Fusarium Wilt of melons
(Watermelon, rockmelon
and honeydew)
Christine Horlock, Agency for Food and Fibre Sciences, Horticulture.
Figure 1. Rockmelon field affected by Fusarium Wilt
Prior to the introduction of resistant melon varieties in the early 1970′s, Australian watermelon production was severely limited by Fusarium Wilt. Growers were continually moving to newly cleared land or land where melons had not previously been grown to escape the disease. Recently, a surge in the incidence of vascular wilt has been observed in different melon growing regions of Queensland, especially in the southern and northern regions. A range of Fusarium species have been isolated from wilted melon plants, displaying symptoms similar to those of the classical Fusarium Wilt, and others showing symptoms of sudden vine collapse.
Cause
Fusarium oxysporum f. sp. niveum (watermelon) and Fusarium oxysporum f.sp. melonis (rockmelon and honeydew melon) (Figure 1) are responsible for the disease known as Fusarium Wilt. Both fungi are very specific to their respective hosts, with Fusarium oxysporum f. sp. niveum (Fon) incapable of infecting rockmelon, honeydew melon or other commercially grown cucurbit crops. Similarly, Fusarium oxysporum f.sp. melonis (Fom) is unable to infect watermelons or other commercially grown cucurbit crops. The ability of Fon and Fom to infect prickly paddy melons, pie melons and other wild cucurbits is unknown.
There are three known races of Fon (0, 1 and 2), two of which have been detected in Australia. There are four known races of Fom (0, 1, 2 and 1,2). All four races of Fom have been identified in France and South Africa and belong to the same vegetative compatibility group (0134), but the current race status in Australia isunknown. Races are determined by the specific resistance genes, found in differential melon varieties, that the pathogen overcomes.
**Identifying Fusarium from a sample does not
necessarily mean it is Fusarium Wilt **
It is important to distinguish between Fusarium Wilt and Sudden Wilt (see DPI Note on Sudden Wilt), as Sudden Wilt affected plants can also be infected by Fusarium species. However, Fusarium species associated with Sudden Wilt are usually not Fusarium oxysporum. Sudden Wilt affected plants are generally infected by a number of different fungal species including Pythium, Fusarium, Rhizoctonia, Macrophominaand others. Fusarium species typically associated with Sudden Wilt are Fusarium solani, F. equiseti and members of the F. moniliforme species complex.
Symptoms
Environmental and soil conditions are important for infection and symptom expression of Fusarium Wilt. Disease severity is maximum at soil temperatures of 17-25oC and declines dramatically above 30oC.
Plants affected early in their development suffer greater injury than those infected later in the season. In seedlings, the cotyledons or seed leaves lose their healthy lustre and wilt, followed by the complete collapse of the plant. If affected at an early enough stage, plants will rot (or damp-off) at the soil line and die. Older plants exhibit a temporary wilt which appears repeatedly in the middle of the day. Some recovery may occur at night, but the plant finally dies. Infected plants are often stunted and yellowed (Figure 2). Fruit from affected vines (if any are produced) tend to be small, with poor flavour and colour.
Leaves
Older plants can show wilting and yellowing of leaves near the crown. As the disease progresses, the leaves show tip-browning. Leaves often have dead areas which can mimic nutrient deficiencies (Figure 2).
Figure 2. Stunted rockmelon plant with yellowed leaves and necrotic spots.
Stems and runners
Wilt symptoms develop in one or more lateral vines, starting at the tip with runners dying back towards the crown. Some runners may remain healthy (Figure 3), and if environmental conditions change to those which do not favour the fungus, the plant may recover, and fruit normally. However, if conditions continue to favour Fon infection, the whole plant will eventually die.
Figure 3 Watermelon plant with wilted and healthy runners.
Longitudinal cracking with an associated gummy ooze also often occurs on the stems. This ooze, which often dries out to form a brown necrotic lesion (Figure 4), can be mistaken for gummy stem blight (caused by the fungus Didymella bryoniae). A similar ooze can also be produced by insect injury.
