Olive anthracnose: the disease of the mummified fruit left on the branch

If you see shrivelled, leathery brown olives still hanging on the branch as harvest approaches, that is not the olive fruit fly — it is anthracnose. In bad years it costs up to 50% of the crop, and even the sound-looking fruit that goes to the mill with it raises the oil's acidity and peroxide value. It damages you in the grove and again in the mill.

The pathogen and its cycle

The causal agent is a fungus, Colletotrichum gloeosporioides (formerly Gloeosporium olivarum).

• It overwinters as mycelium and conidia in mummified fruit, leaves and thin shoots.
• Conidia can survive a full year in mummified fruit at low temperatures. Last year's rotten fruit still on the branch is this year's inoculum.
• Spread is by raindrops and wind. Autumn rains determine whether the disease breaks out.
• Infection occurs through wounds but also directly through the skin — no wound is needed.

Symptoms: what to look for

The main damage is on the fruit, though leaves, thin shoots and flowers are also affected.

1. As the fruit ripens, regular or irregular browning begins.

2. Lesions are usually sunken and depressed at the fruit tip.

3. As the fruit grows, lesions merge and can rot it completely.

4. The rotting fruit loses moisture and mummifies; the tissue hardens to a leathery texture and stays hanging on the branch.

That last point is the diagnostic key: a mummified fruit still on the branch is the signature of anthracnose. Fruit fly damage tends to make fruit drop, and the sting hole is visible.

Effect on the oil

Anthracnose-affected fruit hurts twice:

• Table value goes to zero. Blemished, shrivelled fruit cannot be sold as table olives.
• Oil quality falls. Oil from this fruit shows higher free acidity and peroxide value — risking loss of extra virgin classification.

Separating the harvest and the milling in an affected grove — collecting mummified fruit separately where possible — directly protects the oil's grade.

Cultural measures (this is where the work is)

• Collect and destroy fallen infected leaves and fruit. This is the single most effective measure; it removes the source.
• Prune out and destroy infected branches. Piling them at the field edge does nothing — the source stays.
• Prune for air and light. A dense, closed canopy holds 90% humidity inside it.
• Avoid close planting. Same reason.
• Control the olive fruit fly. Its wounds open the door to anthracnose; the two control programmes reinforce each other.
• Do not delay harvest. A late harvest gives secondary infections time.
• Control weeds — it lowers humidity under the canopy.

Spray timing

The technical instruction gives two points in time:

The timing of the first spray is critical: before the rain, not after. The first autumn rain disperses spores from mummified fruit; protection must already be in place.

⚠ The Integrated Pest Management technical instruction notes that no licensed plant protection product suitable for the programme exists for olive anthracnose. Consult your Provincial/District Directorate of Agriculture and Forestry for products and rates, and check the current licensed product list.

Host range

The pathogen is not confined to olives; almond, strawberry, apple, avocado, lemon, orange, mango, coffee, cucurbits, tomato and aubergine are all hosts. A neighbouring orchard can be the source.

In short

Anthracnose is the disease of autumn rain, and its source is last year's mummified fruit still on the branch. Clearing that source — collecting fallen fruit, removing mummies, pruning out infected wood — matters more than spraying. In bad years it takes half the crop; in clean years it can still cost you the oil's grade.