I really wish that glossy gardening magazines would, every once in a while, devote some space to capturing some of the soul-crushing disappointments of tending plants. Can we please have a cover of Sunset Magazine featuring an aphid and slug infested cabbage? Frustrations are compounded when a beloved perennial plant you’ve been growing for years comes down with a fatal disease. Such was the case when my flame seedless grapevine, which was planted to cover our backyard arbor, contracted Pierce’s disease, caused by an incurable bacteria (Xylella fastidiosa) spread by an insect called the sharpshooter. Pierce’s was discovered in 1892 in Anaheim and is basically the reason we no longer have many vineyards in Southern California. Once a vine gets Pierce’s it will die within a few years. You have to admit failure and rip it out, which I plan to do soon.
To confirm that my vine had Pierce’s I called Jerry Turney, plant pathologist at the Los Angeles County Agricultural Commissioner’s Office. When I described the symptoms, Turney agreed that it sounded like Pierce’s. The signs of Pierce’s, as described in UC’s pest management guide, are:
“(1) leaves become slightly yellow or red along margins in white and red varieties, respectively, and eventually leaf margins dry or die in concentric zones; (2) fruit clusters shrivel or raisin; (3) dried leaves fall leaving the petiole (leaf stem) attached to the cane; and (4) wood on new canes matures irregularly, producing patches of green, surrounded by mature brown bark.”
Turney described the life cycle of the sparpshooter, one of the main carriers of Pierce’s, who spread the diesase by feeding off the sap of infected plants. Sharpshooters live in riparian areas and when a stream goes dry they fly off in search of irrigated plants to feed on. Sharpshooters also like to spend the winter in citrus groves which, while not susceptible to Pierce’s, provide habitat. Our hot dry summers, which dry out local streams and rivers, and abundant citrus trees, make inland Southern California an especially bad place to try to grow grapes. Why nurseries continue to sell vines suseptable to Pierce’s here is a mystery to me.
The only hope for long term control, as Turney sees it, is by breeding hybrid grape varieties resistant to Pierce’s. Turney strongly advised against trying to grow wine or table grapes in Southern California. After losing three table grape vines in ten years, I can attest to the wisdom of Turney’s advice. To grow grapes in the warm southern parts of the U.S., you simply have to plant Pierce resistant varieites such as the native Vitus californica or muscadine grapes. The contrast between our flame seedless and our Vitus californica vine, in fact, is stunning. The flame is stunted and diseased, while our Vitus californica is so vigorous that I have to beat it back on a daily basis to prevent it from subsuming our house and the neighbor’s.
And while we’re working on resistant grapes we may need to start hybridizing citrus as well. While the strain of Pierce’s that took down my grape vine is harmless to citrus, there is a variant of the bacteria that is currently ravaging the citrus of Brazil and Argentina which causes a disease called Citrus Variegated Chlorosis. It’s also fatal and has the potential to spread to North America via the shipment of infected trees. With a global economy and porous borders it’s bound to show up someday. Might as well get ready.
Gardening is a humbling lesson in evolutionary biology. It’s all about survival of the fittest. Work with evolution by selecting for immunity to pests and disease and you’ll harvest the rewards. Resistance is futile.
More info at piercesdisease.org.