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Bacterial Blight and Bacterial Pustule

 

Bacterial Blight

Bacterial blight is the most common bacterial disease of soybean in Ohio. Symptoms related to this disease are often observed early season, allowing the plant to recover as it puts on new growth throughout the season. For this reason, yield losses are minimum when the disease occurs. However, highly susceptible cultivars, and highly favorable conditions can cause yield losses of up to 40 %.

Causal Agent:

This disease is caused by the bacteria Pseudomonas savastanoi pv. glycinea. There are at least 9 known races of the pathogen. The most frequently isolated is race 4, which is known to be pathogenic on almost all soybean cultivars. Several avirulence genes have been identified since the first was discovered in 1984. The pathogen grows well on King’s B medium and will produce a weak glow under a blacklight. In culture, colonies will be white, circular, smooth, and may produce mucus.

Symptoms and Signs:

Symptoms caused by P. savastanoi pv. glycinea can be observed on the leaves, pods, and stems of the plant, with the greatest disease pressure in the mid toupper canopy. At early stages, cotyledons may have brown spots on the leaf margin and die prematurely if infection reaches the apex. As the plant matures, infection will appear on the young leaves, as these are the most susceptible. The most common symptoms on leaves are brown, angular lesions with chlorotic halos. Necrotic spots will merge together and form a large patch that will make the foliage appear broken-down. Pods that are infected will have small and water-soaked lesions that will later turn necrotic. Infected seeds will be discolored and may have a slimy coating due to the bacteria creating a film on the seed surface. If the disease is severe, stunting is also observed at later growth stages.

Disease Cycle:

The bacterium, overwinters in dried crop residue and seeds. Rain and high winds disperse the inoculum onto plant tissues where bacterial cells are introduced into the plant through natural wounds or stomata (Figure 1). Water must be present in order for the bacteria to disperse and infect the leaves.  Necrotic lesions appear 3-7 days after infection occurs, when the bacteria produces toxins that prevent chlorophyll production. Wet and cool temperatures (70-80°F) can promote faster disease development.  Pathogen transmission from a sick plant to a healthy plant can occur when the two plants make contact in high winds, or from human field activity (such as spraying), which can move water droplets containing bacterial cells from plant to plant.

On the left, the underside of a leaf showing symptoms of bacterial pustule. On the right, a leaf with irregular lesions caused by the bacterial blight pathogen.

As a common diagnostic test, a cross section of a lesion is submerged in water and observed under the microscope. The dark cloud emerging from the leaf tissue contains millions of bacterial cells, commonly referred to as bacterial streaming.

Bacterial Pustule

Bacterial pustule has been reported in Ohio, but with less abundance than bacterial blight. It is more prevalent when warm and wet conditions are present and symptoms may appear later in the growing season. Symptoms can appear very similar to symptoms of bacterial blight and soybean rust, so knowledge of the differences between these diseases is critical for accurate diagnosis.

Causal Agent:

Bacterial Pustule is caused by the bacteria Xanthomonas axonopodis (syn. campestris) pv glycines. In culture, this bacteria is gram negative and forms small, deep yellow, circular colonies on rich media.

Symptoms and Signs:

Symptoms related to bacterial pustule are often confused with soybean rust due to the lesion color and pattern. Pustules caused by X. axonopodis pv glycines first appear as pale green spots on leaves The spots, which are commonly observed in the main veins, can develop in either the upper or lower surface of the leaf. Pustules can merge together to form large lesions with irregular patterns. A distinct feature to differentiate bacterial pustules from soybean rust are the raised necrotic centers and the lack of spores when observed under a microscope.

Disease Cycle:

Similar to bacterial blight, bacterial pustule overwinters in crop debris. Wind-driven rain and water droplets can disperse the inoculum from one place to another. This bacterium uses natural openings, stomates, or wounds to enter the plant and colonize. It can also be spread when performing other agricultural practices. Optimal temperatures to grow range from 86-92 degrees F.

Bacterial Disease Management

Although, bacterial disease are often less prevalent than other diseases, growers should consider the following management techniques if a bacterial disease is present in soybean fields.

Resistant Cultivars: The use of a resistant cultivar should be considered to reduce the disease incidence in a highly infested field. There are at least 4 known resistance genes against bacterial blight. As the most economical management strategy, resistant varieties should be used where fields are known to have significant presence of bacterial disease.

Pathogen-free Seed: Infection can be seedborne, so it is important to use seed that is certified to be pathogen-free. Do not use seed from infected plants.

Cultural Practices: Avoid any cultivation practice when plants are wet, since this will spread the inoculum in the field.

Chemical Control: To date, no field in Ohio has warranted sprays, but there are Copper-based sprays that can be used to control bacterial diseases of soybean.

Crop Rotation: This will significantly reduce the amount of inoculum present in the field.

Tillage: This will help eliminate the inoculum present in crop residue where the bacteria usually overwinters.