New citrus selections from Cleopatra mandarin x Poncirus trifoliata with resistance to Tylenchulus semipenetrans Cobb
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The response of four resistant selections of Cleopatra mandarin x Poncirus trifoliata to increasing inoculum densities of a population of the Mediterranean biotype of Tylenchulus semipenetrans was determined in microplots established at two sites. One-year-old trees of the resistant selections and of a susceptible Carrizo citrange were inoculated with 0, 1, 5 or 10 x 10(4) eggs per plant (Pi). Nematode reproduction on the resistant selections was consistently less than on Carrizo citrange. Increasing initial inoculum densities had no effect on the relative resistance in three selections (03.01.42, 03.01.5 and 03.01.16). Selection 03.01.42 was highly resistant (no females and eggs per g fresh root), and 03.01.5 and 03.01.16 both had good resistance (less than or equal to15% as many females and eggs per g fresh root as on Carrizo citrange). The fourth selection (03.01.18) expressed resistance at Pi 1 and 5 x 10(4) eggs per plant but was moderately susceptible at 10 x 10(4) eggs per plant (>15% females and eggs per g fresh root). Deposits of a lignin or suberin-like material were more abundant in resistant selections 03.01.5 and 03.01.18 than in susceptible Carrizo citrange. The proportional increase in trunk diameter of inoculated and uninoculated trees of each rootstock was similar at both sites.