White tip on leeks — symptoms and treatment

Severity: medium

What is White tip

White tip disease is caused by Phytophthora porri. It causes characteristic white dieback of leaf tips. The pathogen is a soil-borne water mould rather than a true fungus, and that explains almost everything about it: it needs free water, it comes up out of the ground rather than in on the wind, and it appears after wet spells. Severity is rated medium — it rarely wipes out a crop, but it ruins the look of the leaves and opens the door to secondary soft rot.

Symptoms

The name describes it accurately. Leaf tips turn white and papery — bleached, dry and thin — and the damage progresses downward along the leaf. That downward march from the tip is the signature. In wet weather you can often see the leading edge advancing: a water-soaked, greasy-looking lesion in the green tissue just below the white zone. Where the weather stays wet, bacteria move into the damaged tissue and cause secondary soft rot, and the plant becomes slimy and foul-smelling.

Key signs:

• Early: white, papery dieback of the leaf tips
• Early: water-soaked lesions below the whitened tissue in wet weather
• Advanced: white dieback progressing downward along the leaf
• Advanced: secondary soft rot in the damaged tissue

Frost damage and tip burn can also whiten leek tips, but do not produce water-soaked advancing lesions that keep progressing down the leaf.

Causes and conditions

Phytophthora porri survives in the soil and in infected crop debris between crops. When the soil is saturated it produces swimming spores that are splashed up onto the lower leaves by rain, and the infection then works its way through the foliage. Rain splash from bare wet soil is therefore the main route — the pathogen is under the plant, not around it. Prolonged wet weather, saturated or poorly drained ground, compaction and low-lying parts of the field all favour it. Growing alliums in the same ground season after season builds the pathogen up.

Treatment

Cultural measures do the heavy lifting here, because the pathogen lives in the soil and arrives by splash — drainage, rotation and debris removal reduce the source.

Metalaxyl — chemical

During wet weather. Apply systemic fungicide during prolonged wet weather. Few products registered for this disease. Metalaxyl targets water moulds and goes on in the wet conditions that let the pathogen move — a protective response to the weather, not a rescue for leaves already gone white. Damaged tissue does not turn green again; the aim is to protect the healthy leaf below it. Pre-harvest interval: 14 days. Pesticide registrations vary by country — check local approval before use.

Prevention

• Rotate away from alliums — the pathogen persists in the soil.
• Improve drainage and relieve compaction so water does not stand around plants.
• Avoid planting leeks in the wettest, lowest-lying parts of the plot.
• Water at the base, and do not irrigate heavy soil that is already wet.
• Clear and destroy leek debris after harvest instead of ploughing it back in.

Frequently asked questions

Can I still eat leeks with white tip? Yes — trim off the white, papery tips and use the sound tissue below. Once secondary soft rot has set in, that part is slimy and should be discarded. After spraying, respect the 14-day pre-harvest interval.

Is it contagious to my other leeks? It spreads within the bed by rain splash from infested soil rather than blowing across the garden. Plants standing in the same wet ground are at risk; the same variety on well-drained ground elsewhere is largely not.

When should I treat? During prolonged wet weather, when the pathogen is active and moving. Treating in dry conditions achieves nothing, and treating after the tips go white will not restore them — it only protects what is still green.