Pea blight

Pea blight (P. syringae pv. pisi) is a bacterial plant pathogenic pathovar of Pseudomonas syringae, which only infects Pea plants. It infects the plant through stomata and wounds. For the development of serious blight, wounds (e.g. those formed by hail, frost or wind-blown sand) appear to be a prerequisite. After infection the bacterium spreads through the intercellular spaces and enters parenchyma cells of the cortex and pith. Bacterial cavities are formed and vascular tissues are also attacked from where the bacteria may spread to stipules, leaflets and inner sides of pods and, via funicle and micropyle, into the seed. Seeds may also become infected at harvest by contact with diseased material. Dry weather and high temperatures prevent disease spread, while frost may enhance it. Disease has been observed to develop more readily on soils with a high moisture content. Infections can occur through contact of diseased and healthy foliage and insects may have a role in transmission.

Symptoms
Symptoms on peas may be found on all aerial plant parts, including stipules, leaflets, petioles, stems, tendrils, flower buds and pods, but those on stems and stipules are most characteristic.

In dry weather, with occasional frost, symptoms usually appear on the stem near the soil as water-soaked and later olive-green to purple-brown spots. The infection extends upwards to the stipules and leaflets, where veins turn brown to black and adjacent tissues become diseased in a fan-like pattern. The interveinal tissues may become watersoaked and then yellowish to brown, finally drying out and becoming papery.

In rainy weather, lesions on leaflets and pods begin as small, round, oval or irregular dark-green water-soaked spots at first, and later enlarge and coalesce but are sharply defined by the veins. A cream-coloured bacterial ooze may be found on the lesion surface which, on drying, gives a glossy appearance. The leaflets later become yellowish and the spots brown and papery. Ripening pods become twisted and dry, lesions on them sunken and greenish-brown. Lesions on the pod may be limited to a narrow band on the sutures. When pod invasion is mainly along the dorsal suture, the seed inside may be covered with bacterial slime. Infected seeds show a water-soaked spot near the hilum and/or are shrivelled, with a brown-yellow discoloration. Infection often takes place on sepals, spreading to the flowers, and flower buds may be killed before they open.

If the infection spreads all over the plants, they may wither and die. Under warm dry climatic conditions, however, the infection stops and the upper parts of the plants remain green and produce healthy flowers and pods. This may also be true for new axillary growth from the base of diseased plants.

Treatment
Pea blight cannot be controlled by chemical or biological methods at present. There is no suitable method to eliminate the bacterium from the seed.

Prevention
Disease prevention can be obtained through production of disease-free seed by growing seed under semi-arid conditions and/or certification, and through late sowing and avoidance of frost-pockets to reduce infection chances, or by sanitation.