Issue No. 18, Article 5/July 28, 2006
Ragged, tattered, brown-speckled soybean leaflets are showing up in some fields in the central part of Illinois. These symptoms are characteristic of the bacterial leaf disease known as bacterial blight. We don't see this disease all that often, so keep an eye open for these telltale symptoms.
Bacterial blight is caused by the bacterium Pseudomonas syringae, which overwinters on residue and sometimes spreads via infected seed. The bacterium is transported to the plant when it is blown and splashed onto the surface of soybean trifoliates. P. syringae remains on leaflets until proper moisture conditions prevail, when it then will infect the plant. During wet weather, the bacterium enters the leaflet either through the water-regulating stomata or, more typically, through wounds. The bacterium moves into the spaces between the cells and multiplies. At the same time, the space between the cells is filled with a slimelike bacterial excretion, while a toxic bacterial by-product inhibits the formation of chlorophyll. The latter two substances result in symptoms easily observed with the naked eye within five to seven days of infection. The bacterium spreads best when storms are particularly windy.
Bacterial blight initially speckles the soybean leaflet with angular yellow spots that eventually turn light brown. Those lesions initially appear slightly water soaked. Leaflets appear slightly puckered, similar to the puckering observed with some soybean viruses. As the tissue encompassed by the lesion dies, the lesion turns a dark red-brown to black color. These necrotic lesions appear all the more stark because they are surrounded by a yellow halo. The lesions usually are seen on soybean leaflets but can appear on other portions of the plant as well. As bacterial blight progresses, the disease looks even more unsettling. Given enough moisture and optimum temperatures for development of the bacterium (70° with a bacterial envelope of 39° to 95°), the black lesions coalesce, forming large regions of necrotic tissue. The interveinal appearance of these necrotic areas may resemble well-aged sudden death syndrome. Given a little more time, the necrotic regions begin to drop from the leaflet, forming large, gaping, ragged holes. When combined with a little wind, the ragged appearance of leaf tissue intensifies.
Exceptional bacterial blight symptoms.
In-season management of bacterial blight is nonexistent. However, hot, dry weather stalls development of P. syringae. Some resistance does exist in soybean varieties, which restricts yield losses to a less than double-digit range. Those varieties without resistance may see yield losses near 15% when the disease is prevalent and ideal conditions prevail. Since farm implements provide both a means of transportation for the bacteria and the slight mechanical injury needed for infection, cultivation and other operations should proceed with caution in those fields exhibiting more intense bacterial blight symptoms. Where conservation agreements allow, deep plowing may bury residue, thus decreasing symptoms during the years following a bacterial blight outbreak. As usual, the use of high-quality (pathogen-free) seed may also aid the producer hoping to make the appearance of bacterial blight a once-in-a-lifetime event.--Matt Montgomery