Bacterial spot on plum — symptoms and treatment

Severity: medium

What is Bacterial spot

Bacterial spot of plum is caused by Xanthomonas arboricola pv. pruni. It affects leaves, fruit and twigs — three phases that feed each other, which makes it persistent rather than merely unsightly. The twig cankers carry the bacterium through winter on the tree itself, so the disease does not have to arrive again each spring: it is already in place, waiting for warm wet weather. Rated medium severity, it seldom kills an established plum but marks the crop and, in bad years, defoliates the tree.

Symptoms

Leaf symptoms come first and are distinctive in shape. Spots begin water-soaked and angular — squared-off rather than round, because the expanding bacteria are stopped at the leaf veins and spread into the corners between them. They darken to brown and are frequently ringed by a yellow halo, giving infected leaves a mottled look. Dead spot centres may fall out, and affected leaves yellow and drop, sometimes stripping branches. On fruit, the bacteria produce sunken pits and, as the plum swells beneath the rigid damaged skin, cracks that split the surface and open the flesh. Twigs develop dark cankers, often at the tips or around buds, and these are where the bacterium overwinters.

• Early signs: small angular, water-soaked spots on the leaves, bounded by the veins
• Advanced signs: brown angular spots with yellow halos, leaf yellowing and drop, pitted and cracked fruit, twig cankers
• Confusable with: fungal leaf spots — the angular, vein-bounded shape and water-soaked margins indicate bacterial spot

Causes and conditions

The bacterium overwinters in twig cankers and buds. In spring these ooze bacteria, which are carried onto young leaves and developing fruit by rain splash and wind-driven rain, entering through natural pores and any break in the surface. Warm, wet, windy weather produces a severe year — wind spreads the bacteria and simultaneously abrades leaves and fruit, creating the wounds it needs. Exposed sites suffer most, and blowing sand makes it worse. Prolonged leaf wetness from overhead irrigation, crowded canopies or heavy dews extends every infection period, and young expanding tissue is the most vulnerable.

Treatment

Copper hydroxide — chemical

Timing: dormancy. Apply copper sprays during dormancy and early spring. Limited efficacy during growing season. The dormant and early spring applications are the ones that count: they target the bacteria overwintering in and oozing from the twig cankers, before the population builds. Once the tree is in leaf, copper does much less — and repeated copper on foliage risks injuring it. No spray cures infected tissue, so the aim is to keep the bacterial population low rather than eliminate it. Pre-harvest interval: 21 days. Pesticide registrations vary by country — check local approval before use.

Prevention

• Prune out twigs carrying cankers in dry weather — they are the overwintering source each spring
• Plant less susceptible plum varieties where available; susceptibility varies considerably
• Water at the base and avoid overhead irrigation that keeps leaves and fruit wet
• Prune to an open canopy so foliage dries quickly after rain
• Shelter exposed sites from prevailing wind, which both spreads the bacterium and wounds the tissue

Frequently asked questions

Can I eat plums with these spots? Yes. The pitting is on the skin and the fruit beneath is safe — cut away the damaged areas. Cracked plums spoil quickly because the split skin lets rot organisms in, so use them promptly.

Will it spread to my other trees? It spreads readily to other plums and to related stone fruit such as peaches and apricots, which host the same bacterium. Apples, pears and unrelated trees are not affected.

Why does spraying in summer not help? Because copper has limited efficacy once the tree is in leaf and the bacteria are established inside the tissue. The disease is managed in the dormant and early spring window.