A very small bacteria affecting most garlic in the upper Midwest in 2012.
(Updated September 4, 2012) Many garlic growers in the upper Midwest have reported yellowing leaves and premature browning resulting in both crop loss and smaller garlic heads at harvest. Some growers had 100% crop loss while others have had little or none. Plum Creek Garlic experienced about a 70% crop loss by weight.
A very small bacteria called Phytoplasma has been identified as the most likely culprit via DNA testing. The specific type of Phytoplasma infecting garlic in Minnesota has a common name of Aster Yellows and is transmitted from plant to plant only by the Aster Leafhopper which carries the disease from infected plants. It is not completely clear why this happened in 2012 although very large Aster Leafhopper infestations in late April and early May were documented in a University of Minnesota Extension report. One of the authors of this report said they were finding as many as 70 Aster Leafhoppers per square foot in Barley. If you do the math this is over 3,000,000 Aster Leafhoppers per acre! This author thought the warm spring in the south and several large rain events in late April and early May brought them north in extremely large numbers.
Normally Leafhoppers are not considered a garlic pest and many remedies for Leafhopper problems on vegetables often include garlic as a key ingredient to repel them.
While the garlic cloves themselves are fine and taste great, the wrappers typically have a deep purple or dirty brown color. Aster Yellows infects the growing parts of the plant and does not infect the soil or move through the air.
Reports across the Midwest indicate many crops, flowers and vegetables have been infected by the Phytoplasma disease this year. North Dakota State University reported on an unusual Phytoplasma infestation in wheat in 2012. Read about it here. Canada also reported large losses due to Phytoplasma in wheat and you can read about it here. It appears Phytoplasma affected garlic production in Edmonton, Alberta approximately 13 years ago and in Argentina garlic a few years earlier than that. Phytoplasma are very common and found in many plant species although the Aster Leafhopper is considered the only insect vector for Aster Yellows which is the specific type that infected garlic this year. See the USDA Phytoplasma Resource Center for more information.
Phytoplasma will winter over in infected perennial plants and it is likely infected garlic bulbs will carry the disease into the next year although no one knows if this will happen or not. Some growers are using their seed stock for planting this fall. They are counting on the garlic having developed a resistance or being able to select garlic that did not acquire the disease. After having some additional testing and with an abundance of caution, we are not recommending using any of our garlic for planting this fall. Using our garlic for planting will not hurt the soil or cause any harm, however you risk not having a healthy crop next year.