Sudden wilt

Sudden wilt is also known as mature plant collapse in capsicum and vine decline in curcubits.

Sudden wilt is a common and serious disease of rockmelon and honeydew melon in Queensland and some parts of Western Australia.

Sudden wilt can cause serious capsicum crop losses during hot, dry weather or when crops are grown in plastic mulch, which results in soil temperatures greater than 35ºC. In North Queensland, losses are most likely in crops planted in early autumn when temperatures are high, while losses in southern Queensland usually occur when crops are planted into black plastic mulch to increase soil temperatures and enhance the early growth of winter-planted crops.

Cause

An interaction between soil-borne pathogens such as Pythium and Fusarium spp, high soil temperatures and soil-plant-water relationships.

Symptoms

Capsicum plants are healthy until fruit set begins. Early symptoms are plants wilting during the warmer parts of the day and becoming stunted, leaves yellowing and some fruit shrivelling. Plants then wilt completely, lose their leaves, and set only small fruit. Affected plants often die. When above-ground symptoms are first noticed, the root system is generally severely rotted. Rotting occurs first on the small roots and progresses to the larger roots, destroying most of the root system.

Vine decline is the most common symptom on rockmelon and honeydew melon, which are the most susceptible curcubits to the disease. The vines appear to be developing normally, then suddenly turn yellow, wilt and die prematurely from fruit set. The rapid wilt of plants may result in complete death in less than 2 weeks, leaving most of the crop immature and resulting in total crop loss. The sudden appearance of wilting and death of the plants has resulted in the common name of sudden wilt for this disease.

How it spreads

Although several pathogens have been isolated from the roots of plants affected by sudden wilt, 2 species of Pythium (Pythium myriotylum and P. aphanidermatum) are consistently associated with the disease and destroy the root system of capsicum plants. This destruction is more severe and rapid under high soil temperatures, resulting in plants wilting rapidly once fruit set begins. The plant is then unable to obtain the large quantity of water required during the fruit set period.

The Pythium species occur widely in most crop production areas and infect the root systems of many crop and weed species. High soil temperatures and waterlogged, compacted soils with low oxygen levels favour the activity of Pythium.

The precise nature of sudden wilt on cucurbits in Australia has not been fully determined. Elsewhere, in several overseas countries the fungus Monosporascus cannonballus has been directly implicated in the cause of the disease on melons. This fungus is soil-borne and spread by the movement of contaminated soil, by water, equipment, potted plants and vehicles. The disease is associated with fungal root infection by fungal mycelium or spores and plant stress occurring at fruit set.

Several other soil-borne pathogens, especially species of Pythium and Fusarium, as observed in Australia, destroy the feeder roots and invade larger roots, restricting water uptake. The high demand for water during fruit enlargement cannot be met, and the vine wilts. Fungi causing root rotting develop most rapidly in saturated or poorly aerated soil and are favoured by hot, dry weather conditions.

These conditions develop if soil structure is destroyed and soil compaction increases during bed formation. Varieties differ considerably in their susceptibility to sudden wilt, with those having vigorous root systems showing greater tolerance to the disease.

Crops affected

Capsicum, rockmelon and honeydew.

Control

• Maintain soil structure using green manure crops and other measures. Compacted soils with poor structure, poor drainage and low aeration favour soil-borne pathogens.
• When preparing hills for beds with plastic mulch, ensure soil depth and drainage is adequate.
• Use white, reflective plastic mulch instead of black mulch during hot weather.
• Do not plant oversized, root-bound seedlings as deep planting predisposes plants to sudden wilt.
• Use irrigation scheduling to ensure plants are neither over-watered (which causes waterlogging and low oxygen levels), nor stressed through under-watering. Avoid over-watering, particularly at fruit set.
• Avoid highly susceptible curcubits varieties.

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.