constitutes a serious obstacle to the cultivation of many solanaceous plants in both tropical and temperate regions. The greatest economic damage has been reported on potatoes, tobacco and tomatoes. It can sometimes cause total crop losses. Disease severity mostly increases if R. solanacearum
is found in association with root nematodes. In tobacco, nematode infestation changes the physiology of the plants, causing susceptibility to bacterial wilt. Experiments in India showed that the combined pathogenic
effects of R. solanacearum
and root-knot nematodes (Meloidogyne javanica
) were greater than the independent effects of either (CABI, 2005)
The bacterium is a quarantine organism. The occurrence of different races and strains of the pathogen with varying virulence under different environmental conditions presents a serious danger to European and Mediterranean potato and tomato production. Absence of the bacterium is an important consideration for countries exporting seed potatoes.
Primary hosts: Lycopersicon esculentum (tomato), Nicotiana tabacum (tobacco), Solanum melongena (aubergine), Solanum tuberosum (potato), Musa (banana), Musa paradisiaca (plantain) and Heliconia.
Secondary hosts: Anthurium, Ricinus communis (castor bean), Zingiber officinale (ginger), Arachis hypogaea (groundnut), Capsicum annuum (bell pepper), Colocasia esculenta (taro), Curcuma longa (turmeric), Gossypium (cotton), Hevea brasiliensis(rubber), Ipomoea batatas (sweet potato), Manihot esculenta (cassava).
Wild hosts: Solanum nigrum (black nightshade), Galinsoga parviflora (gallant soldier), Portulaca oleracea (pussley), Urtica dioica (stinging nettle).
The disease causes rapid wilting and death of the entire plant without any yellowing or spotting of leaves. All branches wilt at about the same time. When the stem of a wilted plant is cut across, the pith has a darkened, water-soaked appearance. There is a greyish slimy ooze on pressing the stem. In later stages of the disease, decay of the pith may cause extensive hollowing of the stem. Bacterial wilt causes no spotting of the fruits. Affected roots decay, becoming dark brown to black in colour. If the soil is moist, diseased roots become soft and slimy.
|Water test for detection of bacterial wilt. Note bacterial strands oozing from infected tissue.
|© A. M. Varela and A. A. Seif, icipe
Water test: To distinguish this wilt from others, take a thin slice or sliver of the brown stem tissue and place it on the inside of a glass of water at the water level. If bacterial wilt is present, a milky bacterial stream (strands) flows from the lower cut surface of the sliver within seconds.
|Bacterial wilt (rotting of vascular ring)
|© A.A. Seif
Bacterial wilt of potato: The infested leaves wilt during the (sunny) day and sometimes recover during cool hours. The wilting is similar to the result of lack of water. During the rapid development of the disease, the entire plant wilts quickly without yellowing. Othersymptoms could be wilting of only a part of the stem, or one side of the leaf/stem. The stem wilts or dries up completely and the remainder of the plant remains healthy. When the diseased tuber is cut, it shows a browning of the vascular ring and the immediate surrounding tissues On the cut surface, a creamy fluid usually appears on the vascular ring.
Bacterial wilt of tomato/eggplant: Bacterial wilt of tomato/eggplant: The initial symptom is a wilting of the terminal leaves, which after 2 to 3 days becomes permanent when the whole plant wilts due to the active development of the disease. Then the whole plants wilt and die suddenly. Total collapse of the plant usually occurs when temperatures reach 32°C and above. Plant wilts while still green. In the case of a slow development of the disease, the plant stunts and produces large numbers of adventitious roots on the stem. Bacterial wilt diagnosis in the field can be done easily. Cut a piece of the stem 2 to 3 cm long from the base. Suspend the cut stem in clear water in a glass container. Hold the stem with an improvised tong to maintain a vertical position. Within a few seconds, milky bacterial threads are discharged from the cut stem.
Moko disease of banana: Initially one of the youngest three leaves turns pale-green or yellow in colour and breaks down at the petiole and the pseudostem. Later all the other leaves collapse around the pseudostem. An infected finger or fruit shows dry and rotted pulp that is coloured brown or black, and the presence of bacterial discharges.
Affected plant stages
Vegetative growing stage.
Affected plant parts
Leaves, roots, seeds, fruits, stems, vegetative organs and whole plant.
Growing points: wilting.
Stems: internal discoloration; creamy exudates; wilt.
Vegetative organs: internal discoloration.
Whole plant: plant death; dwarfing.