Bean Plant Diseases

Types

• Bean plant diseases are caused by bacteria, viruses and fungi. Bacterial infections consist of bacterial blight, halo blight and bacterial brown spot--often, the symptoms of each are similar, with brownish spots on the plant. Bean common mosaic virus, or BCMV, is a major disease of bean plants, while alfalfa mosaic virus and soybean mosaic virus are other serious ailments. Bean anthracnose, caused by fungus, also results in serious crop losses throughout the world.

Effects

• Bacterial blights typically start as angular, discolored, water-soaked or translucent spots on the bean plant leaves. The spots grow and merge quickly in warm and wet conditions until the leaves wither and drop off. Bean pods develop lesions that turn into reddish brown blotches and can shrivel and die. BCMV causes a downward cupping along the main vein of leaflets, which can lead to green veinbanding, blistering and malformation in the plant's leaves. These plants tend to be smaller, with mottled and malformed pods.

Geography

• Bacterial blights are more common in warm, humid areas, most in temperatures over 82 degrees, although halo blight can take hold in temperatures as low as 64. In semiarid areas such as in Idaho and California, humidity and rainfall is usually too low for bacteria to infect the bean plants. BCMV affects beans worldwide, but commercial varieties of beans are genetically resistant. Fungus-based diseases are also found worldwide but can reach epidemic proportions in places that receive frequent rainfall.

Transmission

• Bean-infecting bacteria can survive six to 18 months in plant refuse, in field piles, or on plants associated with previous crops. It even lives on weed surfaces. The primary infection is when seedlings are invaded as they emerge from the soil, while a secondary infection results when the bacteria is spread from the primary source to other plants. Viruses are transmitted via aphids, but some are carried in the seeds or other hosts. They can also be transmitted through cultivated plants and weeds.

Prevention/Solution

• To avoid major bacterial infections, it is crucial to use non-infected seed when planting beans. Reputable companies treat seeds with streptomycin to prevent contamination. Planting equipment should be sanitized with disinfectants after being used in an infected field, while crops should be grown in the same spot once in three years. Use of virus-free seed is recommended to prevent major virus outbreaks, which is also aided by growing resistant plants. For fungus, similar methods of control are recommended.