bacterial spot of stone fruit

Symptoms

Angular water-soaked spots on leaves with yellow halos, dark sunken pits + cracks on fruit (looks like shothole), twig cankers oozing amber bacterial slime in spring, premature leaf drop + defoliation, fruit becomes unmarketable due to cosmetic damage and acts as inoculum reservoir.

Easily confused with

• shothole / coryneum blight of stone fruit

  How to tell them apart: Xanthomonas arboricola pv. pruni (bacterial spot) makes angular, water-soaked leaf lesions confined by the veins; they look greasy and translucent when backlit, start on the lower/shaded leaves, turn dark purple-brown, and finally drop out leaving ragged shot-holes often with a yellow halo. Fruit shows shallow, dark, water-soaked pits that crack and exude clear-to-amber gum, and you may see darkened, sunken twig cankers that ooze in spring. It spreads in warm, wet, windy weather. Stigmina carpophila (coryneum/shot-hole blight) instead makes small ROUND spots with a distinct reddish-purple margin and a pale-tan centre; the dead centre falls cleanly out leaving neat circular holes, and in damp conditions tiny dark fungal pustules (sporodochia) sit in the lesions. Its fruit lesions are rough, scabby, raised spots and its bud/twig cankers are gummy. Round + purple ring + clean holes = fungus; angular + greasy/veiny + halo + gum = report-worthy bacterium.

• Sharka (plum pox)

  How to tell them apart: Plum Pox Virus (sharka) produces SYSTEMIC, painless patterns: pale-yellow chlorotic rings, arcs, lines and blotches on leaves with NO necrosis, and on plum/apricot fruit smooth concentric rings or bands, surface dimpling and internal flesh browning; apricot stones often show pale rings. There is no oozing or shot-hole. Xanthomonas arboricola pv. pruni (bacterial spot) instead gives DISCRETE, angular dark-brown to black leaf spots that are water-soaked at first, frequently fall out leaving a ragged shot-hole, and on fruit make sunken, cracked, scab-like pitted lesions often with amber gum (gummosis). Rule of thumb: virus = soft ring/line pigment patterns without dead tissue; bacterium = hard necrotic spots, shot-holes and cracked, gummy fruit pits.

• red leaf blotch of plum
• almond red leaf blotch

  How to tell them apart: Xanthomonas arboricola pv. pruni (bacterial spot, EU-quarantine) starts as small, angular, water-soaked spots on the leaf confined by the veins; these turn dark purple-brown to almost black, often with a yellow chlorotic halo, and the dead centres frequently drop out to leave a ragged 'shot-hole' appearance. Lesions also appear on twigs (cankers) and fruit (sunken, cracked spots), and the disease spreads fastest in warm, wet, windy weather. Polystigma amygdalinum (red leaf blotch) instead forms larger, rounded-to-irregular blotches that are bright orange-yellow at first and become a thick, leathery, glossy reddish-brown to brick-red stroma raised on the leaf, with no shot-holing and no twig or fruit lesions. If you see angular, vein-bounded dark spots with shot-hole and twig/fruit involvement rather than solid orange-red leaf patches, suspect the bacterial spot — a notifiable quarantine organism. Report any suspected Xanthomonas arboricola pv. pruni to your national plant protection organisation (NPPO) immediately and do not move plant material.

• American brown rot of stone fruit

  How to tell them apart: Xanthomonas arboricola pv. pruni (bacterial spot) on peach fruit shows many small (1-3 mm), dark brown to black, angular and slightly sunken spots, often with cracking, pitting or a tar-like exudate; lesions stay firm, surface-bound and do NOT spread into a soft rot, and the same pinprick spots with yellow halos appear on the leaves. Monilinia fructicola (brown rot) instead starts as one or a few rapidly enlarging soft brown circular rot patches that engulf and collapse the whole fruit within days, and on the rotten surface it grows tan-to-grey powdery sporulating tufts arranged in rings, finally shrivelling the fruit into a hard wrinkled mummy. Key tell: countless tiny firm black cracked dots (+ leaf shot-holes) = bacterial spot; expanding soft brown rot with fuzzy grey-tan spore cushions and mummification = brown rot.

• European brown rot / blossom blight

  How to tell them apart: Xanthomonas arboricola pv. pruni (bacterial spot) shows small, angular, water-soaked-then-necrotic dark spots on leaves that often fall out leaving a 'shot-hole' look; on fruit it makes shallow, dark, sunken pits with cracks but never any fuzzy growth, and on twigs it forms dark, slightly raised cankers. Lesions stay dry and never bear pustules. Monilinia laxa (brown rot / blossom blight) instead causes rapidly expanding soft brown fruit rot that, in humid weather, erupts with tan-to-buff cushiony tufts of spores arranged in concentric rings; blighted blossoms and shoot tips turn brown and wilt, often with an amber gum exudate. Key tell: any powdery/fuzzy tan sporulation or soft mummifying rot means Monilinia, while dry pitted shot-holes and angular leaf spots without sporulation point to Xanthomonas.