IDENTIFY AND TREAT ALLIUM RUST
By David Marks
Rust is a fungal disease which attacks the allium family of plants the worst affected being leeks, onions and chives but onions are also sometimes affected.
The Latin name for the fungus is Puccinia allii and it first makes an appearance in mid to late June time. It thrives in areas of high humidity where air circulation is poor. It prefers lush green growth rather than harder and older growth. Once established it cannot be cured using chemical available to amateur gardeners.
LIFECYCLE OF ALLIUM RUST
There are endless types of rust affecting a huge number of plants, this article is aimed specifically at allium rust affecting leeks, garlic and chives. Onions can also be affected but to a lesser degree. Understanding the lifecycle of this disease will greatly help in preventing it. The most important factor in the lifecycle of rust is that it cannot live on dead plant matter. This is key to controlling it from one year to the next.
Surprisingly little is known about the life cycle of allium rust maybe because different strains are present in different parts of world. The key facts for the UK though are outlined below.
MID SPRING TO EARLY SUMMER
When temperatures begin to rise in mid spring to early summer rust spores are released from infected living plants which land on other plants and infect them. The ideal temperature range for spore release and germination is 10°C to 20°C / 50°F to 68°C although it will still occur outside of these ranges. Spores are transferred by wind and rain drops.
EARLY SUMMER TO LATE AUTUMN
When the spores land on a suitable plant they germinate. Rust fungus then infects the new plant and begins to feed on it. The time between the spores infecting a plant and the visible signs of the infection appearing can be as short as a week but as long as a month. It all depends on the temperature and humidity levels.
It’s important to realise that infected plants may not always show immediate signs of infection. The infection easily spreads form one part of the plant to another (and to nearby plants) over the summer and autumn.
In winter, as the temperature falls, the infection rate slows down, the fungus effectively overwinters on infected living plants, leeks especially because they are quite capable of living through winter and well into the next year.
DAMAGE CAUSED BY LEEK RUST
The damage is unmistakable for any other pest or disease. The leaves develop tiny brown / orange coloured, slightly raised spots on the leaves.
Click the picture above to enlarge it and see the damage more clearly. The above picture shows a leek which is moderately affected by rust. Over the next month or so the symptoms will become worse with lots more infected areas.
Leek rust will not directly kill a plant but the damage is caused to leaves reduces their ability to convert sunlight to energy, slowing the growth rate and opening up the plant to other infections as well. Severe cases may result in some of the leaves dropping off.
Leeks, garlic and onions affected by rust are still perfectly edible because it’s the bulb part you eat and not the green leaves. Chives are different because it’s the leaves which are consumed and aside from the rather unsightly appearance that rust gives them it may not be good for you to eat the affected leaves.
PREVENTING LEEK RUST
The conditions required for leek rust to thrive are set out below. Avoiding these conditions as we suggest will restrict the disease greatly.
- Rust thrives in humid conditions and this can be minimised by growing plants at the recommended distances apart. Crowding the plants restricts airflow which in turn increases humidity.
- Rust does better when plants are in low levels of sunlight. This causes the foliage to grow “soft” which is good for rust. To avoid this grow plants which can be affected in full sunlight positions.
- Rust prefers lush foliage and this is encouraged by the application of nitrogen fertilisers so use them sparingly.
- Rust cannot live on dead plant matter so ensure that all parts of onions, garlic and chives are removed from the soil in autumn at harvest time.
- The reason that leeks are affected the worst is because they are often grown until June the next year allowing rust to effectively overwinter and begin to spread again to new plants the next year. If you have rust try and break the chain from one year to the next by harvesting all your leeks by January.
- Practice good crop rotation as suggested in our article here.
- Try growing rust resistant varieties to minimise the effects of this fungus. Evidence is thin on the ground about this but the RHS do recommend the leek variety Apollo as being resistant to rust. There is some evidence that the garlic variety Early Purple has some resistance
TREATING LEEK RUST
There are no chemical treatments available to the UK gardener for any form of rust including that which affects leeks, garlic and onions.