White rust on spinach — symptoms and treatment

Severity: medium

What is White rust

White rust is caused by Albugo occidentalis. It creates raised white blisters on the undersides of leaves — and despite the name it is not a true rust, which is worth knowing because those white blisters look nothing like the orange dust growers associate with rust diseases. Spinach is grown for its foliage, so a disease that blisters and distorts the leaf attacks the product directly. Severity is rated medium: the plant survives, but affected leaves are downgraded.

Symptoms

Seen from above, the first sign is yellow spots on the upper leaf surfaces — pale patches that give no clue to their cause and are easy to put down to something else. The answer is underneath: raised white pustules on the undersides, sitting below the yellow patches. They are blister-like and physically raised, so you can feel them by running a leaf between finger and thumb before you can convincingly see them; as they mature the blisters rupture and release a chalky white spore mass. In severe cases the leaves become distorted, curling and buckling as the blistered tissue grows unevenly against the healthy tissue around it.

• Early: yellow spots on upper leaf surfaces
• Early: raised white pustules on the undersides, below the yellow spots
• Advanced: distorted, curled and buckled leaves in severe cases

Downy mildew produces the same yellow spotting from above but a soft gray-purple fuzz underneath, not raised white blisters. The texture test — flat fuzz versus a firm raised bump — separates them.

Causes and conditions

The pathogen belongs to the same group as the downy mildews rather than the true rusts, and it behaves accordingly. Spores released from the ruptured white blisters are carried by wind and splashing water onto healthy leaves, and infection depends on leaf wetness and humid air. Cool damp weather, heavy dew, fog and overhead irrigation therefore drive it, and a dense spinach stand makes it worse by trapping moisture at leaf level long after the air above has dried. The pathogen also carries over on infected crop residue and on volunteer spinach left in or around the bed, which is how an outbreak survives from one planting to the next.

Treatment

Metalaxyl + Mancozeb — chemical

At first symptoms. Apply fungicide at first symptoms. Apply preventively in wet weather. The timing has two halves and both matter: react as soon as the first yellow spots and white pustules appear, and move to preventive cover when the weather turns wet, because that is when infection runs ahead of what you can see. Pre-harvest interval: 14 days — plan applications around it, since spinach is harvested young and the window is not generous. Pesticide registrations vary by country — check local approval before use.

Prevention

• Sow at a spacing that lets air move through the stand so leaves dry after rain and dew.
• Water at the base rather than overhead, and water early enough for foliage to dry.
• Clear and destroy infected crop residue and volunteer spinach around the bed.
• Rotate away from spinach and avoid sowing a new planting next to an infected one.
• Inspect the undersides of leaves in cool damp spells — the blisters are only visible there.

Frequently asked questions

Is spinach with white rust safe to eat? The blistered and distorted leaves are a quality problem rather than a safety one, but badly affected leaves are not worth eating. Observe the 14-day pre-harvest interval after spraying.

Is it the same as rust on other crops? No. Despite the name, white rust is not a true rust — it produces raised white blisters rather than the orange pustules of leek or bean rust, and it is closer in behaviour to downy mildew, which is why wet conditions favour it.

When should I spray? At first symptoms, and preventively when wet weather sets in. Once leaves are blistered and distorted they do not recover, so the spray protects the leaves still to come.