White rust

About

White rust makes chalky white blisters on the undersides of leaves, mostly on leafy cabbage-family greens such as arugula, mustard, bok choy and turnip greens, and also on spinach. The same products that control downy mildew also work on it (UC IPM).

Also called white rust

How to recognize it

Symptoms

Raised, white, blister-like pustules form on the undersides of leaves, often with pale yellow spots above them. The blisters break open and release white powdery spores, and heavy infections twist and deform growth and kill leaves.

Often confused with

Downy mildew makes a fuzzy gray growth rather than firm white blisters.

Worse when

Cool, wet conditions; UC IPM says it is consistently worse in winter and early spring.

How it spreads

Spores spread in wet weather, and thick-walled resting spores survive in soil and crop residue.

When

Cool, wet weather in fall, winter and early spring.

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. Avoid infested beds Prevent

   Do not plant susceptible greens where white rust has been a problem, and clear crop residue after harvest.

2. Keep leaves dry Prevent

   Space plants and water at the base; UC IPM notes dry leaves may reduce severity even though skipping sprinklers will not fully prevent it.

3. Remove infected leaves By hand

   Pick off and destroy blistered leaves early.

4. Copper Organic spray

   UC IPM lists copper compounds among products that work on white rust, as preventives.

   Copper only protects leaves it coats before infection; it will not heal spots already there. It can burn young leaves, especially in weather above 85°F, and some plants are sensitive (Purdue lists celery, strawberry, apple, pear and plum among them). PNW 591 rates copper hydroxide toxic to bees, so spray when bees are not foraging. Follow the label, including the days to wait before harvest.

5. Water-mold fungicides Conventional spray

   UC IPM lists mefenoxam and fosetyl-aluminum; products that control downy mildew generally also control white rust.

   Follow the label, including which crops it may be used on and how many days to wait before harvest.

Plants it affects

Arugula, Bok choy, Broccoli raab (rapini), Brussels sprouts, Cabbage, Cauliflower, Collards, Mustard greens, Napa cabbage, Radish, Rutabaga, Salsify (oyster plant), Spinach, Turnip