Australia

Wilfire (Angular leaf spot)

Pseudomonas syringae pv. tabaci

The name of the disease, wildfire, results from the burnt appearance of heavily infected tobacco plants. Initial wildfire symptoms are small circular pale-green areas (ca 0.5 cm in diameter) which become brown in the centre owing to necrosis of parenchymatic tissues. The necrotic process advances rapidly and the chlorotic halo extends; within a few days spots enlarge up to 3 cm in diameter, with a water-soaked zone surrounding the necrotic centre. Spots may coalesce and dead tissues expand irregularly. Heavily infected leaves grow badly and become distorted. The formation of the chlorotic halo is due to the diffusion of toxins released by the pathogen at the border of the necrotic area. Therefore the extension of the spots depends on its capability to colonize the adjacent cells, which is also regulated by weather conditions and host susceptibility.
In seedbeds the disease develops in irregularly distributed strains. Under high moisture or crowded conditions, leaves can be completely infected and rot rapidly. In the field, wet weather allows the formation of large necrotic spots with a thin halo. Spread of the disease is usually observed after rain storms, with the direction of spread determined by the wind. Yellow symptoms have been described in Japan on tobacco plants damaged by a hailstorm; those symptoms were reproduced through artificial inoculation of the bacteria in the stem or leaf midribs (Ono, 1980). Although leaves are the elective site of the disease, lesions can occasionally be observed on stalks, flowers and capsules.
The course of disease symptoms on soyabean is similar (Allington, 1945). The incidence of wildfire on the latter crop is often associated with lesions caused by two other bacterial pathogens, Xanthomonas campestris pv. glycines [X. axonopodis pv. glycines] and Pseudomonas syringae pv. glycinea [P. savastanoi pv. glycinea], which act as infection courts (Chamberlain, 1956).
In the case of blackfire or angular leaf spot, necrotic areas are angular in shape and darker than described, almost black; the chlorotic halo is absent. Necrotic tissues in the centre of the spots may break away. Deformation of the leaves results as a consequence of the uneven growth of healthy and infected areas. When epidemics arise, diffused dropping of necrotic tissues occurs and veins only may remain in place of the leaves. On a number of cultivars, leaf spots may be circular and show concentric circles.
Significant differences between strains causing wildfire or blackfire were recorded by Deall and Cole (1986) in localization of spots with reference to plant age and stalk position, lower leaves of young plants being more susceptible to wildfire while top leaves of older plants were more susceptible to angular leaf spot. However, at the end of the season it is often difficult to distinguish between the two described syndromes, which can occur in the same field at the same time.