The fungus flourishes when the weather is cool, sunlight is low, and humidity is high. If you live in this type of climate or had a very wet/gray season, avoid watering your plants late in the day, and especially avoid watering the leaves if they won’t have a chance to dry out before evening.
Use drip irrigation to water only the root zone of your plants and minimize water drifts or splashes.
Grow your garlic and onions in the sunniest spot possible, and allow enough space between plants for air to circulate among the foliage.
Fertilize adequately, but not excessively. Over-application of nitrogen (which results in lush foliage growth) has been found to make allium plants more susceptible to rust infections.
Rotate your crops and do not grow garlic, onions, leeks, and other alliums in the same area where rust appeared in the previous three years on any allium crop.