Alternaria leaf spot (cabbage family)

About

A leaf-spotting fungus of broccoli, cabbage, cauliflower, kale and their kin. It is most damaging on broccoli and cauliflower, where it can also spot and rot the heads.

Also called Alternaria leaf spot, Alternaria leaf spot of brassicas, Alternaria head rot

How to recognize it

Symptoms

Tiny black specks on older, lower leaves grow into round brown spots with target-like rings and often a yellow halo. Spot centers can crack or fall out, spots merge into large dead patches, and badly hit leaves drop. Broccoli heads and cauliflower curds get small brown to black spots and then break down quickly.

Worse when

Mild to warm, humid weather (about 60 to 78°F) with long spells of wet leaves.

How it spreads

Comes in on infected seed and lives on in crop debris and in mustard-family weeds; spores move on wind, rain splash, tools and insects such as flea beetles.

When

Summer into fall, especially in wet spells.

What to do, gentlest first

Prevention works best; most sprays only protect healthy leaves and can't cure infected ones. Always read and follow the product label.

1. Clean seed and tolerant varieties Prevent

   Start with clean seed and don't save seed from spotted plants. For June-planted broccoli, UMN suggests Wolfman, Green Magic and Eastern Crown.

2. Rotate and weed Prevent

   Wait three years before growing cabbage-family crops in the same spot, skip brassica cover crops like tillage radish, and pull shepherd's purse and wild mustards. Space plants for air flow.

3. Remove sick plants and debris By hand

   If only a few plants are spotted, pull them as soon as you see it. Bury or remove crop leftovers after harvest, and wash hands and tools after handling sick plants.

4. Chlorothalonil or azoxystrobin (rarely needed) Conventional spray

   Farms use protectant fungicides such as chlorothalonil and azoxystrobin, starting before spots spread.

   UMN says most home gardeners don't need fungicides, and sprays won't undo spots already there. Follow the label, including days to harvest.

Plants it affects

Bok choy, Broccoli, Broccoli raab (rapini), Brussels sprouts, Cabbage, Cauliflower, Collards, Kale, Kohlrabi, Mustard greens, Napa cabbage, Radish, Rutabaga, Turnip