Bacterial diseases in vegetable crops

Bacterial diseases

Bacterial diseases – an overview:

This page provides an overview of the bacterial diseases in vegetable crops. The related tools listed at the end of the page provided detailed information about the identification, symptoms, and management of bacterial diseases. It is important to have a plant diagnostics laboratory confirm the pathogen causing any diseases in a crop so that the disease can be appropriately managed.

Pathogenic bacteria cause many serious diseases of vegetables. They do not penetrate directly into plant tissue but need to enter through wounds or natural plant openings. Wounds can result from damage by insects, other pathogens, and tools during operations such as pruning and picking.

Bacteria only become active and cause problems when factors are conducive for them to multiply. They are able to multiply quickly. Some factors conducive to infection include: high humidity; crowding; poor air circulation; plant stress caused by over-watering, under-watering, or irregular watering; poor soil health; and deficient or excess nutrients.

Bacterial organisms can survive in the soil and crop debris, and in seeds and other plant parts. Weeds can act as reservoirs for bacterial diseases. Bacteria spread in infected seed, propagating material and crop residues, through water splash and wind-driven rain, and on contaminated equipment and workers' hands. Overhead irrigation favours the spread of bacterial diseases.

Warm, wet weather favours the development of some bacterial diseases, while others are favoured by cool, wet conditions. Development is often arrested by hot, dry conditions, but may exacerbate symptoms once the plant is already infected (e.g. Bacterial wilt caused by Ralstonia solanacearum).

Sometimes bacterial ooze may be seen on diseased plant tissue. However, symptoms of bacterial diseases may be confused with those caused by fungal diseases. It is important to have diseased tissue examined in a plant diagnostics laboratory to confirm the type of pathogen causing the disease.

Different strains (pathovars – pv.) of bacterial diseases affect different types of vegetable crops or cause different diseases in the same crop. For example: Xanthomonas campestris pv. vitians in lettuce and X. campestris pv. cucurbitae in cucurbits; and in beans Psuedomonas syringae pv. syringae and P. syringae pv. phaseolicola cause different diseases.

Common bacterial diseases and crops affected:

Some examples of common fungal diseases of vegetable crops are provided in the table below with some typical symptoms.

Bacterial disease	Factors conducive to spread	Crops affected	Symptoms
Black rot (Xanthomonas campestris pv. campestris)	Warm, wet conditions.	Brassicas.	Light-brown to yellow V-shaped lesions on the leaf, which become brittle and dry with age. Vein blackening with the necrotic area.
Bacterial canker (Clavibacter michiganensis pv. michiganensis)	Moderate temperatures and high humidity.	Tomato; capsicum; chilli	Seedlings may die and older plants may wilt and die eventually. Older plants have leaves that turn yellow and wilt only on one side. Cankers on stems and fruit. Tissue inside stems becomes discoloured.
Bacterial soft rot (Pseudomonas spp., Erwinia spp.)	Warm, wet conditions.	Wide range of vegetables, including lettuce; brassicas; cucurbits; tomato; capsicum; potato; sweetpotato; carrots;herbs.	Wet, slimy, soft rot that affects any part of vegetable crops including heads, curds, edible roots, stems and leaves. May have a disagreeable odour.
Bacterial leaf spot/Bacterial spot (Xanthomonas campestris - various strains)	Overhead irrigation and windy conditions.	Range of vegetables including lettuce; cucurbits; tomato; capsicum.	Lettuce – Large brown to black circular areas that start as small translucent spots; usually on outer leaves. Tomatoes and capsicums – Greasy spots on leaves and stems that go from tan to black; fruit may have circular spots with central scab. Cucurbits – Begin as small water-soaked/greasy spots on underside of leaves with corresponding yellowing on upper side; fruit may produce light-brown ooze from water-soaked markings.
Bacterial wilt (Ralstonia solanacearum)	High temperatures, high soil moisture and poor drainage. Once infection has occurred, severity of symptoms is increased with hot and dry conditions, which facilitate wilting.	Potato; tomato; capsicum; eggplant.	Wilting, yellowing and stunting of plants but they may wilt rapidly and die without any spotting or yellowing; vascular tissue appears brown and water-soaked; a white ooze appears when pressure is applied to affected tubers or stems.
Bacterial leaf spot/Bacterial spot/Bacterial blight (Pseudomonas syringae - various strains)	Long periods of leaf wetness.	Beet; spring onions; leeks; rocket; coriander.	Beet – irregular, round leaf spots with a grey centre surrounded by a purple margin. Spring onions/shallots – pale yellow to light-brown lesions with a water-soaked appearance around the margins; outer leaves wither and die and youngest leaf turns lemon to light-green. Leeks – brown streaking on the shank.
Bacterial blight (Pseudomonas syringae pv. pisi)	Cool, wet, windy conditions.	Peas.	Water-soaked spots on leaves and stipules which become dark-brown and papery in warm weather or black in cool weather. Water-soaked spots on pods that become sunken and dark-brown.
Bacterial speck (Pseudomonas syringae pv. tomato)	Humidity and overhead irrigation.	Tomato.	Small dark spots surrounded by a yellow halo on leaves; dark raised specks on fruit.
Bacterial brown spot (Pseudomonas syringae pv. syringae)	Cool, wet, windy conditions.	Beans.	Tan to reddish-brown spots on leaves. Water-soaked spots on pods which enlarge and become sunken and tan with distinctive reddish-brown margins.

Other bacterial diseases of vegetables include:

Peppery leaf spot

Pseudomonas syringae pv. maculicola (brassicas)

Varnish spot

Pseudomonas spp. (lettuce)

Corky root

Rhizomonas suberifaciens (lettuce)

Angular leaf spot

P. syringae pv. lachrymans (cucurbits)

Bacterial pith necrosis

Pseudomonas corrugata and other bacteria (tomatoes)

Common bacterial blight

Xanthomonas campestris pv. phaseoli (beans)

Halo blight

Pseudomonas syringae pv. phaseolicola (beans)

Black leg

Erwinia carotovora pv. atroseptica (potatoes)

Management:

Disease management strategies aim to favour the host plant’s growth and development while attacking vulnerable stages in the lifecycle of the pathogen to prevent or restrict its development. The key means of bacterial disease management include:

Exclusion or eradication of the pathogen (quarantine and use of pathogen-tested seeds and propagated materials)

Use of clean transplants

Monitor crops regularly and use predictive models

Reduce the pathogen levels by crop rotation

Remove weeds and incorporate crop residues that can host the disease

Protect the host plant by using resistant plant varieties

Minimise mechanical damage to crops and damage by insect pests

Avoid working in crops when they are wet

Spray with a registered bactericide when weather conditions favour disease development to prevent infection

Understand chemical resistance and rotation of chemical groups

If the plants are already infected, isolate and destroy them and prune infected leaves, but avoid excessive handling of diseased plants; if the disease is systemic and has spread throughout the plant, the plant cannot recover and should be destroyed (burning or burying)

Use correct temperatures and packing conditions during transport and storage.