Leek rust is caused by a fungus. It affects all of the allium family, including onions, garlic, chives etc. There are actually several sub forms of rust that cause allium rust and only two seem to be apparent in the U.K.
But that doesn’t really matter much if you have rust. And though one form seems to infect some alliums and the other form affects other alliums is also of little consequence if you get rust in your crop.
Rust is a biotroph. It lives inside the plant and takes nutrients from the plant cells. It isn’t in the rusts interest to kill the plant … though sometimes it happens. The rusty orange marks we see on the leaves are just the top of the iceberg.
I read advice that too much nitrogen can be a problem, that it causes allium rusts to explode. And I read that more potash helps. I’m not convinced that either is wholly true.
I also read that we need to remove affected leaves and burn them … never compost them. Again, I’m not convinced.
Research on prevention and cures are mixed. Commercial growers can use chemical control methods. Though in ten years of growing leeks I never resorted to chemical control. Nor did I cut off infected leaves and burnt them. Try doing that on 90-100,000 leeks!
What Causes Leek Rust?
As mentioned above, leek rust is caused by a fungus. So if we ensure the conditions fungi like don’t exist we get no rust. That’s difficult, but we can do a lot to reduce the conditions that the rust prefers.
Cultural Control of Leek Rust
From my experience the best control methods are cultural. This applies to all allium crops, leeks, onions, shallots, chives and garlic.
Don’t plant the crop too densely. It creates the humid conditions the rust needs to infect the plant. This sounds contrary to my advice to plant many crops densely in No Dig beds to get Hugh yields and suppress weeds. But leeks need special care.
Leeks need airflow along the rows to keep humidity levels down. In row they can be denser. And I often use multiseed modules, but then thin them as they grow. Just harvest the biggest and eat them. The others will continue to grow. It’s like getting two or three crops out of each area of land!
Healthy vigorous plants don’t succumb to disease so readily as poor unhealthy plants. So my traditional growing practice was to give a good base dressing of fertiliser at planting time, plus additional nitrogen as the crop started to form a canopy.
The nitrogen made the crop shoot away and I maintain a healthy crop is less likely to get problems.
As for using potash. I’m for it. It gives a healthy root system and a healthy plant. That’s what is needed to reduce plant diseases.
However, a touch of rust on alliums is almost inevitable. It’s not going to set the crop back much. And as the weather gets cooler rust growth reduces and the crop keeps growing during the milder weather. Leeks grow all winter in southern England and in milder weather further north.
Allium Crop Hygiene
Gardeners often advocate removing infected leaves. As the fungi is already in the plant I can’t see it’ll make much difference. It just creates work with no advantage except feeling we’ve done our best!
They also advocate burning infected leaves. I’m not convinced. The disease still persists in the plants that are growing. Certainly I’m not against crop hygiene. It makes sense, but don’t expect it to make much difference. Allium rust is everywhere and travels on the wind. If you cleared crop debris and didn’t get a problem next year it us not likely to be down to clearing the leaves. It’s more likely to be down to better weather conditions.
Microclimates Affect Allium Rust
Microclimates make huge differences. Leeks planted in areas where there is restricted air movement tend to show rust symptons first. These areas are more humid and favour the rust. More exposed sites get less allium rust.
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