horse chestnut leaf blotch

Symptoms

Irregular reddish-brown dead patches with a bright yellow border spread inward from the leaf edge and between the veins, often merging until the leaf curls and falls early; tiny black fruiting dots speckle the dead tissue. Unlike the leaf miner there is NO mine, no caterpillar and no insect inside - it is a fungus, and the dead patches have a distinct yellow margin.

Easily confused with

• horse chestnut leaf miner

  How to tell them apart: On horse chestnut, both brown the leaves by late summer but the cause and clue differ. Horse chestnut leaf miner (Cameraria ohridella): an INSECT - hold a brown blotch to the light and you see a hollow mine with a tiny caterpillar or pupa inside; blotches start pale then reddish-brown and the leaf litter is the overwintering source. Horse chestnut leaf blotch (Guignardia aesculi): a FUNGUS - irregular reddish-brown dead patches with a distinct BRIGHT YELLOW border spreading from the leaf edge, speckled with tiny black dots, but NO mine and NO insect inside. Management is the same for both (rake up and destroy fallen leaves), so the distinction is mainly reassurance - neither usually kills the tree.

• drought stress

  How to tell them apart: Horse chestnut leaf blotch (Guignardia aesculi) shows irregular reddish-brown blotches with a bright yellow halo and a wavy dark margin, often starting between veins or at the leaf tip, and on close inspection the dead tissue is dotted with tiny black fruiting bodies (pycnidia) visible as black specks; blotches enlarge and merge through wet summers and the leaf curls and falls early. Simple drought stress (abiotic) instead produces a uniform pale-brown scorch confined to the leaf margins and tips of many leaflets at once, with a smooth, even gradient from green to brown, NO yellow halo and NO black specks, worsening on the sunniest, most exposed side of the tree during hot dry spells. The black pycnidia and the discrete haloed blotches are the decisive signs for the fungus; an even marginal burn across whole crowns with no fruiting bodies points to drought.

• bacterial blight / canker (multi-host)

  How to tell them apart: On horse chestnut (Aesculus hippocastanum), bacterial blight (Pseudomonas syringae pv. syringae) shows small, water-soaked, angular dark-brown spots that are sharply confined by the leaf veins, often with a thin yellow halo, and the tissue may ooze or feel slightly slimy in wet weather; lesions appear scattered, including in the leaf interior, and can be linked to twig/bud cankers. Leaf blotch (Guignardia aesculi) instead begins as reddish-brown spots that enlarge into large, irregular, dry papery blotches typically starting at the leaf margin or tip, surrounded by a bright yellow border, with curled crispy edges and tiny black pinhead fruiting bodies (pycnidia) speckling the dead areas in late summer. If you see vein-limited angular wet spots, suspect the bacterium; if you see spreading brown papery blotches from the margins with black dots, suspect the fungus.