What causes rust to form on garlic?
It could be the variety that is planted and is caused by weather conditions.
It is best to plant in a new position in the garden each season, crop rotation is important. Make sure there is good air circulation around the garlic and that the soil is well drained, heavy clay soils coupled with wet weather provide ideal conditions for rust to form. Avoid overhead watering at night, water during the day so that the foliage has time to dry out. Plant fresh corms, not the ones from last season crop that could have overwintering rust spores in the bulbs. Pick off the infected leaves when you first see signs, that can help slow the spread.
Unfortunately wet weather and heavy soils contribute to rust being a problem. Don’t compost infected plant material, dispose of in the rubbish or burn. Don’t plant too close together, remove infected foliage if bulbs are not ready to harvest, don’t water at night with overhead watering as this increases the humidity around the plants and creates an ideal environment for rust spread.