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.