Olive knot

What you see

Rough, warty, woody galls, from pea-sized to fist-sized, on twigs, branches and occasionally leaf stalks. Growth beyond a heavily galled section is weak, shoots die back, and yields fall.

Conditions

Enters through wounds - pruning cuts, hail damage, frost cracks, olive fly punctures and leaf scars - during wet weather. Rain splash and pruning tools spread it through the tree.

Treatment

Prune out galled wood in dry summer weather and burn it. Disinfect the saw and secateurs between every cut, which matters more here than almost anywhere else. Copper after any hail or heavy storm.

Prevention

Prune only in dry conditions. Disinfect tools constantly. Copper at leaf fall and after storms. Do not plant nursery trees with any galls on them.

Months: October, November, December, January, February, March, April

Plants

Olive

Remedies

Bordeaux mixture (copper)

The thing people get wrong

Pruning is how olive knot travels, and a grower working through a grove with one pair of secateurs on a damp morning will inoculate every tree they touch. Disinfecting between trees - and between cuts on a galled tree - is not fussiness, it is the main control measure. And prune in summer: the bacteria need water, and a dry July cut heals before they can enter.