungus. "Once the new pathogen could infect one plant, it could infect many," he says.

Blame the Rain

But Hall is unconvinced. She speculates that heavy rain in southern Australia was probably enough to cause the epidemic. "I would suspect the fungus has always been there in a low level, but because the fungus likes wet conditions it became much worse," she says.

According to Hall, fungal outbreaks in several crop types have risen in southern Australia this year. For example, "it's been a bad year for grapes", she says, thanks to the fungus Botrytis cinerea which causes botrytis bunch rot in the fruit.

The pistachio disease might have been prevented by spraying the nuts with fungicides as they grew, says Nelson, and by removing fallen twigs and nuts, which can harbour the fungus. But because the outbreak was unprecedented, farmers were unprepared.

Original New Scientist article