Blight
Blight is a rapid and complete chlorosis, browning, then death of plant tissues such as leaves, branches, twigs, or floral organs. Accordingly, many diseases that primarily exhibit this symptom are called blights. Several notable examples are:
Bacterial blight of cotton
The bacteria can affect the cotton plant during all growth stages, infecting stems, leaves, bracts and bolls. It causes seedling blight, leaf spot, blackarm (on stem and petioles), black vein and boll rot. On cotyledons small, green, water-soaked rounded (or irregular) spots form which turn brown. Cotyledons can be distorted if the infection is intense. Black and elongated lesions can girdle the hypocotyls and kill seedlings. On the leaves, scattered small dark-green, water-soaked, areolate spots, form measuring 1–2 mm on the lower surface, which appear translucent against transmitted light. The spots increase in diameter to 5 mm, become angular (due to leaf veination), brown and later turn dark brown to blackish, becoming visible on the upper surface. On susceptible cultivars numerous spots can occur, causing chlorosis, necrosis and distortion, and eventually defoliation. the black arm symptom is characterized by dark brown to black lesions which may coalesce and the extended necrosis then girdles the stem with blockage of the vascular system at infected sites which hinder the movement of water and nutrients though the plant system. Drooping of leaves is associated with this hindrance of water and nutrient movement. Cracking of the stem and gummosis is also observed in infected plants. The resultant breaking of the stem which hang typically as a dry black twig is referred to as the ‘black arm’. The pathogen also reportedly causes blackening of the veins and veinlets, giving a typical ‘blighting’ appearance.