The first three cause rootstock diseases, but psorosis causes bark shelling on the trunk and branches of trees older than 8 to 10 years. These four viruses are bud-transmissible. Certain aphids also are responsible for spread of tristeza virus. Damage caused by virus diseases varies with rootstock-scion combination.
Some viruses result in only a slight reduction in growth rate.
Others result in yield loss, severe stunting, decline and eventual death. Because citrus viruses can be transmitted by budding, careful selection of budwood is an effective control method. Exocortis This virus causes bark-shelling and stunting of trees on trifoliate orange, trifoliate hybrids and Rangpur lime rootstocks. In the early stages of the disease, gum exudes from pustules at the base of the trunk and may extend from below the soil line to the bud union. New bark forms beneath the pustules, and the outer bark sloughs off, forming the characteristic bark-shelling.