Bacterial fruit blotch

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 honeydew was identified and these crops must now be considered at risk from this disease.

Cause

The bacterium Acidovorax avenae sub. sp. citrulli.

Symptoms

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 watermelon 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 three-quarters 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.

How it spreads

Acidovorax avenaecan be seed-borne and local and overseas experience points to seed-borne inoculum as the starting point for severe outbreaks. 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 that move through the crop.

Fruit are most susceptible to infection when young, before the protective waxy layer (watermelon) or corky netting (rockmelon) has formed.

Crops affected

Watermelon, honeydew and rockmelon.

Also weeds such as wild cucurbits, paddy melon and prickly paddy melon in the vicinity of watermelon plantings can also be diseased during times of wet weather.

Tomato and eggplant can become infected under glasshouse conditions but are unlikely to be affected in the field.

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. Sow the treated seed 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.

Destroy volunteer watermelon and rockmelon plants and wild cucurbits in the vicinity before planting, and maintain a buffer zone around the crop 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 are 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 many 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 contain this disease to sporadic occurrences.

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.