REIDSVILLE, NC – Bill Cline is North Carolina’s blueberry Extension horticulturalist as well as an NC Extension plant pathologist who specializes in the diseases of small fruit crops. Recently, Cline discussed blueberry diseases found in North Carolina.

  • Exobasidium – This is a fungus which causes blueberry fruit to have a green spot which doesn’t ripen. “It’s not rain” which is causing those green spots, Cline said. “It’s the pathogen.”

The variety Premier is particularly susceptible to Exobasidium. Treatment is a standard fungicide like Captan. Spray from earliest budbreak through bloom, three, four or five times. If treating organically, lime sulfur can be used one to two weeks before budbreak.

  • Mummy Berry – Mummy berry is a disease caused by a fungus which can affect a blueberry plantation year after year. Fruit will get white but not a deep blue.

The lifecycle of the disease is complex. Infected berries turn hard, fall to the ground and overwinter. They are the “mummies.” In February or March, small, brown, cup-shaped mushrooms emerge from the mummies, full of millions of spores. When it rains, the rainfall splashes the spores up onto emerging leaf shoots. The leaves will then get blighted and produce their own spores.

Signs of mummy berry in infected mature stage fruit: the berries have not taken on a dark color. On the right, an infected berry is cut in half. Photo courtesy of Bill Cline, NCSU

The blighted leaves are UV-radiant, which fools pollinators like bees into thinking they are flowers. Bees land on the infected leaves, take spores on to their bodies, then move on to actual blueberry flowers, depositing spores in the blossom.

The bee-introduced spores grow within the blueberry fruit, which later in the year turn white or pale blue rather than dark blue. The infected berries get hard, fall to the ground and the cycle repeats.

Control can be aided by adding an inch of mulch under bushes in winter or spraying fungicide once leaf shoots emerge – “when they are unfurled green spears” – to protect the leaves from becoming infected with spores. The fungicide must be sprayed early, as it acts in a protective, not a curative, manner.

“Mummy berry is a very host-specific crop,” Cline said. “It just infects blueberries … Mummy berry is endemic to North Carolina but it is controllable. We produce millions and millions of pounds of blueberries because we control it.”

  • Ripe Rot – Ripe rot, the common term for anthracnose fruit rot, is cause by post-harvest handling techniques. The vast majority of post-harvest rot occurs at the stem scar of the berry – which means the disease occurred after the fruit was picked.

Manage ripe rot by picking the fruit when it is dry and cool it quickly. A study was done to observe what happens when healthy fruit is handled under different conditions. Fruit which was field packed dry did not have much decay. Fruit which was sorted in a handling facility in both a clean environment and one which was intentionally contaminated with fungus also did fairly well. The worst conditions were those where moisture was introduced to the sort surface, both clean or intentionally contaminated.

“Handle fruit dry,” Cline said. “If you handle it wet, you’re going to have problems, no matter what.”

Management techniques included harvesting blueberries on time – every seven days. Fungicides will not make up for post-harvest infection.

For more information about controlling disease in blueberries and other small fruits, check with your location Extension agent.

by Karl H. Kazaks