This disease, caused by the bacteria Pseudomonas syringae pv. pisi and P. syringae pv. syringae, is a serious disease of peas. Recently, the pathogen P. syringae pv. syringae has been considered as the main cause of the disease in pea crops. Bacterial blight is widespread in field peas in southern New South Wales and Victoria, but its severity varies greatly from crop to crop and between seasons. The disease is seed borne and is more prevalent after frost events; multiple frosts can cause epidemics resulting in significant yield loss.
- bacterial blight first appears as small, dark green, water-soaked spots on leaves and stipules, often near the leaf base. The spots enlarge and merge but are often limited by the veins
- the leaf spots turn yellowish and later brown and papery, (Figure 7.5)
- spots on pods are sunken and olive brown
- spots can develop on the stem near ground level. These begin as water-soaked areas that later turn olive-green to dark brown. Stem lesions may coalesce, causing the stem to shrivel and die
- stem lesions may spread upwards to the stipules and leaflets. In this case, a fan-like lesion is formed on the stipule. Spots can merge causing the stem to shrivel and die, (Figure 7.6).
Pre-emergence and post emergence damping-off may occur and even advanced plants may be killed. Heavily infected seed may be discoloured, but light infection has no visible effect on seed.
The symptoms of bacterial blight caused by Pseudomonas syringae pv. pisi or Pseudomonas syringae pv. syringae are indistinguishable from each other.
Figure 7.5 Leaf lesions caused by bacterial blight
Figure 7.6 Water soaked lesion spreading into the leaf from the base caused by bacterial blight
Bacterial blight commonly becomes established within a field by sowing infected seed or from infected pea trash that is nearby. During wet weather, bacteria spread from infected to healthy plants by rain splash and in wind-borne water droplets, (Figure 7.7).
Infection may occur at any stage of plant growth. The pathogens can remain on the surface of plants without causing symptoms. However, following rain, heavy dew, frost or other forms of damage to plant tissues, symptoms can develop. Damage to field peas provides entry of bacteria into the plant tissue. Early infections may lead to epidemics but later infection can also cause yield losses. Because the disease depends on wet conditions, bacterial blight is most severe in wet seasons. A combination of excessive rainfall and strong winds provides the most favourable conditions for spread of the disease within crops.
Pseudomonas syringae pv. pisi is largely restricted to field peas but P. syringae pv. syringae has a wide host range including clover, common beans, faba beans, lentils, chickpeas and vetch which act as alternate hosts.
Figure 7.7 Disease cycle of bacterial blight in field peas. Illustration by Kylie Fowler
Bacterial blight can be avoided by using an integrated approach to management that encompasses planting disease-free seed, crop rotation, variety selection and avoiding early sowing.
This is the main control measure recommended. The use of clean seed will minimise the possibility of disease, provided the land has not been cropped to peas for several years. Do not use seed from crops identified with bacterial blight during field inspections. A field inspection should occur at mid to late pod fill. Bacteria remain viable on seed for at least 2 years. Seed testing for bacterial blight is available from: AGWEST Plant Laboratories, Department of Agriculture Western Australia and AsureQuality 3-5 Lillee Crescent (PO Box 1335) Tullamarine Vic 3043.
To obtain a blight-free crop, peas should not be sown on land sown to peas in the previous year or adjacent to pea stubble. Where possible, peas should not be grown on the same land more than once in three years. If disease occurs the rotation should be extended to once in four years.
Stubble can be a significant source of inoculum. Destroy by burying, baling or burning infected stubble. The survival time of inoculum is significantly reduced by burying pea trash 10 cm below the soil surface.
Early sown crops are more vulnerable to bacterial blight infection than late sown crops; never sow earlier than recommended for your district. In areas prone to bacterial blight avoid early sowing.
Bacterial blight is often associated with physical crop damage such as hail, frost, strong winds, sand blasting or machinery damage. Physical damage enables bacteria to enter plant tissue. Minimise the use of post emergence herbicide sprays, if possible, as the severity of bacterial blight can increase if plant tissue is damaged. Avoid paddocks where sulfonylurea residues may be present and the more frost-prone paddocks.
The frequency of bacterial blight can be reduced by avoiding varieties susceptible to Pseudomonas syringae pv. syringae, (Table 7.1) and sowing those that have better resistance.
When bacterial blight is detected, steps should be taken to prevent the spread of disease. Where possible, harvest infected crops last to avoid contaminating healthy crops and machinery used in an infected crop should be cleaned thoroughly and washed with disinfectant after use. Likewise, machine operators and farm workers should only move from crop to crop after taking precautions against the spread of bacteria. This is best achieved by wearing rubber boots and waterproof trousers that are washed with disinfectant immediately after leaving an infected field. Crops should never be inspected when they are wet as this increases the chance of spreading disease.
Fungicides and seed treatments are designed to be active against fungal diseases and are ineffective in the control of bacterial diseases. There are copper based compounds, registered for use in field peas against bacterial blight, but evidence for their effectiveness in Australian field pea crops is limited and inconclusive.
ASCOCHYTA BLIGHT (BLACKSPOT) OF FIELD PEAS
DOWNY MILDEW OF FIELD PEAS