Resistance of hybrid citrus rootstocks to a Mediterranean biotype of Tylenchulus semipenetrans Cobb

The response of 52 citrus hybrid rootstocks to a Mediterranean biotype of T ylenchulus semipenetrans Cobb was determined in greenhouse tests. Seven sel ections of the cross 'Cleopatra' mandarin [Citrus reshni Hort. x Poncirus t rifoliata (L.) Raf], and one of Citrus volkameriana Pasq. x P. trifoliata d id not support nematode reproduction and were considered as highly resistan t to the citrus nematode. The nematode showed very low infectivity and repr oductive potential on seven additional selections of 'Cleopatra' mandarin x P. trifoliata, one of 'King' mandarin x P. trifoliata, and two C. volkamer iana x P. trifoliata. These selections were considered as nematode resistan t. All the selections with 'Troyer' citrange [Citrus sinensis (L). Osbeck x P. trifoliata (L.) Raf] in their parentage supported nematode reproduction but showed different levels of susceptibility.