By Kendall Johnson and Phil Brannen
Peaches are one of the most economically important fruit crops for the state of Georgia. Growers provide a large variety of high-quality fresh-market peaches for not only the Southeast, but for much of the U.S. Recently, growers have reported an increase in a disease called phony peach, and this threatens production. The disease results in stunted tree growth and lowered fruit quality and quantity. Though this disease has been known for over a hundred years, warmer winters may be to blame for increased disease pressure. Without regard, these reports sparked the need for additional research to determine disease prevalence and early detection methods to limit the development and spread.
PHONY PEACH DISEASE
Peach trees are plagued by several different diseases throughout the growing season, but this reemerging disease is causing noticeable losses within orchards in Georgia. Phony peach disease is an increasingly important disease for the southeast caused by the bacterium Xylella fastidiosa. Phony peach was first documented in peaches in 1890 near Marshallville, Georgia, and has been a challenge for growers ever since, with a few large documented outbreaks that were observed particularly following warm winters. The disease is vectored by sharpshooter insects, and these are prevalent in warm southeastern climates.
The geographic distribution of this disease ranges roughly from North Carolina to Texas and points south, though there are historical accounts of the disease further north. Although the disease occurs throughout the Southeast, it is a significant problem in Georgia due to several factors such as year-round vector activity and greater bacterial survival in the tree. This disease is of particular importance because it can be one of the primary limiting factors associated with peach orchard longevity.
Phony peach disease causes several symptoms that can be used for identification, though other causes can mimic these symptoms – making accurate identification difficult to the novice. Phony peach trees have shortened internodes that can result in a nearly flat leaf canopy on top, dark green foliage, and a generally denser and shorter tree. Infected trees also tend to undergo bloom earlier and have a delayed leaf senescence in the fall. Fruit quality is also substantially diminished, as phony peach reduces the tree’s ability to produce the equivalent size and number of peaches as compared to noninfected trees. Fruit produced from trees infected with phony peach also tends to ripen before healthy peaches.
Xylella fastidiosa infects peach trees either through root graft transmission or more often through insect-vectored sharpshooter transmission. As mentioned, the bacterium is transmitted by sharpshooter insects, which are prevalent throughout Georgia. Bacteria slowly migrate downward from the point of infection through the xylem of the tree. Bacteria tend to set up housekeeping primarily in roots, and it can take more than eighteen months before phony peach symptoms develop.
Unfortunately, phony peach is not a curable disease, thus control is directed towards prevention or minimizing the spread of this disease. To date, the most often used detection method is visual assessment. Growers or consultants walk or drive through orchards and flag trees they think are symptomatic of the disease. In the fall, after phony peach trees have been flagged, the trees should be rogued to prevent further spread of the disease and to extend the orchard’s productive life. Additionally, peach orchards should not be planted within three hundred yards of host plants, such as wild plums, as leafhoppers could travel from infected plums and further infect a peach orchard. It is therefore also suggested to eradicate wild plums near peach orchards. A good weed control program with a weed free strip and mowed row middles will help to reduce habitat for the insect vectors that transmit the bacterium.
Kendall Johnson, a Ph.D. student at the University of Georgia, is testing various molecular and applied methods to determine the prevalence of phony peach infected trees in Georgia. This is a collaborative effort with Drs. Clive Bock and Chunxian Chen at the USDA SE Fruit and Tree Nut Research Laboratory in Byron, GA. Currently, the lab is collecting different plant tissues from trees to extract DNA and conduct a quantitative PCR (qPCR) protocol for the observation of pathogen presence and quantification. The molecular results are compared to visual disease assessments made by experts in the field. So far, we have found that root samples provide the best and most consistent results for DNA quality, quantity, and Xylella detection.
Another detection method we are testing is the use of Agdia’s AmplifyRP XRT+, which is an isothermal DNA amplification and detection system. This is another method that proposes several advantages such as utilization in the field, reduction of detection time, and a seemingly more cost effective approach. This method still requires further testing and needs confirmation of a reliable option before suggesting to growers or county agents.
Finally, the last detection method involves use of a drone carrying a 3DR Solo UAV with Micasense Rededge spectral imager; this is a collaborative effort with Dr. Glenn Rains (UGA, Tifton Campus). Kendall hopes to develop an analytical technique to combine crop stress indices and identify Xylella fastidiosa infected peaches. The UAV will take spectral images of the peach orchard with the potential to correlate foliage color and disease presence. Tissue samples from peach plots flown by the UAV will be collected and analyzed using qPCR to determine the presence of the bacterium. Infected plants will be compared to non-infected plants to identify the spectral characteristics of disease symptoms that can be detected from aerial imaging.
Kendall has already determined that current visual methods are very poor at identifying the disease in the field, as experts can’t accurately identify symptomatic trees, and non-symptomatic trees are often infected. Both UGA and USDA are committed to finding better early-detection methods for phony peach that will allow for the continued success of Georgia’s peach industry, even in the face of changing climate patterns.