Anthracnose on walnut — symptoms and treatment

Severity: medium

What is Anthracnose

Walnut anthracnose is caused by the fungus Gnomonia leptostyla, also known by its asexual stage name Marssonina juglandis. It is a leaf disease of walnut whose defining consequence is premature defoliation — the tree strips its own canopy before the season is over. The fungus does not rot the kernels directly, but a tree that loses its leaves early has fewer weeks of photosynthesis left to fill them, and nut quality drops as a result.

Symptoms

The first sign is spotting on the foliage — irregular dark brown spots scattered across the leaflets, without the neat circular outline that many other leaf diseases show. In wet weather the spots coalesce, running together into large dead brown areas until a leaflet is more blotch than green tissue. Leaves that reach that stage do not recover: they yellow and fall, and defoliation works upward until a badly affected tree stands half-bare. The nuts are of reduced quality, because they were filled by a canopy that shut down early.

• Early: irregular dark brown spots on the leaflets
• Progressing: spots coalesce in wet weather into large dead blotches
• Advanced: premature leaf drop and a thinning canopy; reduced nut quality

Walnut foliage also browns from drought scorch, which burns the leaf margins fairly evenly; anthracnose begins as distinct spots within the blade and only merges later. Walnut blight is the other lookalike, but it is bacterial and attacks the catkins and young nuts — if the nuts themselves are turning black and shrivelling, look there instead.

Causes and conditions

The fungus overwinters in fallen leaves beneath the tree. Spores are released from that litter in spring and splashed up into the new canopy by rain, which is why the disease keys off wet spring weather and why infection moves upward from the bottom of the tree. Spores need a film of water on the leaf to germinate, so long rainy spells drive the disease and dry springs hold it back. Once lesions establish they produce spores of their own, and each rain spreads another round. A dense, poorly ventilated canopy stays wet longer and is hit harder.

Treatment

Sanitation — cultural

Remove and destroy fallen leaves in autumn to reduce inoculum. Prune for air circulation. This attacks the disease at its source: the litter under the tree is where the fungus survives winter, so clearing it removes next spring's spore supply. Do not compost those leaves back onto the same ground. An open canopy dries faster after rain, and dry leaves are infected less.

Copper + Mancozeb — chemical

Apply preventive sprays in spring during wet weather. Repeat every 10-14 days. These are preventive materials that protect leaf tissue before the spore lands, so a spray applied after the spots appear will not erase them. Aim to have the new canopy covered through the wet spells. Pre-harvest interval: 21 days. Pesticide registrations vary by country — check local approval before use.

Prevention

• Rake up and destroy fallen walnut leaves every autumn — the single most useful thing you can do
• Prune to open the canopy so it dries quickly after rain
• Avoid wetting the foliage with overhead irrigation
• Begin sprays in spring wet weather rather than waiting for spots to appear

Frequently asked questions

Are the nuts still safe to eat? Yes. The fungus attacks the leaves, not the kernel. What you lose is quality — nuts filled by a canopy that dropped early.

When should I treat? In spring, during wet weather, before symptoms show. Preventive sprays protect healthy leaves; they cannot undo spots already there.

Will it kill my tree? Not outright — a mature walnut survives it. The cost is cumulative: repeated early defoliation weakens the tree and keeps nut quality down.