Mid-Columbia cherry tree growers are being asked to be alert for signs of viruses infecting trees so they can stop their spread.  Oregon State University plant pathologist Dr. Jay Pscheidt says evidence of Cherry Leaf Roll has been found in isolated areas of the region.  Pscheidt notes he does not think it is an intensive problem at this time, but they want to make sure the viruses do not spread.  Pscheidt says growers are asked to look for certain symptoms, including enations on leaves and fruit that does not ripen or color properly and has little flavor.  Once the disease is established, tree removal is the only way to stop it.