Gummy stem blight

Cause

The fungi Stagonosporopsis cucurbitacearum (syn. Didymella bryoniae), Stagonosporopsis citrulli and Stagonosporopis caricae.

Symptoms

The disease can affect all above-ground parts of the plant at any growth stage from seedlings to mature vines with fruit. Infection and symptoms can occur on all plant parts except roots. Symptoms on seedlings occur as light to dark brown spots on the cotyledons. Infection of the cotyledons or hypocotyl causes a water-soaked, brown discolouration of the tissues, followed by tissue desiccation and collapse. Seedlings die rapidly after infection of either the hypocotyl or cotyledons.

On older plants, leaf symptoms appear as small, circular tan spots up to 5mm in diameter, sometimes surrounded by a yellow halo. Under favourable conditions, the leaf lesions may enlarge rapidly and become irregular in shape. When the lesions coalesce, the entire leaf may become blighted. The spots dry, become cracked and may tear, giving leaves a tattered appearance. Infection often begins at the leaf margins.

Stem infections consist of brown, oblong, water-soaked lesions. Main stem lesions enlarge and slowly girdle the main stem resulting in a brown canker that produces a characteristic red or brown gummy fluid. Tiny black pimple-like fruiting bodies of the fungus (pycnidia) develop within the infected tissue. Cankers can girdle the entire stem and result in foliage wilting and affected areas dying. Vine wilting is usually a late symptom of the disease.

The most important form of the disease is crown rot, which may kill plants. At first, pale brown, then bleached areas develop, and a reddish gum oozes from cracks. Affected areas are studded with pycnidia of the fungus.

Fruit develop water-soaked, small oval to circular spots that are a greasy green colour and turn dark brown as the spots enlarge. Gummy exudate and black fruiting bodies may develop on the spots. Affected fruit may eventually become black in colour.

How it spreads

The fungus is seed-borne and can survive in soil, weeds and on crop residues. The fungal fruiting bodies contain large numbers of spores that spread in wind and splashing water. Warm, wet weather favours the disease.

Crops affected

Gummy stem blight is a major disease of cucurbits, particularly in tropical and subtropical areas. The disease can cause serious losses in watermelon, rockmelon, honeydew, squash, pumpkin and cucumber.

The specific fungi and their hosts are listed below.

Cucumis spp. (e.g. cucumber, honeydew melon, rockmelon, winter melon).

• Stagonosporopsis cucurbitacearum (syn. Didymella bryoniae)
Stagonosporopsis citrulli
• Stagonosporopis caricae

Citrullus spp. (e.g. watermelon, gourd).

• Stagonosporopsis cucurbitacearum (syn. Didymella bryoniae)
• Stagonosporopsis citrulli
• Stagonosporopis caricae

Cucurbita spp. (e.g. pumpkin, squash, zucchini).

• Stagonosporopsis cucurbitacearum (syn. Didymella bryoniae)
• Stagonosporopsis citrulli

While Stagonosporopis caricae occurs on Cucumis and Citrullus species, it is found almost exclusively on papaya.

Control

Rotate cucurbits with other crops on a two-year cycle. Apply the recommended fungicides, particularly if wet weather occurs.

Destroy all organic debris from previous cucurbit crops by deep ploughing to reduce sources of inoculum carrying over to new plantings.

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.