Blueberry Stem Blight and Cankers — Causes and Treatment

Stem blight in blueberries is a serious disease that can kill whole canes or even entire bushes within a single growing season. Unlike the shoot-tip dieback caused by Phomopsis, stem blight caused by Botryosphaeria species spreads rapidly down from an infection point and can girdle large woody canes in a matter of weeks. Recognising it quickly and cutting it out promptly is the most important response.

What stem blight looks like

Stem blight typically starts at a wound, a pruning cut, a dead bud or a spot of frost or mechanical damage on the cane. From there, the fungus spreads rapidly downward, killing the tissue as it goes. The first visible sign is rapid wilting of the leaves on the affected cane, which quickly turn brown and then remain attached to the dead stem — they do not fall as they would in normal autumn leaf drop. The infected bark may appear discoloured, reddish-brown or have a water-soaked look. If you cut into the cane at the border between dead and living tissue, the wood inside is often dark brown rather than the white of healthy wood.

How the disease spreads

Botryosphaeria fungi are present in most gardens, living in dead wood and plant debris. They infect through wounds — pruning cuts made with dirty tools, storm damage, frost damage to unprotected shoots, insect feeding wounds. Water stress dramatically increases susceptibility: a blueberry that has been drought-stressed, over-cropped or growing in poorly draining soil is far more likely to suffer serious stem blight than a healthy, vigorously growing plant. The spores spread in rain splash and on contaminated tools.

Pruning out infected wood

Cut affected canes well below the discolouration — look for the boundary between brown-stained and clean white wood and cut at least fifteen centimetres below it. Sterilise your secateurs with a dilute bleach solution or methylated spirits between every cut. Remove all prunings immediately from the garden and do not compost them. If the blight is found at the base of a main cane, the entire cane should be removed at ground level.

Prevention through plant health

A healthy, well-watered blueberry with good soil pH, adequate nutrition and open canopy structure resists stem blight far better than a stressed one. Make all pruning cuts clean and angled to shed water, ideally during dry weather. Avoid wounding the canes through rough cultivation near the base. In areas where stem blight has been a recurring problem, applying a copper fungicide or wound paint to larger pruning cuts provides some protection against infection.