Purple Blotch
Sunken oval lesions on onion and garlic leaves that begin as small white-centred water-soaked spots and enlarge into purple to brown patches with faint rings and a yellow edge. The purple colour and the sunken oval shape separate it from downy mildew, which makes fuzzy grey-violet patches without a defined margin.
Disease checklist
Four things to look for. If most of them match what you are holding, you are in the right place.
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.
1Rotate alliums off the ground
Wait at least three years before onions or garlic return to a bed, since the fungus overwinters on old allium debris.
Try this first2Clear the old tops
Remove and bin allium leaves and culls after harvest instead of leaving them at the row end.
3Manage thrips
Thrips feeding wounds give the fungus an easy entry, so keep thrips numbers down.
blue sticky traps thrips4Water at the base in the morning
Drip irrigation keeps the leaves from staying wet through the humid part of the day.
drip irrigation kit for garden5Protectant fungicide in humid spells
Chlorothalonil or mancozeb on the labelled schedule, applied with a spreader-sticker because allium leaves shed spray.
garden fungicide chlorothalonil6Cure the bulbs properly
Dry the crop with the necks intact in a warm airy place before storing, and do not store bruised bulbs.
mesh onion storage bags
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Which plants get purple blotch
Every plant we cover that gets purple blotch, 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.
perennials4
annual1
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 Amaryllidaceae, with 8 of the 8 affected plants.
- Amaryllidaceae8 plants
When to treat
Protect during warm humid weather once the first outer-leaf lesions show, and check the neck at harvest: bulbs bruised at lifting can rot in storage from this same fungus.
Timing only — the label is the law
This page tells you when to watch for purple blotch 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.
Other problems on the same plants
If a plant gets purple blotch, these are the problems it most often gets as well, so they are worth checking for while you are out there. They tend to appear together; one does not cause another.