Rust (Puccinia allii) is a fungal disease that attacks garlic, onions, chives, shallots and other alliums. Infestation becomes apparent in mid to late spring when weather is moist and warm. Yellowish orange flecks soon turn to bright orange pustules on the undersides of leaves. Then the pustules open, releasing spores. The rust spreads. This is debilitating to the plant and can result in smaller, inferior garlic bulbs.
It is important to deal with rust before it can take hold and spread. Prevention is the best method:
FULL SUN AND PLENTY OF AIR CIRCULATION — Full sun helps dry out foliage; so does a good, stiff breeze. Plant in areas that allow you to take advantage of sun and air circulation, and check on plants to monitor their condition, especially after mornings thick with dew.
ROTATE PLANTINGS — If you have had a rust infection in a certain area in the past, never plant garlic there again. Spores hide out in the soil. Raindrops splash onto the soil and transport spores to leaves.
CLEAN, DRY MULCH — Straw keeps soil and water from splashing on leaves. Rust spores live in soil, so keep a 3-inch-thick layer of mulch over the garlic.
SPRAY WITH OIL — Light horticultural oils will help prevent rust from taking hold. This means that if there has been a problem with garlic rust in your yard it is wise to begin spraying regularly before infection becomes evident. Coating leaves with oil prevents spores from taking hold. Spraying after a rain is crucial. This can become a bit of a chore during the late spring when rains may be frequent. Oil sprays will not help if a rust infection has already attacked.
PLANT IN CONTAINERS — Garlic will grow well in containers of fresh potting soil. Rust is rarely a problem. Plant six to eight cloves in large pots, 10 to 15 gallons.
WATERING — Avoid watering the leaves if they will not have a chance to dry out before evening.
FERTILIZING — Because garlic uses only a moderate amount of nitrogen and possibly some phosphorous, excessive fertilization makes garlic more vulnerable to rust damage. Infection is worse on nitrogen-rich soils with low potassium, so take care with fertilizer applications
IF YOU SPOT RUST, cut off the infected leaves. Even if you have to cut off most of the leaves, you’ll still get a crop, but probably with smaller bulbs. Even if your crop is plagued by garlic rust this season, all is not lost — you can still use the cloves as seed garlic for next season
You can try spraying the remaining leaves with a homemade fungicide: Mix 1 tablespoon each of baking soda, light-colored vegetable (cooking) oil and mild dishwashing detergent in 1 gallon of water. Spray on leaves. Treat plants weekly in early spring to help prevent rust from invading the garden or to control the spread of rust and other diseases already present.