Alternaria leafspot (Alternaria spp.)

Symptoms of Alternaria leafspot include circular lesions with sunken centers surrounded by bright yellow chlorotic halos with ragged margins. These lesions first form on older leaves and gradually turn dark brown. Cool, wet weather favors disease development. Alternaria leafspot also infects Brussels sprouts and causes brown rot of cauliflower. Alternaria can be transmitted on the seed and in plant debris.

Blackleg (Leptosphaeria maculens)

This fungal disease of cole crops does not usually cause losses in the field unless it is introduced by using infected seed. The first symptoms of this fungal disease are spots on the leaves and a depressed light brown canker near the base of the stem. Numerous black dots form on the canker and on the leaves. The stem canker spreads down to the roots, which are eventually killed, stunting the plant. With high rainfall, the disease can spread regardless of temperature, although it is more active at coo ler temperatures. Blackleg can also infect the crop in storage.

Blackleg will not survive on seed that is soaked in 122 degrees F water for 25 minutes. Unfortunately, hot water treatment reduces germination so seed should be tested before planting. Growers can purchase seed tested (indexed) for low incidence of this pathogen. Infected transplants are also often sources of blackleg.

Methods of reducing infection from transplants are:

• rotating transplant beds out of crucifer production for at least 4 years,

• ensuring good air drainage and rapid evaporation of dew,

• avoiding spraying or dipping transplants in water, and

• using new containers for shipping transplants.

Sheep manure can be a source of blackleg inoculum if the sheep have grazed on infected plants. Blackleg symptoms are worse on plants also damaged by herbicide.

Black rot (Xanthomonas campestris pv. campestris)

Black rot is caused by a bacteria to which all brassicas are susceptible. This bacteria is seed-borne, but can also overwinter on weeds and crop debris. Worldwide, black rot is considered to be the most important disease of crucifers. Infected leaves develop a wedge shaped yellowing on the margins, often followed by blackening of the veins and dying of V-shaped sections of the leaves. Under warm, humid conditions, symptoms appear 10 to 14 days after infection. Black rot may not appear when temperatur es are low, but spreads rapidly during periods of rain and high temperatures.

The following practices are suggested as controls for black rot:

1. three-year rotation of the seedbed and field,

2. hot water disinfection of the seeds (122 degrees F for 25-30 minutes),

3. using black rot tolerant or resistant cultivars, and

4. using certified disease-free transplants.

Clubroot (Plasmodiophora brassicae)

Clubroot is not a serious problem in most of the south because it is favored by cool, wet soils. Symptoms of clubroot on cole crops are an enlarged or club-bed root system and a wilted or stunted shoot. Younger plants can be killed and older plants weakened to the extent that they will not produce a marketable crop. The fungus can be introduced on transplants or by irrigation from ponds receiving drainage water from infected fields.

Because clubroot is so long-lived, a 7-to-10-year rotation out of crucifers is required for control. There are relatively few other control options. Raising soil pH to 7.2 by broadcasting and incorporating hydrated lime into the soil at a rate of 1500 pounds per acre 2 or 3 days before transplanting offers some control. This liming should not be done more than once every 3 years to keep the soil from becoming too alkaline, nor is it very effective on light, sandy or loose muck soils. Clubroot-resist ant cultivars of kale, turnip and rutabaga are commercially available.

Experiments in Australia compared fumigation and solarization and a combination of the two for efficacy in controlling clubroot. Plastic sheeting for solarization was installed at the time of methyl bromide fumigation, and within 4 hours of dazomet applic ation. For all solarized plots, plastic was left in place for 7 weeks. In clay soil, solarization combined with low rates (90 pounds per acre compared to the standard rate of 670 pounds per acre) of fumigants (dazomet and methyl bromide) controlled club root in cauliflower better than either fumigant alone at low or high rates (90 or 670 pounds per acre) or solarization alone. On sandy soils, however, the fumigants alone gave the same control as solarization plus fumigant.

Downy mildew (Peronospora parasitica)

Downy mildew is of minor importance in the field, but can become systemic (move through the whole plant) and kill seedlings in seedbeds. This fungal disease of cole crops causes upper leaf surfaces to become chlorotic with black spots, while diffuse white fungal growth is found on the lower surfaces. Spread is most rapid during rainy periods and at temperatures of 50 to 60 degrees F. Two-year rotations and clean, well-drained soils can lower disease potential. See the table at the end of the Cole Crop chapter for resistant cultivars.

Fusarium yellows (Fusarium oxysporum f. conglutinans)

This fungal disease of cole crops results in an uneven yellowing of plants progressing upward from the lower leaves. It is most severe when soil temperatures are above 70 degrees F, and least severe below 60 degrees F. The yellows pathogen persists in the soil for many years, but Fusarium yellows is effectively controlled by use of resistant cultivars.

References

• Lambe, R.C. and G.H. Lacy. 1982. Controls for two important diseases: black rot and black leg of cabbage. American Vegetable Grower, March. pp 10-12.

• IPM Manual Group. 1985. IPM for cole crops and lettuce. Univ. of CA Publication 3307.

• Porter, I.J., P.R. Merriman and P.J. Keane. 1991. Soil solarization combined with low rates of soil fumigants controls clubroot of cauliflowers, caused by Plasmodiophore brassicae Woron. Aust. J. of Exp. Ag. 31:843-51.