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Bacterial Diseases 101 (06/30/16)

The reports of high winds, hail and rain throughout the last two weeks has contributed to the frequent reporting of bacterial diseases.

Bacterial Diseases 101

The reports of high winds, hail and rain throughout the last two weeks has contributed to the frequent reporting of bacterial diseases. Most crops grown in North Dakota are hosts to bacterial diseases, and although differences do occur among host crops, most bacterial diseases in the state have similarities in spread and management.

Wounding

Bacterial pathogens need a wound or opening to invade plant tissue. They cannot create their own entry point like fungal leaf pathogens. Wounding can occur from hail, wind, sand blasting, insects, humans and several other causes. Most of the wounding events are beyond our control, however at least one can be controlled. If a bacterial disease is apparent in a field, avoid unnecessary traffic when the field canopy is wet to prevent the wounding of plant tissue when there is a favorable environment to spread of the bacterium.

Water

Rain and dew promote the spread of bacteria by creating a conducive environment for disease development. The greatest epidemics of bacterial diseases often coincide with a wind driven rain events or severe thunderstorms

(See Bacterial blight article on dry beans on page 8).

Symptoms and Signs

Bacterial diseases will often have vastly different symptoms and signs when compared to fungal diseases. Early bacterial lesions usually appear water-soaked (greasy) (Figure 1) and turn necrotic after a few days resulting in premature death of the entire leaf (defoliation) (Figure 2). A diagnostic sign of bacterial pathogens is bacterial ooze (Figure 3). The ooze is the primary source of inoculum that can be spread during rain events and through other types of movement in a wet canopy. Eventually the ooze will dry giving the infected area a shiny appearance resembling a glazed donut (Figure 4).

Fungicides Are Not Recommended

Fungicides do not provide satisfactory management of bacterial diseases. Some research conducted in other states has shown that some chemicals, like cupric hydroxides, can suppress new infections, however most research conducted in North Dakota has not shown an economic benefit from foliar applications. Therefore, disease identification is important when making the decision to apply a fungicide. Misdiagnosing a bacterial disease for a fungal disease will greatly reduce chances of disease control and return on investment.

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Sam Markell

Extension Plant Pathologist, Broad-leaf Crops

Andrew Friskop

Extension Plant Pathology, Cereal Crop


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