apple blotch

Symptoms

Dark-brown angular leaf spots merging into large blotches, intense yellowing + premature leaf drop from June onward, completely defoliated trees by August in epidemic years.

Easily confused with

• apple scab

  How to tell them apart: Apple scab (Venturia inaequalis) starts as olive-green, fuzzy, velvety spots with diffuse, feathery margins on the upper leaf surface, often along veins; lesions later turn dark brown to black, corky and may distort the leaf, and matching scabby cracked lesions appear on the fruit. Marssonina blotch (Marssonina coronaria) instead produces distinct dark brown to greyish-green rounded blotches with a fine green-yellow halo or mottling between the dark spots; tiny black fruiting dots (acervuli) form in the centre, leaves rapidly turn yellow and drop from mid-summer, usually with little or no scabbing on the fruit. Scab favours cool wet spring weather, whereas Marssonina blotch peaks in warm humid mid-to-late summer.

• European pear-juniper rust

  How to tell them apart: Marssonina coronaria (apple blotch) shows dark brown to nearly black, irregular blotchy spots scattered across the upper leaf surface, often with a faint green halo; tiny black fruiting dots (acervuli) appear within the spots and the whole leaf yellows and drops early. Gymnosporangium tremelloides (a rust) instead begins as bright yellow-orange round spots on the upper surface, and on the underside develops raised cushions that split into characteristic tube- or fringe-like aecia releasing orange spores. Look at the leaf underside and spot colour: brown-black flat blotches with black dots point to Marssonina; orange spots with tube-like protrusions point to the rust. The rust also needs a nearby juniper host to complete its cycle, which Marssonina does not.

• fire blight

  How to tell them apart: Fire blight (Erwinia amylovora) is a bacterial SHOOT and blossom disease: young shoots wilt and bend into a characteristic black 'shepherd's crook', the blight runs downward from the tip through the petiole into the woody twig, killed leaves turn blackish-brown and stay firmly attached (a scorched, 'burnt-by-fire' look), sunken cankers form on the bark, and in warm humid weather amber-to-milky bacterial ooze beads out. Apple blotch (Marssonina coronaria) is a fungal LEAF-SPOT disease: it makes discrete dark-brown to greenish-grey blotches on the leaf blade itself, often with a dendritic/star-shaped margin and minute black fruiting dots (acervuli) inside the spots, surrounded by yellow chlorosis, and it triggers heavy premature leaf-fall in late summer. Look for shoot wilting, cankers and ooze for fire blight; look for blotchy leaf spots with black dots plus mass defoliation but healthy non-wilting shoots for apple blotch.