David M. Kopec, Extension Turfgrass Specilaist
Over the last eighteen months, you have read newspaper articles and have seen short TV segments about the wheat quarantine in Arizona due to the presence of the Karnal bunt disease. Karnal bunt is a fungus that infects the wheat kernel, seriously affecting quality or destroying the seeds completely. It belongs to a relatively unique group of fungi, which are called "obligate parasites." This means that it is extremely difficult for the fungus to be cultured in a living state, away from the host plant. The "bunts", "smuts" and "rust" diseases are generally grouped as obligate parasites.
The bunt fungus has two spore phases. One spore phase is a hard resting spore. This spore is what has been found on wheat in Arizona. This resting spore germinates and releases the fungus body, which can damage the wheat kernel. At the end of the wheat crop, the fungus returns to the resting spore form again. No one knows how long (or if) the resting spore can survive in the Arizona soil, or infect plants in a relatively dry climate that we have.
More recently, the Arizona Department of Agriculture has quarantined the inter-state importation of annual and perennial ryegrass seed from the Pacific Northwest into Arizona. Why? Because the resting spores have been identified on perennial ryegrass seeds from Oregon. The spores probably have come from infected wheat plants in fields next to, or near ryegrass seed fields. The Arizona Department of Agriculture has established the quarantine, not authorities in Oregon.
The Arizona Department of Agriculture is waiting for USDA test results in Maryland to see if the "bunt" found on ryegrass is capable of infecting wheat. Likewise, they are testing at the same time to see if "bunt" spores from wheat can cause the actual disease on ryegrass. This will clear up the picture.
The "rust" and "smut" fungal diseases are known to be very specific in the host plants they attack. For example, leaf rust on tall fescue will not infect leaf rust on Kentucky bluegrass, or vice-versa. In fact, different races of the same rust exist and they are specific to the varieties they can infect within the single turf species. Now that's specific! No one is certain about the specificity in the case of Karnal bunt.
All is not doomed. There is large case of circumstantial evidence that may make a "pathological liar" out of Karnal bunt on ryegrass for turf.
1. Known cases of host specificity among related fungi (above).
2. Karnal bunt spores have been noted on perennial ryegrass from Oregon since 1993 but no epidemic or symptoms have ever been seen in turf situations.
Recently, the state of Georgia released its quarantine on perennial ryegrass importation from Oregon. New Mexico has no quarantine, nor does California (which grows lots of wheat!). These states are waiting for the results of the USDA spore-transfer-infection test, as well.
What happens if the "resting spore" from Oregon grown ryegrass infects Arizona wheat? My guess is that the odds of this are extremely remote. However, a seed treatment with a suitable fungicide should take care of this. There are fungicides which are very effective as seed treatments for bunt fungi, but they are not currently labeled for ryegrass seed. A special local needs (SLN) action would be needed to make this happen. I will let you know ASAP on the test results and Arizona's decision!
Don't forget, creeping red fescue makes a fine overseeding as well.
Issued in furtherance of Cooperative Extension work, acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, James A. Christenson, Director, Cooperative Extension, College of Agriculture & Life Sciences, The University of Arizona. The University of Arizona is an equal opportunity, affirmative action institution. The University does not discriminate on the basis of race, color, religion, sex, national origin, age, disability, veteran status, or sexual orientation in its programs and activities.