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.
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