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The fungi that creates garlic rust is prolific around the country. It's the 'spring sorrow' of any garlic grower. It's a frustrating time to see one's healthy garlic leaves begin to get white round flecks then turn into rusty orange spores that spread.
There are 7,000 species of rust, but garlic rust (Puccinia porri) is a real threat to garlic crops. This pathogenic fungi is prevalent, and seems to have infested every corner of our country. This fungal disease generally spreads from spores in the wind (it can travel long distances), animals, people and their machinery. Puccinia allii also infects other garden vegetables too such as leeks, onions, spring onions and chives, so it's best not to plant these in the same soil each year.
Rust spores are microscopic. Rust spores from the soil can reach the leaves from disturbance of the soil, rainfall splashing and other transfer methods. If possible, is best to water the soil via a dripline and not overhead. Rust can be transported from the leaves and bulb into the following years crop. Consider a pre-treatment of the clove before planting.
The presence of rust is highly variable from year to year and between locations. Garlic rust thrives when soil nitrogen levels are high, close planting, high humidity (wet leaves for 4+ hours), poor drainage, stressed plants or when plants are in their bulbing stage, and in temperature ranging from 12-24°C. It seems that growers at higher altitudes (>500m) or in very cold climates are less affected by garlic rust. We have found this to the the case for our plot located well above sea level (300m+) in a dry mountain environment which gets regular wind and air movement between the beds.
The first sign of rust are small white spots which occur on the leaves. At this stage you can pick off the leaves and dispose of them (not in the compost). Do not pick too many leaves as each leaf contributes to 10% of bulb weight, and the last few are a protective cover for the bulb.
The garlic rust stages progress as the white spots turn into a rusty orange appearance as the rust reproduces. The final garlic rust stage is destructive. This is where the leaf is covered in many orange or advanced black spots. Minor rust will not affect bulb size, moderate rust will, while severe rust will result in tiny or no bulbs. It is awfully disappointing to the grower as the stages progress.
While there are inorganic treatments (systemic fungicides - normally two treatments in the early stages), there is no organic treatment to kill rust. Organic treatments using sulphur, sodium and potassium bi-carbonate based sprays on contact with the rust pustules will only sterilise the surface and reduce the spread to surround leaves and soil. However the internal pathogen in the leaf will remain.
One organic method to reduce the impact and spread is by mixing four teaspoons of sodium bicarbonate (baking soda) or potassium bicarbonate per litre of water (plus a teaspoon of vegetable oil to make it stick and a drop of dish detergent to penetrate the leaf).
When sprayed onto the leaf it makes the surface highly alkaline, anti-fungal and as a result slows the spread of rust growth for a week before reapplication is necessary. Do not spray any acidic formulas (like vinegar) as this will feed the rust. If it is raining alot it can be a very frustrating time to reapplying regularly this treatment. Another option is using hydrogen peroxide with water following the manufacturers instructions and spray on leaves every few weeks.
Some growers elect to plant their cloves earlier to hopefully harvest earlier before garlic rust takes hold. Another consideration is to grow an early harvesting group such as turban or asiatic garlic types for the same reasons. This has mixed results with growers using this technique, others believe the strongly bolting garlics that are harvested later are a more robust in fighting rust. We have found planting early has not helped our crop, and the best resistant to date has been our later harvesting types. Micro-climates and seasonal weather variability certainly alters the presence of this persistent fungi on garlic leaves.
Orange spores from leaves than reinfect soils can live there for 1-2 years before the soil biota will break it down. So for this reason it is best to have a three year rotation on garlic beds particularly if you experience rust. Black rust spores (acute stage) can live in the soils for 5-10 years, so it is best to remove the emergent black rust spores leaves early.
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