Anthracnose on blackberry — symptoms and treatment

Severity: medium

What is Anthracnose

Blackberry anthracnose is caused by the fungus Elsinoe veneta. It attacks the canes first and foremost, but also spreads to leaves and to the fruit itself, where it spoils individual drupelets. Because the fungus damages the cane tissue that carries water and nutrients to the fruit, a badly infected planting loses vigour year after year rather than in a single season. It is one of the most common cane diseases of blackberry and closely related brambles.

Symptoms

The first sign is usually on the new canes: small purple spots appear on the young green growth. As the season goes on these spots enlarge and their middles turn grey, so the lesion ends up looking like a sunken grey patch with a purple rim. Where many lesions run together the cane bark cracks and the cane becomes brittle. On the fruit, individual drupelets fail to fill — they dry out and harden instead of ripening, so a berry ends up lopsided and gritty rather than plump. Leaves develop spots as well.

• Early: small purple spots on new canes; scattered leaf spots.
• Advanced: cane spots enlarged with grey centres; cracked, brittle bark.
• On fruit: drupelets that dry and harden instead of ripening; uneven, misshapen berries.

Cane spotting can be mistaken for other cane blights or for winter injury. The combination of purple-rimmed, grey-centred cane lesions plus dried drupelets points to anthracnose.

Causes and conditions

The fungus survives from one season to the next in lesions on old canes left in the row. In spring, spores are released from that overwintering cane tissue and are carried onto new growth mainly by splashing rain and by water running down the canes. Infection needs the tissue to stay wet, so wet spring weather, heavy dew and overhead irrigation all favour it. Dense, crowded rows that dry slowly after rain keep the canopy wet longer and make the disease considerably worse. New canes are most vulnerable while they are still soft and green.

Treatment

Pruning and cane management — cultural

Timing: post-harvest. Remove and destroy old floricanes after harvest — these carry the overwintering fungus and are the source of next spring's infection. Thin the canes for air circulation so the canopy dries quickly. Avoid overhead irrigation.

Copper + Lime sulfur — chemical

Timing: dormancy and bud break. Apply lime sulfur at the dormant stage. Follow with copper sprays in early spring at bud break. Pre-harvest interval: 14 days.

Pesticide registrations vary by country — check local approval before use.

Prevention

• Cut out and destroy old floricanes as soon as harvest is finished; do not leave prunings in the row.
• Thin canes and keep rows narrow so air moves through and foliage dries fast.
• Water at the base with drip or a soaker hose rather than wetting the canes overhead.
• Start with clean, disease-free planting material from a reputable nursery.
• Keep the dormant lime sulfur application in the routine where anthracnose has been a problem before.

Frequently asked questions

Will it spread to my other berry plants? It spreads readily to other blackberries and related brambles growing nearby, since rain splash carries spores from infected canes to healthy new growth. It is not a threat to unrelated plants such as tomatoes or beans.

Can I still eat berries from an infected plant? The dried, hardened drupelets are simply poor quality rather than dangerous, and sound berries from the same planting are fine to eat. If you have sprayed, respect the pre-harvest interval given above before picking.

When should I treat? The key timings are the dormant stage and bud break — before the fungus moves onto the new canes. Spraying after cane lesions are already established does far less good than pruning out the infected canes.