Bacterial fruit blotch of watermelon
A paper recently published in Plant Disease presents the situation of bacterial fruit blotch of watermelon in United States. This disease of watermelon (Citrullus lanatus) seedlings and fruit is due to Acidovorax avenae subsp. citrulli. In USA, it first appeared in commercial fields in 1989, in Florida (see EPPO RS 519/10, 1992). Early season outbreaks can result in total loss of fruit at harvest. Although extensive losses have been only reported on watermelon, symptoms on melon (Cucumis melo) have been seen in the fields. The disease seems to be limited to Cucurbitaceae, but little information is available on possible other hosts among this family. Characteristic symptoms on fruits appear as dark olive green stain, or blotch on the upper side of the fruit. The epidermis of the rind can then rupture and frequently bacterial ooze is produced. On leaves, small, dark brown, somewhat angular and water-soaked lesions can appear, but they are generally inconspicuous. On seedlings, water-soaked areas on the underside of cotyledons can be seen, and lesions on the hypocotyl cause collapse of the emerging plant. The pathogen is very likely transmitted by seeds. It can then be further spread by overhead irrigation in the transplant-producing units. After transplantation in the field, the disease can spread to neighbouring leaves and plants. Fruit can be infected at a very early stage and then seeds associated with the diseased fruit can remain in the soil. Local sources of inoculum that may contribute to outbreaks in succeeding crops could include volunteer seedlings from infected seeds, buried infected rind and perhaps cucurbit weed hosts. Control methods mainly rely on sanitation practices to apply during production of transplants and crop rotation. However, considering that host range is probably limited and that survival in the field within infested rind and volunteer plants may be limited in time, the authors felt that eradication of the disease from production regions may be possible.
According to this paper and the IMI description, Acidovorax avenae subsp. citrulli is present in the following countries:
Australia, Guam, Northern Mariana Islands (Tinian), USA (Arkansas, Delaware, Florida, Georgia, Iowa, Indiana, Maryland, North Carolina, South Carolina). The pathogen may also be present in Malaysia and Indonesia. In the American paper, it is however noted that in some states (Delaware, Iowa and Maryland) the disease was seen in the field in 1989, but has not been found since.
Latin, R.X.; Hopkins, D.L. (1995) Bacterial fruit blotch of watermelon - The hypothetical exam question becomes reality.
Plant Disease, 79 (8), 761-765.
IMI Descriptions of Fungi and Bacteria, No. 1213, CABI, Wallingford (GB).