Bacterial fruit blotch of melons
(Watermelon, rockmelon and
honeydew)
R G O'Brien and Christine Horlock, Agency for Food and Fibre Sciences, Horticulture .
Figure 1. Bacterial fruit blotch of watermelon
In Queensland, the fruit blotch disease of watermelon has occurred sporadically over the last 25 years. We have regarded it as a minor disease but recently fruit blotch has shown that, given suitable conditions, it can cause severe damage. Since mid 1995, blotch has been noticed in the Ayr, Bowen, Chinchilla and Emerald districts. The severity of the disease is greatly modified by weather conditions and outbreaks are often associated with periods of wet, windy weather. Hail damage can greatly increase disease severity. In 1996, a second strain which also infects rockmelon and honey dew was identified and these crops must now be considered at risk from this disease.
Figure 2. Rockmelon leaf showing angular bacterial blotch lesions
Symptoms
(Links to more symptom images at end)
Seedlings
The disease can be seed borne. Water-soaked areas appear on cotyledons, these collapse after a few days to become pale-brown coloured, dry areas. If the disease progresses, the water-soaked areas enlarge, affecting the stem which may collapse causing seedling death. There can be a rapid spread of the disease in crowded production areas.
Field plants
Leaf spots may not be very conspicuous. They are often angular and tend to elongate along veins. Young spots and the margins of older spots are greasy or water-soaked in appearance, especially when viewed from below. Leaf spots sometimes occur along the edge of leaves or at points of damage on the leaf. Spots are brown to dark brown in colour becoming pale and tattered as they dry out. With showery weather, a white bacterial ooze may cover the lesion.
Fruit
Distinctive symptoms develop in fruit. On watermelons large areas of rind are covered by a water-soaked blotch. This also occurs on young rockmelon fruit before netting. Infected areas on young fruit are more severely affected than old, and may show water-soaking over almost the whole surface. Abortion of fruitlets can occur.
Infection of fruit after they are ¾ developed leads to smaller external lesions. On rockmelon, there may be just a small depressed skin blemish, easily overlooked.
Internal fruit symptoms vary with age of fruit, and age when infected. There is generally a reddish brown discolouration of flesh where infection passes through the rind. In fruitlets, all internal tissues may look water-soaked but are still firm. In more mature fruit, small cavities develop in the flesh or the seed cavity can be partly lined with reddish brown discoloured tissue. Secondary rots, which enter through skin cracks, rapidly break down the flesh to a watery pulp.
Other hosts
The causal bacterium (Acidovorax avenae sub. sp. citrulli) can affect some other plants. Tomato and eggplant can become infected under glasshouse conditions but are unlikely to be affected in the field.
Surveys in 1997/98 revealed that the wild cucurbits, paddy melon and prickly paddy melon, in the vicinity of watermelon plantings were also diseased during times of wet weather.
Disease cycle
Acidovorax avenae can be seed-borne and local and overseas experience points to seed-borne inoculum as the starting point for severe outbreaks. The presence of strains which affect rockmelon as severely as they do watermelon suggests rockmelon seed, as well as watermelon, could serve to introduce the disease. Other possible sources of initial crop contamination include cucurbits (especially volunteer watermelons and wild cucurbits) and trash from a previous crop. It seems that once trash is decomposed the causal bacteria do not survive for long. Secondary spread, either in plant house or field, is associated with humid conditions and free moisture on the leaves. Bacteria can be re-distributed from infected leaves by rain or irrigation splash or carried on machinery, clothing, boots or on animals which move through the crop. Fruit are most susceptible to infection when young, before the protective waxy layer (watermelon) or corky netting (rockmelon) has formed.
Control
Disease-free seed is the basis for a control program. Many consignments of watermelon seed are now tested by seed companies for the presence of the organism. A certificate of nil disease detection gives a degree of surety that the seed is clean. Rockmelon and honeydew seed is currently screened for disease freedom by several seed companies, and the disease status of seed lots should be confirmed with seed suppliers. If seed has not been rated disease free, an effective seed treatment has been developed. Both internal and external contamination can be eliminated by soaking seed for 25 minutes in a water bath maintained at 55 °C. After treatment, place seed in running tap water to bring the temperature down quickly, then dry seed without delay . Seed should be sown within 2 days. Note that this treatment may lower the germination of some seed lots. Test a sample before committing large quantities of expensive seed.
It is essential that volunteer watermelon and rockmelon plants and wild cucurbits in the vicinity be destroyed before planting, and that a buffer zone around the crop be maintained free of such plants.
Copper sprays will delay the spread of bacterial diseases. It is important that these be applied early to prevent the disease being widespread at fruit set. A range of copper fungicides is registered for use on cucurbit crops and should be used according to label recommendations. Do not apply sprays during the heat of the day as leaf burning may occur.
The sporadic nature of the disease in Queensland over 25 years suggests the causal bacterium does not readily survive under our normal field conditions. Early destruction of crop residues, volunteer melons, and wild cucurbits; rotations; clean seed and the use of copper fungicides will hopefully return this disease to sporadic occurrences.
Further information
Links to more symptom images*
(*Please use the back button on your browser to return to this page)
Link to necrotic blotch lesions on rockmelon plants - old infections
Link to blotch affecting netting on young rockmelon fruit
Link to internal cavities in rockmelon fruit
Link to rockmelon leaf with yellow haloes surrounding necrotic lesions
Link to blotch lesions on prickly paddy melon - a susceptible weed
Link to prickly paddy melon fruit with watersoaked and necrotic blotches
Link to honeydew fruit with necrotic blotches
Links to other melon disease notes
Fusarium Wilt 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:
|
Heidi Martin |
Christine Horlock |
Telephone the DPI Business Information Centre on 13 25 23 for the cost of a local call (Queensland residents); 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.
File No: H0144
Last updated 27 January 2004
