Leek Rust: This is a fungal disease causing bright yellow spots on the leaves. It is often worse in long, wet spells.
Remedy: Mild attacks of rust won’t affect the plant, but serious infections may cause leaves to shrivel and affect yield. There is no control for rust once you have the infection. Make sure you don’t crowd plants, as this increases humidity and increases the likelihood of infection. Dispose of any badly affected plant material, and don’t grow garlic, leeks or onions in the same spot for three years.
Onion white rot: This fungus causes the leaves to wilt and turn yellow. Under wet conditions, the plants may not wilt but will become loose in the soil. If you lift the plants, you will see a white fluffy growth on the base.
Remedy: Throw out any infected plants, and don’t grow onions, garlic or leeks in that spot again for at least eight years. This is a very persistent fungus that survives in the soil for a long time. There is no chemical control.
Leek moth: This is a relatively new pest of leeks and onions and thought to be mainly concentrated around the south-east coast of the UK, although it has been found further inland and north. Caterpillars tunnel into the leaves, causing whitish-brown patches to develop on leaves. In severe cases, leaves may turn yellow and rotting occurs within them.
Remedy: Once you see the damage, there is nothing you can do to control it. Remove and destroy infected plants. When planting out, cover leeks with horticultural fleece (like Enviromesh) to prevent adult moths from laying eggs.