Alternaria Leaf Spot (Brassicas)
Risk near you, next 48h
See whether your local forecast favours this disease right now.
When it strikes
Treat, then re-check in 5–7 days
Pick off the worst material, apply the organic option below, and look again after five to seven days. If the spread continued, escalate rather than repeating the same treatment.
Rain and irrigation splash
Water splashing off soil and infected leaves carries it upward and along the row. Watering at the base and mulching the soil surface break the route.
It carries over to next season in soil, debris or seed — which is why rotation matters here, not just treatment.
Hot compost only
A heap that reaches and holds 55–65°C throughout will destroy it. A cool corner heap will not, and will return the problem to the beds with the finished compost.
When and where to look
Scout the planting while temperatures hold between 18°C and 28°C, with humidity above 85%, after rain or heavy dew, while the leaves are still wet. Risk builds after roughly 9 unbroken hours of those conditions, so the morning after such a night is the moment to look.
Rule these out first
Same category, and at least one host crop in common — so they are genuinely confusable in the same part of the garden.
The signs that lead here
Full problem lists for these crops
About
Alternaria brassicae and A. brassicicola cause dark leaf spot on cabbage, cauliflower, broccoli, kale and swedes. It is seed-borne as well as splash-spread, which is why an outbreak can appear in a bed with no brassica history.
Symptoms
Round dark brown to black spots with concentric rings, like a tiny target, often with a yellow halo. Spots enlarge, merge and shred in wind. On cauliflower and broccoli the curd develops sunken black specks that make it unusable.
Organic Treatment
Remove and destroy affected outer leaves as they appear, and clear all brassica debris after harvest. Hot-water seed treatment at 50°C for 20 minutes kills seed-borne infection on saved seed.
Chemical Treatment
Protectant fungicides such as difenoconazole or azoxystrobin, applied at first spotting and repeated on a 10–14 day interval in wet spells, hold it. Rotate active ingredients.
Prevention
Buy certified seed, rotate brassicas on a four-year cycle, space widely for airflow, water at the base, and never work among wet brassica foliage — the spores travel on hands and clothing.
Crops at risk
Related problems
Alternaria Leaf Spot (Brassicas) risk by location
Each location has a month-by-month profile computed from the weather archive: how many hours a year actually meet this disease’s thresholds.