Blight is a term used to describe a large spectrum of plant diseases which cause wilting, drying and eventual death of the infected area. The term bacterial blight describes blights which are caused by bacteria. These diseases occur in various plants worldwide including trees, flowering plants, cotton, food crops and grains.
The bacterial blight which affects soybeans is also known as soybean rust. This infection, caused by the bacteria Pseudomonas savastanoi pv. glycinea, starts with small yellow spots on the leaves. The spots are surrounded by a watery clear or white circle that looks like a halo. As the disease progresses, the lesions turn brown and affected parts of the leaves will dry and eventually drop off.
Gardeners may discover lesions on the leaves of their green beans or butter beans. These spots may start out as pale green areas which eventually turn brown, and usually appear a few days after a windy rain or hailstorm. If the affected leaves are not removed the disease can spread into the stem of the plant. This type of blight is referred to as leaf spot, stem rot or black rot.
Blights on cotton plants initially appear as spots with a red or brown border on the leaves. The bacteria can spread into the bolls and destroy the cotton. Bacterial blight is contagious and if it is allowed to progress unchecked significant crop losses can occur. Fortunately, farming techniques appear to have limited the spread of the disease in cotton, and only small outbreaks have occurred in recent years.
Grain crops such as wheat, barley, oats, rye, and rice can be affected by several forms of the disease. In wheat, bacterial blight may appear as bacterial stripe, black chaff, basal glum rot, bacterial black point and bacterial leaf blight. Barley can be infected with bacterial streak or black chaff; oats with halo or strip blight; rye with bacterial blight.
The grain which is the most susceptible is rice, probably because rice is grown in moist conditions, which are perfect breeding areas for bacteria. The disease was first discovered in Japan in the late nineteenth century and has been found in Asia, Africa, Australia, and America. Bacterial blight can spread quickly through a rice field significantly reducing the crop yield. Unlike other bacterial plant infections, some progress has been made developing chemical treatments for affected rice.
Flower gardens and some trees are also susceptible to bacterial disease. Geraniums, roses and lilacs can all become infected. Trees are most susceptible during the early spring when conditions are wet and if they have wounds such as broken branches or insect holes. Younger trees appear to be more likely to be diagnosed with bacterial blight than more mature trees. For example, the disease rarely appears in hazelnut trees which are more than ten years old.
Bacterial blight grows best under damp, humid conditions and can be spread through wind and rain, soil and infected seed. In grain crops, the germs can live for months in the stubble and straw which remains after the harvest. The primary means of controlling the disease is through crop rotation and proper irrigation. Watering from the ground near the base of the plant and leaving adequate space between plants to allow for proper ventilation is very helpful in preventing infection.