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Bacterial Blights
Four types of bacterial blight occur in North Dakota:
Common blight (Xanthomonas campestris pv. phaseoli)
Fuscous blight (A variant of Xanthomonas campestris pv. phaseoli)
Halo Blight (Pseudomonas syringae pv. phaseolicola)
Brown Spot (Pseudomonas syringae pv. syringae)
Blight bacteria cause plant defoliation with discoloration and shriveling of seed. Because blight bacteria are highly contagious, they can easily cause epidemics with losses exceeding 75 percent of the potential yield.
Halo blight is most evident during cool, rainy weather, especially early in the season. Common and fuscous blights occur during warm, wet weather. Brown spot is more serious in protected areas where plants dry slowly. Blight bacteria are seed-borne and enter the seed through natural openings or wounds. The bacteria survive in seed many years. Certified seed should contain few blight bacteria.
Disease generally begins from infected seed or from bacteria surviving in debris of previous crops. The bacteria cause small spots on cotyledons or young foliage. From these spots, bacteria exude onto the leaf surface during periods of rain or dew. The bacteria are then spread from plant to plant by rainsplash or overhead irrigation.
Blight bacteria can enter plants through natural openings but enter most easily through wounds. Hailstorms or blowing sand accompanied by rainstorms are ideal for spreading bacteria.
Blight bacteria also spread by contaminated equipment, by insects, or by animals, including man. Reducing movement through fields and cultivating after plants are dry helps reduce spread of blight.
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