What it is
Ring spot is a fungal disease of cabbage and other brassicas caused by Mycosphaerella brassicicola. It produces circular, brown-grey to black spots (up to 20 mm) with light centres containing black fruiting bodies arranged in concentric rings, and a yellow halo. Spots are often angular because they are limited by leaf veins. It kills leaves early, penetrates several leaf layers in storage, and can blacken whole plants under severe attack.
Symptoms to look for
- Circular brown-grey to black spots (up to 20 mm) with lighter centres
- Black fruiting bodies arranged in concentric rings inside the spots (visible as black dots)
- A yellow halo around the spots; spots often angular where limited by veins
- Older, lower leaves affected first; symptoms worse if younger leaves are hit
- Early leaf death; in storage the fungus penetrates several leaf layers
When it spreads
Cool, moist weather with heavy dew or wet spells lasting one to two weeks, around 16 to 20 degrees C. It is worst in areas of intensive brassica production with overlapping crops. The fungus survives in crop debris and on brassica weeds, is seedborne, and spreads by windborne spores and rain splash.
Treatment
Field practices
Use clean or hot-water-treated seed, establish nurseries away from diseased fields, use two-to-three-year rotation away from brassicas, avoid overlapping brassica crops, control cruciferous weeds, use wider spacing for airflow, and collect and burn crop debris after harvest.
Organic options
Remove and destroy infected leaves and crop debris, and check seedlings in the nursery, removing infected leaves or discarding badly affected seedlings. Copper fungicides give partial protection when applied preventively and repeated every 7 to 14 days in cool, wet weather. Improve airflow and keep foliage dry.
Active ingredients
- copper hydroxide
- copper oxychloride
Chemical options
Protectant fungicides such as chlorothalonil or mancozeb applied preventively on a 7 to 14 day schedule give the best control, since ring spot is difficult to stop once established. Copper is also used. Alternate modes of action to slow resistance and treat seed with thiram or hot water (50 degrees C for 30 minutes).
Active ingredients
- chlorothalonil
- mancozeb
- copper hydroxide
- thiram (seed treatment)
Safety
Wear gloves, a mask, and eye protection when spraying fungicides, and follow the label pre-harvest interval before harvest. Keep children and animals away from treated plants until dry, avoid drift onto food crops, store products safely out of reach, and wash heads before eating.
Preventing it next season
Start with clean or hot-water-treated seed and disease-free transplants raised away from infected fields. Rotate crops, avoid overlapping brassica plantings, control brassica weeds, keep foliage dry with good spacing, and remove debris. In cool, wet, intensive-production areas, preventive fungicides may be needed.