Soil, Crop and Pest Management

Bacterial Black Rot Of Crucifers

The Plant Disease Diagnostic Laboratory

Prepared by Dr. Robert L. Wick, Department of Plant Pathology, Fernald Hall, Room 111, University of Massachusetts, Amherst, MA 01003. Tel. (413) 545-1045 Fax (413 545-2532.

Revised October 1, 1991 V7

Introduction

Black rot, caused by the bacterium Xanthomonas campestris pv. campestris occurs throughout the world where cruciferous plants are grown. Cabbage, cauliflower, Chinese cabbage, turnip and rutabaga are often severely affected. Black rot also occurs on other cruciferous hosts including ornamentals and weeds. The disease is seed-borne and although the bacterium will not persist for more than a few years in soil, it can overwinter in plant refuse.

Symptoms

The bacterium enters the leaves by colonizing the hydathodes (water pores) and moves from the leaf margins inward. Lesions may also begin at wounds. Diseased tissue is often V-shaped, flaccid, tan to yellow and with blackened veins. The blackened veins are diagnostic and are best seen by holding the leaf up to the sun. When the lesions reach the petiole and stem, the bacterium moves systemically through the plant, resulting in premature leaf drop. At this stage of disease, a cross-section of the stem will reveal a

ring of discolored vascular tissue. Once the bacterium is systemic, heads of cauliflower and cabbage may become invaded or in turnip and rutabaga, the root may become invaded. The bacterium spreads from plant to plant by wind-driven rain, detached leaves, insects, and by working in the field especially when plants are wet.

Management

1. Request hot-water treated seeds; avoid southern-grown transplants. Hot-water treated seeds must be planted soon after treatment or viability will be affected.

2. Direct seeding is less conducive to the spread of bacteria than transplanting.

3. Control cruciferous weeds.

4. When disease is present, avoid working in the field when the foliage is wet.

5. Plow under plant debris after harvest. Rotate out of cruciferous crops for at least two years.

continued on overleaf

Bactericides: (Cabbage only) cupric hydroxide (Kocide 101®): 2 lb/A at seven to ten day intervals. Flecking of the wrapper leaves may occur at this rate.