These pictures were taken on June 4, 2012. These small, raised, blister-like spots (called uredinia) erupting with round orange spores (urediniospores) are characteristic for rust diseases. These spores are dispersed by wind to other plants. The dark brown spots contain another diagnostic type of rust spore called a teliospore. Pustule is another name for rust spots. Small, white flecks are the initial symptom of rust.
Leaves on severely affected plants turn yellow, wilt, dry up, and die prematurely. Garlic bulbs on such plants can be significantly reduced in size and quality, with affected bulbs often lacking their protective dry outer skins thereby leaving them prone to shattering during harvest.
Rust primarily affects garlic. It has occurred rarely in North America. Bulb onion, leek, chive, shallot, and wild species of Allium are also susceptible to some pathogen strains, but may only be affected when growing near garlic with rust. Leek, shallot, and elephant garlic have not been found to be susceptible to rust strains present in North America.
Initial sources of the rust pathogen for a farm or garden are infected planting material and spores dispersed potentially long distances by wind from infected garlic in another planting. Once introduced to an area, the pathogen can survive overwinter in affected crop residue and also in volunteer plants and weeds.
Infection occurs when temperature is between 41 and 77 F (54-70 is most favorable) and relative humidity is at least 97% for 4 hours. At least 9 days after infection, pustules are present with spores. Development of rust is promoted by dense plantings and when conditions are too dry, too wet, nitrogen fertilization is excessive, or plants are otherwise stressed.
Manage rust by using pathogen-free planting material, separating susceptible crops, managing related weeds, rotating land where garlic is grown, and applying fungicides when conditions are favorable and the pathogen is known to be present.