Symptoms and Signs
The characteristic symptom of exocortis is the shelling of susceptible rootstocks. The viroid kills the bark, which dries, cracks, and may lift in thin strips. Droplets of gum often appear under the loose bark.
Comments on the Disease
Exocortis is of minor importance in California today because strict regulations on budwood sources have kept new plantings largely free of this viroid disease. Exocortis is widespread in older plantings, but it is a mild disease that causes only moderate stunting and limited loss of production. The viroid is easily spread on infected budwood and contaminated propagation tools.
Infected trees rarely die, but growth is stunted and productivity slowly declines. Among the commonly used rootstocks, trifoliate is most affected by exocortis.
It is best to remove infected trees from the orchard because pruning clippers and saws can transmit exocortis unless thoroughly disinfected with hypochlorite (bleach); heat does not kill the viroid.