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 c...