Figure 4. Rockmelon plants with brown lesions on the stem.
Vines infected with, or killed by, Fusarium Wilt can be covered with pinkish-white fungal growth in wet weather.
Roots and vascular system
External lesions may develop on roots, accompanied by red gumming at or just below the soil surface, similar to that seen on stems. If the taproot and stem are split open, an orange-brown discoloration of the water conducting tissues can be seen (Figure 5).
Figure 5. Melon crown showing vascular discolouration of water-conducting tissues.
Source of infection and spread
Fo can be seed-borne, but most transmission occurs by the movement of infected soil or plant parts. The fungus is well adapted to life in the soil, and can survive season to season by living (saprophytically) on dead plant material, or on the roots and stems of other plants such as tomatoes, alfalfa and weeds. Season to season survival also occurs through the formation of very robust resting spores (chlamydospores) which can remain viable in the soil for many years.
Chlamydospores are stimulated to germinate by the growth of susceptible host plant roots nearby. Fo can enter the plant through root tips, natural openings, or wounds and grows into the water-conducting tissue. Eventually water movement is reduced sufficiently to produce the wilting symptoms typically associated with the disease.
Both the incidence and severity of the disease increase during warm, dry weather. However, very high temperatures (32-38oC) combined with increased humidity seem to reduce the disease levels, and older plants (even those with some dead runners) can recover from infection and go on to fruit normally under these conditions. Infection by Fo is also reduced by very wet soil conditions.
Distribution and importance
Until 30 years ago, when tolerant melon varieties were introduced, the Australian melon industry was regularly devastated by outbreaks of Fusarium Wilt, caused by Fusarium oxysporum. Currently in Queensland, the disease is most serious in watermelons, but considerable losses can also occur in rockmelons and honeydew melons. Fusarium Wilt occurs throughout all Queensland (and Australian) growing districts to a greater or lesser degree.
Management
Although Fusarium Wilt is caused by different forma speciales (f. sp.) of Fusarium oxysporum in rockmelon / honeydew melon (Fom) and watermelon (Fon) the two pathogens are managed using the same control measures.
- Plant resistant varieties. The degree of resistance is influenced by the populations of the fungi in the soil, and which races are present.
- Avoid introducing the fungus to new areas, asonce Fusarium is introduced the soil will remain infested for very long periods. Fo can be spread on equipment, tools, feet and in surface water contaminated with infested soil. Do not put compost on fields which has been made from diseased plants – compost from such plants will contain the fungus.
- Crop rotation can be helpful in lowering the amount of Fusarium in the soil, as part of an integrated management program using resistant varieties. It is important to rotate to unsusceptible plant species, such as grasses or cereals.
- Soil fumigation may be useful initially, but recolonisation of the soil occurs very quickly.
Disease is difficult to control because of the long survival times of the pathogen in soil. Use of resistant varieties can minimize risk of Fusarium Wilt. Rotation of fields, and the removal and destruction of all affected plant debris at the end of each growing season, may also reduce the incidence of the disease.
Acknowledgement
The author gratefully acknowledges the assistance of Thomas Zitter, Department of Plant Pathology, Cornell University, New York, in the production of this Note. Professor Zitter kindly lent one of the disease images and provided valuable comments.
Further information
Other melon disease notes
Bacterial fruit blotch of melons (watermelon, rockmelon and honeydew)
Powdery mildew of melons (watermelon, rockmelon and honeydew)
Sudden wilt 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 Stanthorope Qld 4380
Phone: (07) 4681 1255
Fax: (07) 4681 1769
Email: christine.horlock@dpi.qld.gov.au
Telephone the DPI Customer Service 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.
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.
Share and Enjoy
More than half of a WA cereal farmer’s crop has lost its organic certification due to contamination from a neighbouring GM canola crop.
