The sky is falling!
(Well, maybe not.)
Jay W. Pscheidt, Ph.D.
Professor of Botany and Plant Pathology and Extension Plant Pathology Specialist, Oregon State University
The new Compendium of Grape Diseases, Disorders and Pests (Wilcox et al 2015) points out the multitude of problems that can beset grapes. Oregon’s grape industry has done well to avoid many of these troubles using geographic isolation, unique climate conditions and planting stock quarantines. Grapes are still susceptible to all these problems, which could arrive and cause havoc on any growing season. When one of these problems does come along, we may sound a lot like “Chicken Little” declaring that the sky is falling. Several disease issues have fallen onto our doorstep that need to be discussed. Although some are very serious and not unexpected, all can be dealt with. These issues include Xylella, sterilizing pruners, fungicide resistance and climate change, which we will address throughout the season.
In October, 2015, the presence of the bacterium Xylella fastidiosa was confirmed by the Oregon Department of Agriculture (ODA) in several pear trees growing in the field germplasm collection at the USDA Repository in Corvallis. Grape growers may be alarmed knowing that Pierce’s Disease is caused by Xylella fastidiosa subsp. fastidiosa. Pierce’s Disease has been a high-profile and rapidly increasing disease in California and other southern states but has not been known to be in the Pacific Northwest. At this time, the preliminary DNA sequence data suggests the bacterium on pear is X. fastidiosa subsp. multiplex, which can cause a chronic leaf-scorching disease in many different species of woody landscape shrubs and shade trees, including oak, elm, and other trees – but not grape. (Whooh!)
There are still a ton of questions that need to be answered in the coming months and years about this find. Keep half an ear open on this problem.
Why don’t find Pierce’s Disease in the Pacific Northwest? Our climate may be too cold for the pathogen to survive. Infected grapevines do not retain the pathogen after a cold dormant season typical of continental climates. Also the majority of leaf hoppers (xylem feeding insects that vector the bacterium) found in PNW surveys are Western grape leafhopper which are not efficient vectors of Xylella. The Blue-green sharpshooter will vector Xylella and has been found in the Willamette Valley, Columbia Gorge, Medford and Milton-Freewater areas of Oregon. This leafhopper is usually found in surrounding vegetation but less in vineyards. The glassy winged sharpshooter, a very efficient vector, has not been found in or around Oregon vineyards nor is it abundant in the PNW.
If you are still worried, you can keep an eye out for various symptoms. Pierce’s Disease first appears as water stress in midsummer and gradually gets worse. Leaves become slightly yellow or red along margins in white and red varieties, respectively, and eventually leaf margins dry or die in concentric zones. Fruit clusters shrivel or raisin. Dried leaves fall, leaving the petiole attached to the cane. Wood on new canes matures irregularly, producing patches of green, surrounded by mature brown bark. ‘Pinot Noir’ and ‘Cabernet Sauvignon’ have highly regular zones of progressive marginal discoloration and drying on blades. Unfortunately, any other problem that blocks, inhibits or limits water from getting to the leaves will produce similar symptoms. Fungal cankers, damaged trunks, girdling roots, gopher damage, herbicide injury and root rots also can produce similar symptoms.
Bottom Line: Finding Xylella on pears in Oregon is not, at this time, a worry for grape growers.
Reference: Wilcox, W. F., Gubler, W. D. and Uyemoto, J. K. 2015. Compendium of grape diseases, disorders, and pests. Second edition. St. Paul, MN: APS Press.