EUGENE, Ore. — What Bill and Anita Cook should have been doing once the weather cleared last Wednesday was pruning their 60-year-old Atlantic, Jersey, Dixie and Stanley blueberry bushes – snipping away at older, weaker limbs crowding out the stronger ones as they prepare to bear fruit this summer.

Instead, the Cooks’ eyes are fixed on the muddy ground beneath the bushes on their one-acre farm in west Eugene. They were preoccupied with a painstaking task: picking up “mummy berries,” one by one, and pocketing them.

This job has become as necessary as pruning because of the growing threat that the mummy berry fungus, Monilinia vaccinii-corymbosi, poses to Northwest blueberry farmers.

With some help from the vinegar fruit flies that found their way to the Willamette Valley a few years back, the fungus has wiped out half of the Cooks’ yield, for two years in a row. Other farms have seen entire blueberry patches devastated.

And because the mummy berry is particularly adept at not just surviving winter but shooting millions of spores that are carried by the winds from one field to the next, every bad year could mean the next one is worse.

Berries attacked by the fungus are called mummies because they look like mummified berries: dried, shriveled and gray-white.

The fungus is discovered only at the same time blueberries are nearly ready to harvest, and they look like what they are – duds, ugly useless fruit. It’s after they drop to the ground that the mummy berry threatens to do the most damage to future crops.

At that point, the mummies act more like zombies. They “overwinter,” lying peacefully in the mud, often obscured by leaves. Come spring, they sprout tiny mushroom-like structures with small cups on the end, called apothecia.

A single cup can produce a million spores in less than a week. Then comes secondary infection, a second kind of spore produced by the infected shoots, called conidia, that moves from blighted shoots to nearby flower blossoms.

It’s the conidia that germinates with pollen and infects the fruit slowly, as it develops, which is why there’s no way to tell if a bush has mummy berry until its fruit fails to ripen.

“Your crop looks great,” said Jay Pschiedt, a plant pathology professor at Oregon State University. “Right up to harvest.”

The ideal climate for this noxious rite of passage: a cold, wet spring. Local blueberry farmers have seen cold, wet springs the past two years.

The timing is particularly frustrating for Oregon’s 300 growers because the global market is starved for blueberries, which are prized for their antioxidant prowess and flavor.

Last fall, Oregon became the first state in the nation to gain approval to export its blueberries to South Korea, adding to a long list of countries to which the product is shipped, from Argentina to the United Kingdom.

The weapons in a grower’s arsenal are weak, especially for those who want their crops organic. While commercial fungicides are mostly effective, organic options don’t seem to work all that well, said Ross Penhallegon, Lane County agent for the Oregon State University Extension Service.

And that’s what reduces Bill and Anita Cook to the mind-numbing task they undertook Wednesday: reaching down to the ground and snatching up one mummy berry after another, by the hundreds.