Photo: Howard F. Schwartz, Colorado State University, Bugwood.org

Alternaria Leaf Spot

Dark brown to black spots on older leaves that grow into larger patches with faint rings inside and a yellow halo around the edge. On cabbage and broccoli the spots are round and sooty; on carrot they start as dark water-soaked patches along the leaflet margins; on melons the rings show only on the upper leaf surface.

Disease17 plants we cover get alternaria leaf spotmostly annuals

Disease checklist

Four things to look for. If most of them match what you are holding, you are in the right place.

Small black spots on the oldest leaves first
Faint concentric rings inside the larger spots
Yellow halo ringing the spot
On carrot, browning that starts at the leaflet edges and works inward until the tops look scorched

What gets rid of it

Gentlest option first. For most backyard cases the first one is enough, so try these in order and stop as soon as it works. If you do spray something, the product label tells you how — please read the notice further down.

1Start with clean seed
Buy certified or hot-water-treated seed, since the fungus rides on brassica and carrot seed.
organic broccoli seedsTry this first
2Three year rotation
Keep brassicas out of a bed for three years and clear crop residue, because the fungus lives on debris.
3Kill volunteer brassicas and weeds
Pull wild mustard, shepherd's purse and last year's volunteers, which carry the fungus between crops.
4Keep the canopy dry
Space plants, avoid overhead watering, and do not work among wet plants.
soaker hose 50 ft
5Protectant fungicide during wet spells
Chlorothalonil, copper or mancozeb applied on the labelled schedule ahead of extended leaf wetness.
garden fungicide chlorothalonil
6Harvest heads promptly
Cut broccoli and cauliflower as soon as they size up rather than letting them stand into wet weather.

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Which plants get alternaria leaf spot

Every plant we cover that gets alternaria leaf spot, grouped by type — tap any one to open its page. A note beside a plant came from that plant’s own entry, because the same disease can be a minor nuisance on one plant and a serious problem on another. Plenty of plants we do not cover get it too.

tree1

PistachioPistacia veraall worse in humidity

biennials2

annuals14

Plant families hit hardest

Related plants often share the same problems, so if one plant in a family has it, check the others. The largest group here is Brassicaceae, with 10 of the 17 affected plants.

Brassicaceae10 plants
Cucurbitaceae4 plants

When to treat

Start protecting at the first spots on older leaves, because on broccoli and cauliflower the fungus moves from the leaves into the head and a rotting head cannot be salvaged.

Timing only — the label is the law

This page tells you when to watch for alternaria leaf spot and when treating is worth considering. It is not a recommendation to apply any particular product. Read and follow the product label before you spray — under federal law the label is the legal instruction, and using a pesticide against it is illegal.

Check the pre-harvest interval. Every product that’s cleared for an edible crop sets a minimum number of days between spraying and picking. Find it on the label before you spray anything you intend to eat.
Check the re-entry interval before you or anyone else works in the treated area again, and use the protective equipment the label names.
Confirm the product is registered for your crop in your state. This page groups many different crops together, and a product legal on one of them is not legal on all of them — state registrations differ too. Your state pesticide agency or local extension office can confirm both.
Rates and mixing instructions come from the label, never from us. We don’t publish them because they differ by product and formulation.