Anthracnose

Anthracnose is one of the most common and serious diseases in horticulture. It requires both pre- and post-harvest treatments.

It is also known as pepper spot disease on avocado twigs, degreening burn in citrus and blossom blight in mango.

Cause

Most commonly Colletotrichum spp., but also Diplocarpon (affecting roses) and Elsinoe (affecting grapes).

• Avocado, cashew, passionfruit – Colletotrichum gloeosporioides.
• Mango – Colletotrichum gloeosporiodes and occasionally C. acutatum.
• Rockmelon and honeydew – Colletotrichum orbiculare.
• Banana – Colletotrichum musae.

Symptoms

These fungal diseases cause the development of dark, sunken spots or lesions, often with a raised rim, on affected foliage, stems and fruit of a wide range of horticultural crops.

Pink spores are followed by black fruiting bodies. Immature fruit do not show infection until fruit ripens. Disease development after harvest is the result of infection of fruit on trees before harvest. The fungus may remain dormant in green fruit for many months.

Leaf spots are large and tan-coloured with dark brown margins. Pinkish spore masses may form on the spots under humid conditions. Leaf spots are extremely rare and generally form only after prolonged wet or humid weather.

Large circular brown spots may form around puncture marks to the skin of the fruit. The spots darken with age, centres become sunken and, in moist conditions, pinkish spore masses may form on the spots. Small spots less than 5mm in diameter may develop around the breathing pores (lenticels). The fungus also causes a major post-harvest problem in ripe fruit.

Internally, the rot penetrates deep into the flesh in a hemispherical pattern.

Pepper spot in avocados is seen as myriad small, dark, raised spots on the fruit's surface. It also affects twigs.

Blossom blight in mangoes is seen as small, black, irregular spots that spread to cause death and shedding of flowers, resulting in poor fruit set.

In passionfruit, small black dots (spore cases) of the fungus appear on the affected area. These areas later take on a dry parchment-like appearance and the skin easily breaks.

In bananas, the spores produce on dead banana material and spread to young fruit in water droplets. The fungus remains dormant in the tissue until the onset of ripening.

How it spreads

This fungus can be seed-borne and carry over on crop residue in the soil. It is spread in water droplets and worse in warm, humid weather.

Crops affected

Rockmelon, honeydew, tomato, chilli, capsicum, avocado, citrus, mango, cashew, passionfruit, banana and most other tropical crops.

Control

The critical phases for disease control are during flowering and fruit set, and after harvest. This disease is most severe during wet weather when new growth flushes are particularly susceptible.

The leaf spot symptom is generally not serious enough to warrant treatment or preventative measures. However, prevention against the fruit rot symptom requires regular spraying and orchard hygiene.

Pre-harvest treatment

Follow a recommended fungicide spray program for your crop from flowering to fruit set. Control fruit-damaging pests such as fruitspotting bug and fruit fly. Pay attention to orchard hygiene by pruning out dead wood before flowering, and regularly removing infected fruit and dead leaves entangled in the canopy. Keeping the canopy open by judiciously pruning and tree shaping helps to reduce the severity of infection. Use regular leaf and soil analyses to keep nutrient levels, particularly calcium and nitrogen, at adequate levels, as this increases the resistance of the fruit to infection. Avoid planting susceptible varieties.

In annual crops, do not plant into soil containing plant residue from a previous susceptible crop. Follow a recommended fungicide spray program and do not save seed from an infected crop.

Post-harvest treatment

Treat fruit after harvest with an appropriate chemical. Pre-cool fruit before transport if the time from harvest to delivery at the wholesale market exceeds two days. Store fruit until sale at the temperature recommended for that crop. The longer the period between harvesting and consumption the worse the disease, so minimise delays in marketing wherever possible.

Handle fruit carefully to avoid damage that can initiate the onset of the disease.

To minimise degreening burn in citrus avoid picking immature fruit and carefully manage the degreening duration, temperature and ethylene concentration. Also avoid over-fertilising with nitrogen fertiliser and maintain even soil moisture close to harvest.

Chemical registrations and permits

Check the Australian Pesticides and Veterinary Medicines Authority database for chemicals registered or approved under permit to treat this pest on the target crop in your location. Always read the label and observe withholding periods.