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Holcus Leaf Spot

Bacteria

Pseudomonas syringae pv. syringae


In a Nutshell

  • Lime-green translucent discolorations appear along the leaf veins of lower leaves.
  • Lesions expand longitudinally and tend to coalesce.
  • The brown necrotic stripes in their centers dry and fall out, giving the leaf a ragged appearance.
  • Bacterial ooze is sometimes perceptible in infected tissues.

Symptoms

Lesions appear initially as lime-green to olive-colored translucent discoloration running along the leaf veins of lower leaves. They also gradually start to show in the upper foliage. In optimum weather conditions, these lesions expand longitudinally and tend to coalesce. Bacterial ooze is sometimes perceptible in infected tissues at early stages of the disease. With age, the lesions develop brown necrotic stripes in their centers, that later dry and fall out, giving the leaf a ragged appearance. In some susceptible corn varieties, chlorotic striping of the whorl leaves and distortion of the upper nodes of the plant can be observed.

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Hosts

Trigger

Symptoms depend greatly on the strength of the pathogen, the corn variety and the environmental conditions. The bacteria survives in crops debris in soils, on numerous alternative hosts and weeds and on volunteer crop plants. It is disseminated between plants by irrigation water, wind, or contaminated workers and equipment. The bacteria enters the plant through natural openings or wounds. It can withstand temperatures between 0 and 35 °C, but it thrives in the range 25-30°C. The disease is worst during periods of wet humid weather. When the disease occurs early in the season, some farmers choose to destroy the entire crop by disking.

Organic Control

To this day, no effective organic treatment is available. Alternative options for control of bacterial spot in corn are limited to the use of preventive measures and good field practices.

Chemical Control

Always plan an integrated pest or disease management, with preventive measures and biological treatments if available. Currently, chemical treatment is limited to copper or copper combined products. Many sprays are only marginally effective, making the disease very difficult to control once an epidemic is underway.

Preventive Measures

  • Use seed material from healthy donor plants or from certified sources.
  • Choose tolerant varieties available in your market.
  • Plant late to avoid wet climatic conditions that spread the disease.
  • Avoid working in fields when foliage is wet.
  • Avoid overhead irrigation.
  • Clear the field of weeds and alternative hosts.
  • Do not compost or leave plant residues near plantations.
  • Disinfect your tools with high-proof alcohol or fire (not both).
  • Remove infected plants immediately and burn their remains.
  • Crop rotation with non-susceptible crops is recommended.

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