Decline of persimmon (Diospyros kaki L.) trees on Diospyros virginiana rootstocks.
Persimmon trees grafted on D. virginiana rootstock seedlings frequently grow very poorly and show several decline symptoms. Rootstock and scion girths of declining trees were smaller than those of healthy trees. Trees grafted on D. kaki rootstocks did not decline. In declining trees, the wood colour close to the graft union changed from yellow to brown, and gum accumulated in the phloem rays and xylem vessels, partly blocking them. The water transport in this area was impeded and the water potential of shoots of declining trees was less than that of healthy trees. Scions from both healthy and declining trees were grafted on the same healthy D. virginiana rootstocks. The healthy scions were influenced by their unhealthy neighbours and grew poorly. Plant material obtained from these originally healthy scions also grew poorly when grafted on healthy D. virginiana rootstocks, indicating that the healthy scions were somehow infected by their unhealthy neighbours. Removing scions of young declining trees and replacing them with healthy scions also resulted in reduced growth, compared with healthy rootstocks grafted with healthy scions. These results suggest the presence of a transmittable biotic factor, both in D. virginiana rootstocks and D. kaki scions, causing decline only in D. kaki scion tissues.