As the weather becomes more conducive to disease development, it’s likely we will see a rise in incidence and severity of Alternaria and black rot in crucifers. As the air temperature cools at night, the relative humidity rises, often reaching the dew point. Dew formation on plant surfaces fosters the development of fungal and bacterial diseases.

The most common symptom of Alternaria diseases is yellow, dark brown to black circular leaf spots with target-like concentric rings. Lesion centers may fall out. Individual spots coalesce into large necrotic areas and leaf drop can occur. Lesions can occur on petioles, stems, flowers, flower pedicels, and seed pods. Infections of broccoli and cauliflower heads can lead to complete deterioration of the heads and loose marketability.

Management best practices include:

• Avoid overhead irrigation if possible. When irrigation is needed, do it in the morning.
• Control weeds, especially those in the family Brassicaceae.
• Avoid working in fields when foliage is wet.
• Promptly incorporate plant debris after harvest, or mow if tillage is not possible in late fall crops.

As both diseases may be seed borne start with certified disease-free seed or plan to use a hot water treatment for future seasons. Practice a 3-year crop rotation with all brassica crops. Use proper plant spacing and row spacing for good air circulation.

Varieties differ in susceptibility to Alternaria. Blues F1 napa cabbage and Mighty Hybrid Brussels Sprouts have less susceptibility to ALS, but otherwise resistance is not very widespread. There are more options available for black rot resistant/tolerant brassica crop varieties including Report and Passat for late season cabbage and Belstar F1 broccoli. Use Cornell’s Disease Resistant Vegetable Crop Varieties as a resource when selecting seeds. The NEVMG also has related information in its Brassica crop section.

Fungicides are ineffective once the crop is infected with black rot. However, use of fungicides such as Howler and Regalia that induce plant resistance mechanisms preventatively can help. Double Nickel and Oso are examples of fungicides that can help control the spread of Alternaria.