Sudden wilt of melons
(watermelon, rockmelon and honeydew)

Christine Horlock & Chrys Akem, DPI's Agency for Food and Fibre Sciences, Horticulture.

Figure 1. Honeydew melon and watermelon crops affected by Sudden Wilt

What is it?

Sudden wilt is a plant disease that mostly attacks rockmelons and honeydew melons and causes extensive losses in crops throughout Queensland (Figure 1). Occurrence of the disease tends to be sporadic and predominates in winter planted fields, although it has also been observed throughout the growing season, especially in northern Queensland.  Affected fields have significantly lower yields and poor fruit quality.  The disease is not unique to Australia.  It has been reported on melons in other countries including the United States, Israel, Japan, Spain and Mexico.

What are the symptoms?

The presence of the disease on melons is characterised by a premature death of crown leaves followed by vine collapse (Figure 2).  Young plants are generally not affected and the symptoms typically occur from fruit set onwards.  Early symptoms are wilting during the warm part of the day, and general yellowing of lower crown leaves. Plants eventually wilt completely, shed all their leaves and are left with small immature and unmarketable fruits.  The root systems of affected plants are typically smaller with few fine healthy rootlets (Figure 3).  If sudden wilting occurs just prior to harvest, fruit may reach a mature size, but will not ripen fully leading to poor flavour and unmarketable quality.

Figure 2.  Premature death of crown leaves

Figure 3.  Melon roots affected by Sudden Wilt

What causes it?

Sudden wilt is a generic term used for any plant that suddenly dies at or around fruit filling. No one is really sure of the exact cause of sudden wilt or exactly why the disease is more severe in some years than others.  Severe onset seems to usually follow a period in which the soil environment is unfavourable to root growth.  Research over the years has established that the disease is caused by a complex of several factors which lead to poor root development and function, and an inability of plants to carry fruit during the stress of fruit filling.  Three factors have been implicated with the cause of the disease complex; ie: agronomic, pathogen and plant factors.  It is the interaction between these factors that seems to be responsible for the seasonal occurrence of the disease especially on rockmelons and honeydew melons.

Source of disease infection and spread

The precise nature of sudden wilt has not fully been determined and be different in different districts.  The disease, however, is associated with fungal root infection and plant stress occurring at fruit set.  Several soil-borne fungi, especially species of Pythium and Fusarium, destroy the plant feeder roots and invade larger roots, thus restricting water uptake.  The high demand for water during fruit enlargement cannot be met following this invasion, and the vine wilts and plant death follows with immature fruits. 

Fungi causing root rotting develop most rapidly in saturated or poorly aerated soil.  These conditions are produced by the destruction of soil structure and increased soil compaction during bed formation.  Varieties differ considerably in their susceptibility to sudden wilt, with those having large and vigorous root systems showing greater tolerance to the disease.  As a whole, honeydew melons appear to be more prone to sudden wilt than rockmelons. There also appear to be considerable differences in susceptibility in rockmelon cultivars.

Although not as common, sudden wilt of watermelons does occur.  Conditions of extreme water stress such as flooding or drought are commonly associated with sudden wilt of watermelon.  The disease is especially devastating if flooding or drought occurs in close succession. 

Control

Research over the years has established certain guidelines for managing and minimising the impact of sudden wilt on melons.  These are mostly practices that favour the growth of strong, healthy root systems.  Some of the issues to consider  for the control of sudden wilt in melons are:

• Proper irrigation: Over-irrigating early in the life of a crop quickly collapses the freshly cultivated soil, resulting in anaerobic conditions.  In this case roots become forced to grow on the surface and can support the plant till fruit set when heavy demands for moisture results in wilting, with the tendency to apply more irrigation water to wilting plants only compounding the problem.  Over-irrigating, particularly at fruit set should be avoided.  This can be a particular problem when using trickle irrigation and plastic mulch.  Accurate management of irrigation is essential to minimise the disease. 

• Adequate drainage: Melon crops grown in well-drained soils are less at risk of developing sudden wilt than those grown in poorly-drained soils.  Green manure crops grown during the off-season can help improve soil structure and drainage.  Beds should be considered on heavy clay soils so that adequate drainage can be maintained.

• Favourable soil temperature: Research has also shown that high temperatures tend to favour the build up of associated pathogens, especially Pythium species.  Planting directly into black plastic mulches in the heat of the summer tend to weaken the roots and make them more prone to infection by soil pathogens.  White or reflective plastic mulches should be considered during the warm summer months.

• Choice of cultivars: Past research has clearly shown that some melon cultivars are more resistant to the disease than others.  This has been associated with the root volume of the different cultivars.  Cultivars with a large volume of roots, in relation to the tops, tend to be more resistant and should be selected for disease prone fields.

• Fumigant/fungicide use: Trials over the years have shown that methyl bromide and methane sodium suppressed the incidence of sudden wilt of melons resulting in higher fruit yields than those plants treated with specific fungicides.  The fumigants however did not eliminate the disease.  More research is needed to identify alternate fumigants and fungicides that can be used to control sudden wilt.

The above control strategies can be more effective when implemented in an integrated disease management program in which one option complements the other. This is largely the direction that needs to be taken in future research for the sustainable management of sudden wilt on melons.

Further information

Other DPI melon disease notes

• Bacterial fruit blotch of melons (watermelon, rockmelon and honeydew)
• Fusarium Wilt of melons (watermelon, rockmelon and honeydew)
• Powdery mildew of melons (watermelon, rockmelon and honeydew)

• Viruses affecting melons (watermelon, rockmelon and honeydew)

Key contacts

Chrys Akem
Plant Pathologist, Horticulture
Agency for Food and Fibre Sciences
Department of Primary Industries
Ayr Research Station
P.O Box 591, Little Drysdale St.
Ayr, Qld 4897.
Phone: (07) 4783 0411
Fax: (07) 4783 3193
Email: chrys.akem@dpi.qld.gov.au

Christine Horlock
Plant Pathologist, Horticulture
Agency for Food and Fibre Sciences
Department of Primary Industries
Applethorpe Research Station
New England Highway
PO Box 501
Via Stanthorpe  Qld  4380.
Phone: (07) 4681 1255
Fax:  (07) 4681 1769
Email: christine.horlock@dpi.qld.gov.au

Telephone the DPI Business Information Centre on 13 25 23 (Queensland residents) for the cost of a local call between 8 a.m. and 6 p.m. weekdays; non-Queensland residents phone 07 3404 6999; email callweb@dpi.qld.gov.au.

Infopest CD for current registered pesticides: email infopest@dpi.qld.gov.au. 

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Information contained in this publication is provided as general advice only. For application to specific circumstances, professional advice should be sought. The Department of Primary Industries Queensland has taken all reasonable steps to ensure the information in this publication is accurate at the time of publication. Readers should ensure that they make appropriate inquiries to determine whether new information is available on the particular subject matter.

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File No: H0236
Last reviewed 21 January 2004