Vegetable Program
Soil, Crop and Pest Management

Alternaria Blight of Crucifers

The Plant Disease Diagnostic Laboratory

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

Revised September 20, 1990 V8

Introduction

The fungi Alternaria brassicae and Alternaria brassicicola cause leaf and pod spots on broccoli, cabbage, cauliflower, horse-radish, kohlrabi, mustard, radish, rape and turnip. On cauliflower and broccoli, brown to black spots may occur on the florets thus rendering them unmarketable. Radish and turnip marketed with the foliage intact, may be rejected because of leaf spots. On cabbage, the oldest leaves are usually affected resulting in little significant damage. Alternaria brassicae is considered to be more pathogenic than Alternaria brassicicola.

Symptoms

When Alternaria attacks seedlings, the stem becomes girdled and damping-off occurs. Leaf spots vary in size from specks to an inch or more in diameter. The lesions often have concentric rings giving them a target or zonate appearance. A. brassicae usually causes a grey, light to dark brown spot while A. bassicicola causes a dark sooty spot. On cabbage, cauliflower and broccoli, the lesions are most common on the older leaves. On brussels sprout, the lesions may extend through several leaf layers of the edible bud. The foliage of turnip and rutabaga may become extensively blighted and on these crops, subsequent spore production often leads to root infection. The root lesions usually show up in storage where they exhibit the typical zonate symptom.

Life history

The two fungi are primarily seed-borne. They colonize the seed coat during the seed development. When the seed germinates, the fungus becomes active. When germinated under mist at relatively warm temperatures (80 F), damping-off may occur. More commonly, the fungus develops slowly and is not detected until leaf spots appear on plants in the field. In addition to being seed-borne, the fungus can survive the winter in crop refuse and cruciferous weeds.

Disease is most prevalent during warm (80-90 F) wet weather. Once sporulation occurs, the spores are easily spread by wind and splashing rain. In cauliflower and broccoli, the most serious losses are due to colonization of the florets. Disease may also develop in storage.

Management

1. Request hot-water processsed seed.

2. Plow under plant debris after harvest.

3. Eliminate cruciferous weeds; rotate with noncruciferous crops.

4. Plant broccoli and cauliflower in a different area than cabbage. Cabbage is a good host for Alternaria and may become a source of the pathogen for the other crucifers.

Fungicides: chlorothalonil (Bravo 720®): 1½ pts/A when disease first occurs and repeat at 7-10 day intervals. Do not apply to Chinese cabbage or Chinese broccoli within seven days of harvest.

Bravo 720 is labeled for Cabbage, Chinese cabbage (tight-headed cultivars only), Cauliflower, Broccoli, Chinese broccoli, and Brussels sprouts.