Purple blotch

About

A leaf disease of onions, garlic, leeks and shallots that makes purple, target-like blotches. Badly hit leaves die early, so bulbs come out smaller and may rot in storage.

Also called purple blotch, Alternaria porri

How to recognize it

Symptoms

Small, sunken, water-soaked spots with pale centers appear on older leaves. They grow into purple to brown blotches, often ringed in yellow like a bull's-eye, and dark spores form on them. Leaves yellow, collapse and can be girdled; stored bulbs may get soft spots that turn reddish-purple, then brown or black.

Often confused with

Stemphylium leaf blight often shows up alongside it and looks much the same (the care is the same). Early spots also resemble iris yellow spot virus.

Worse when

Warm, humid weather (about 77 to 85°F) with leaves staying wet for 12 to 16 hours or more, plus wounds from thrips, hail or other diseases.

How it spreads

The fungus survives winter in old crop leftovers near the soil surface; spores form at night and blow or splash to new leaves in the morning.

When

Midsummer, as plants mature and older leaves age.

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 sets, spacing and rotation Prevent

   Start with healthy seed or sets, space plants, weed often, and go easy on nitrogen so leaves dry fast. Wait at least three years before planting onion-family crops in the same spot.

2. Keep thrips in check Prevent

   Thrips wounds open the door for the fungus, so manage onion thrips (see the thrips page).

3. Clean up and cure well By hand

   Remove or bury old tops and culls after harvest. Harvest in dry weather, avoid bruising the necks, and cure bulbs before cutting the leaves off.

4. A fungicide labeled for purple blotch Conventional spray

   Utah State advises spraying once purple blotch is confirmed to slow its spread; the same treatment also covers Stemphylium. Your state's home vegetable fungicide guide lists current products.

   Make sure the product lists purple blotch and the crop you're growing. Follow the label, including days to harvest.

Plants it affects

Garlic, Leek, Onion, Shallot