Bacterial Leaf Blight

Bacterial leaf blight is often first noticed in fields as brown areas about 3 to 4 feet in diameter. Leaf symptoms appear as irregular brown water-soaked spots surrounded by a yellow halo, often beginning on the leaf margins. Spots grow together and result in leaf death and dark brown streaks develop on leaf petioles. Floral parts may also be blighted. A sticky amber-colored bacterial exudate, which is a diagnostic sign of the disease, may be present on leaves or observed flowing downward on petioles and flower stalks. Infected flower clusters, umbels, may be completely blighted.

Xanthomonas campestris pv. carotae is seedborne and is spread with carrot seed. The bacteria also survive in carrot debris but cannot survive in the soil in the absence of debris. Rain or sprinkler irrigation is required for optimum disease development. Optimum temperatures are between 77° and 86°F; infection does not occur below 65°F. Plant-to-plant spread occurs with splashing water during rainfall, sprinkler irrigation, and may even occur during periods of heavy dews.

In most carrot-growing areas, bacterial blight does not warrant control. In a few areas, such as the Antelope and Cuyama Valleys, severe outbreaks may occur.

Plant Xanthomonas-indexed seed or treat seed in a hot water dip. Use of furrow irrigation rather than sprinklers may aid in reducing disease pressure. Turn under carrot residue to hasten decomposition. Avoid continuous carrot culture and practice a 2- to 3-year crop rotation scheme.

Use cultural controls, hot water dips, and sprays of certain copper sulfate formulations in a certified organic crop.

Assay seed and treat if pathogen is found. Foliar treatments occasionally necessary if rainy weather persists.