Evan de la TorrePierce’s DiseaseCalifornia is faced with a new threat and it is one that is only getting worse witheach passing season. That threat is Pierce’s disease, named after Newton B. Pierce,Special Agent to the Secretary of Agriculture, and California’s first professionally trainedplant pathologist. To understand this new threat that California is facing1 and how geneticengineering can help, it is important to understand some of the background for thisdisease, what non-genetic methods are being used to try and stop this threat before it getsout of control, and why genetic intervention seems the most effective approach in thefuture.Pierce’s disease was first introduced to California in the 1880s when it decimated30,000 acres of vineyards in the Los Angeles Basin near Pomona and Anaheim. It hit thesame area twice more in the 1930s and 1940s with the same ferocity. In 1887 and 1889Pierce’s disease was found in the Napa Valley and northern Sacramento. In 1921 it wasfound in Tulare County and by 1940 it was identified in mostly all the counties in the SanJoaquin Valley. More recently, Pierce’s disease has impacted 25% of the 3000 vineyardsin the Temecula Valley, which has totaled $13,000,000 in damage in this county alone.(Wine Institute) Currently, there are 15 counties that have been labeled as “infected”within California and the number is increasing with each passing year. Furthermore, thetwo areas that have been devastated the most by Pierce’s disease are the San Joaquin 1 Pierce’s Disease is a disease that other states than California has had to face (i.e. Florida), but I will choseto only talk about California for this paper.Valley and the Napa Valley. (Wine Institute) Seeing the trend of rapid and viciousspreading, one might ask how was it possible for this disease to spread so quickly? The rapid infestation of Pierce’s disease throughout California can mostly beattributed to the glassy-winged sharp shooter2. The sharp shooter carries the bacterium,Xylella fastidiosa , which is the cause of Pierce’s disease. When a sharp shooter drinks thesap from one of the vines, it also injects the plant with this bacterium and thus spreads thedisease with am amazing quickness. Furthermore, before the growers knew what thedisease was or how it spread they would send cuttings from one vineyard to another.Some of these cuttings traveled from the San Joaquin Valley to the Napa Valley and thusthe disease was introduced to northern California. The reason that California, and morespecifically the San Joaquin and the Napa Valleys, has been affected with this disease isbecause the sharp shooters and consequently this disease thrive in climates that have mildwinters. (Overview of Pierce’s disease) Now, to further understand this new threat, it isessential that one understands how the sharp shooter transmits the disease and how theplant is affected from it.As mentioned, Pierce’s disease is the bacterium Xylella fastidiosa that thrives inthe plant’s xylem. The sharp shooter, which feeds on the xylem sap, transmits thebacteria from the diseased plant to the healthy plant every time it feeds on a vine. Oncethis happens, the bacteria eventually blocks the water conducting system andconsequently reduces the flow of water within the plant’s cells to the affected leaves.(Overview of Pierce’s disease) Therefore, the drying or “scorching” of the leaves on thevine are the first signs of the disease. The leaves then turn slightly yellow along the 2 There are other sharp shooters, such as the blue and green, that are spreading the disease as well, but theglassy-winged sharp shooter is the most damaging of the insects.margins before drying or die rapidly even when they are still fully green. (PICTURE)Furthermore, the bark matures unevenly; mature dark brown bark can be surrounded byimmature light green bark. (Overview of Pierce’s disease) Since there is a lack of watertransport to the plant’s cells, the vine prematurely produces small, inedible, raison-likegrapes. Consequently, the vine is now unusable and if left alone will die within two years.However, the growers don’t let the plant die on its own because they pull the now uselessplant up before it can die naturally. Clearly, this disease is very threatening for growersaround California so what are growers doing in order to counter this leafhoppingmenace?First and foremost, there have been massive inspections on all transports ofnursery stock and bulk grapes throughout California. Considering that the transportationof vines from one area to another is the easiest way for either infected plants or sharpshooters to infect new areas, a lot of effort has been given to ensure that this doesn’thappen. In fact, in 2001 alone, 50,000 nursery plant shipments and 100,000 grapeshipments were inspected. (Wine Institute) Furthermore, growers are now selectivelyremoving vegetation around the vines that might house the sharp shooters and areconducting heavy pruning of the infected plants. Ground spraying of the infected plantsthroughout the vineyard with insecticide has also been an alternative. However, it isdifficult to find an insecticide that will only target the sharp shooter and not harm theother animals and vegetation in the area. Some insecticides that have shown promise inkilling the sharp shooter but they have also been killing healthy vines. Another method,which has shown more promise, is that of antibiotics. Unfortunately, injecting the drugsinto the hard grapevines requires countless hours of manual labor and it is difficult forgrowers continually inject their numerous vineyards. (Alright Seed) There has also been amethod of biological control that introduces a parasitic wasp that lays eggs in the eggs ofthe sharp shooter. This method has been shown to be an inexpensive, long-lasting andenvironmentally friendly alternative. Consequently, it can be seen in Temecula thatbecause of the implementation of this method, the levels of sharp shooters have slowlydeclined. (Wine Institute) However, using this method also means that there will be manymore wasps to deal with in the vineyards. Consequently, researchers are trying to breed astingerless wasp to deal with this immediate problem. Though these methods have hadsuccess in stopping the sharp shooter, each
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