Influence of the scion on the susceptibility of sour orange rootstock to Phytophthora gummosis and root rot

Snmmary. The influence of the scion on the response of sour orange rootstock towards artifìcìally-induced Phytophthora gummosis and root rot was evaluated under a controlled environment. The citrus cultivars grafted onto sour orange rootstock were: clementine, old line (D.L.) 'Tarocco' orange, nucellar Une (N.L.) 'Tarocco' orange and sour orange, the latter served as a control. The influence of a scion on the susceptibility of the rootstock to gummosis was assessed by rating the development of lesions on the rootstock after artificial inoculation with a virulent isolate of Phytophthora citrophthora. The influence on Phytophthora root rot was assessed by rating root weight, percentage of infected feeder root segments, and inoculum density on plants transplanted in a soil artificially inoculated with chlamydospores of a virulent isolate of P. nicotianae. All grafted citrus cultìvars, other than sour orange, induced a decrease in rootstock resistance to gummosis though the disease was more severe in the graft combinations with clementine and N.L. 'Tarocco', The investigations on the influence of a scion on sour orange Phytophthora root rot gave the following results compared to the control: i) a higher number of propagules of P. nicotianae in the rhizosphere of the sour orange/c1ementine and the sour orange N.L. 'Tarocco' combinations, ii) a higher rate of root infection in the sour orange/N.L. 'Tarocco' combination, The correlation coefficients between inoculum density and root infection were 0.574 and 0.677 for sour orange/clementine and sour orange/N.L. 'Ta­rocco' orange combinations, respectively.