Papaya: Ripe fruit rots

Figure 1: Anthracnose of papaya

Figure 2: Stem-end rot of papaya

Cause

There are many fungi associated with ripe fruit rots, and anthracnose caused by Colletotrichum gloeosporioides is considered the most common.

Other fungal diseases which are no less important include:

• Fusarium fruit rot disease (Fusarium spp.)
• Alternaria fruit spot (Alternaria solani)
• Rhizopus soft rot (Rhizopus stolonifer)
• Penicillium rot (Penicillium digitatum)
• Guignardia spot (Guignardia spp.)
  Cercospora black spot (Cercospora papaya)
• stem-end rot caused by a suite of fungi such as Lasiodiplodia theobromae, Phomopsis caricae-papayae, Mycosphaerella spp. and Phytophthora palmivora.

Minor post-harvest rots include Stemphylium rot (Stemphylium lycopersici) and chocolate spot (Colletotrichum gloeosporioides).

Symptoms

Anthracnose and chocolate spot

Anthracnose and chocolate spot although caused by the same pathogen (Colletotrichum gloeosporioides) have been described as separate diseases. With anthracnose, the first symptoms are small, well-defined dried pink spots on the surface of ripening fruit which later grow to 5cm in diameter, rounded, sunken (3–5mm deep) and brown to black in colour (Figure 1). .

The lesions can be either be water-soaked or dried and hard. Dark fruiting bodies of the fungus are produced in these lesions often in a concentric pattern. Salmon to pink spore masses are released from these fruiting bodies under favourable climatic conditions. Symptoms on immature fruit are uncommon.

With chocolate spot, lesions tend to be superficial and not sunken, are irregular to round in shape, up to 1cm in diameter and reddish-brown in colour. As the fruit ripens, these lesions either remain superficial or grow and become sunken which may resemble anthracnose. Although a single fungal isolate is capable of causing both diseases, little is known as to the factors which cause some lesions to remain superficial while others become sunken.

Stem-end rot

Depending on the identity of the causal organism, symptoms of stem-end rot can be either soft, broad, water-soaked lesions which penetrate deep into the flesh of ripening fruit, or firm, dry, wrinkled and blackened lesions which develop around the stem-end (Figure 2). Fungal growth on the surface of these lesions tends to be dotted with black fruiting bodies of these fungi.

Control

Control of papaya rots for the local market is dependent on the use of post-harvest fungicides. Research has shown that dipping fruit in hot water (52⁰C for 5 minutes) and fungicides used as a hot dip are highly efficacious and that alternate solutions such as chitosan and ammonium carbonate may also be beneficial for low-chemical-input farming systems.

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.