Common bacterial diseases in onion farming-how to control

Understanding the common bacterial diseases in onion farming, how to identify them, and control them can save unnecessary costs.

Bacterial Blight of Leek

Symptoms

The disease first appears as

• Dark-green, longitudinal, water-soaked lesions that form at leaf tips and edges.
• As they elongate, lesions turn orange to brown with surrounding chlorosis and may extend as a narrow strip from leaf tip to the sheath.
• When a lesion extends into the sheath, the affected leaf turns light-green, curls, splits and eventually wilts and dies.
• Severely affected plants are misshapen, undersized and cannot be harvested.

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Conditions for Disease Development:

• Infested seed and infected leek debris from a previous crop are both sources of primary Inoculum.
• The bacterium may infect but remain latent in the plant until environmental conditions favor development of disease.
• Generally, warm temperatures and high humidity encourage symptom expression and disease spread.

Control:

• Sow only clean seed.
• During the growing season, limit overhead irrigation and avoid mowing the crop when plants are wet with dew or rain.
• Removing infected plants and plant debris throughout the season and rotating to a non-host help mitigate the risk of disease.
• Apply soil amendments as needed to increase soil pH to at least 5.5 to reduce the chance of infection.

Bacterial Leaf Streak and Bulb Rot

Symptoms:

The first symptoms observed are;

• Oval, water-soaked leaf lesions, tip-burn and leaf streaking of varying lengths.
• Initially, leaf streaks are green but eventually darken to black.
• As infections become more severe and spread down the leaf, entire leaves collapse and dry.
• Leaf distortion and twisting may also occur.
• Bulb infection is characterized by dark spots on outer scales and reddish-brown
• discolouration of inner scales.
• Symptoms often develop in a ring-like pattern due to restriction of the rot by the scales.

Conditions for Disease Development:

This disease occurs;

• Particularly in winter and spring when temperatures are cool.
• Epidemics are associated with prolonged periods of rain, which favor progression of the disease.
• Excess fertilizer stimulates disease development.
• It is thought that frost damage may predispose onion plants to infection.

Control:

• Applications of fixed copper compounds or streptomycin inhibit spread of this
• disease although bacterial strains resistant to copper may occur.
• Excessive fertilizer applications may increase foliar symptoms and should be avoided.
• Reduce postharvest rot by harvesting onions at the proper maturity stage, by reducing wounding and bruising during harvest and by proper curing of bulbs with forced hot air.

Center rot

Symptoms:

Symptoms first appear as

• Whitish to tan lesions with water-soaked margins, often on interior leaves.
• Foliar lesions can rapidly coalesce, progressing to wilt and dieback of affected leaves. The pathogen moves from the leaves into the neck and bulb causing yellowish to light-brown discoloration.
• With severe infections, all leaves can be affected giving a bleached appearance to plants. 
• Secondary bacterial infections rot interior bulb tissue and produce a foul odor.
• Under conditions favorable to the disease, yield losses may approach 100 percent.

Conditions for Disease Development:

• Both pathogens are seed borne and can survive on a few reported alternate hosts (corn, cotton, melon, pineapple, rice and sugar cane).
• They may also survive epiphytically on weeds and crop debris.
• Spread can occur by wind, splashing water and thrips.
• Infection is favored by moderate to warm temperatures and rainfall during bulb initiation.

Control:

• Seed produced in high-risk areas should be tested for Pantoea ananatis and Pantoea agglomerans before sowing.
• Some onion varieties are known to be more susceptible to this disease than others. Avoid planting these varieties where disease pressure is high.
• Control weeds, volunteer onions and thrips.
• Consider drip rather than sprinkler irrigation if possible
• Avoid working in fields when foliage is wet.
• Avoid excessive nitrogen fertilization.
• If applied preventively, copper-based bactericides may provide control under low
• to moderate disease pressure.
• Initiate sprays two weeks before bulbing and continue every 5-7 days thereafter.
• Deep cultivate after harvest to promote decomposition of crop debris.
• Where this disease occurs, a minimum three-year rotation to non-hosts is recommended.

In part 2 we shall take a look at

• Bacterial Soft Rot
• Enterobacter Bulb Decay
• Slippery Skin
• Sour Skin
• Xanthomonas Leaf Blight

Reference

Lowell B. et al,(2012)Onion Disease Guide; A Practical Guides For Seedsmen, Growers, And Agricultural Advisors. Seminis Plant Health

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