Race identity, pathogenicity and damage threshold of Tylenchulus semipenetrans on sour orange in Jordan

Abstract

Three populations of Tylenchulus semipenetrans, collected from Northern, Southern, and Central Jordan Valley, were differentiated on 'Valencia' sweet orange, 'Troyer citrange', 'Pomeroy' and 'Rubidoux' Poncirus tn/oliata, 'Thompson seedless' grape, and 'Manzanillo' olive. The three populations did not infect olive or P. tri/oliata, and consequently were identified as the 'Mediterranean' race. Pathogenicity tests showed that inoculation of 500 juveniles/pot (kg soil) did not affect the growth of sour orange seedlings. However, initial population densities (Pi) of 1,000 to 20,000 juveniles/pot progressively reduced vegetative growth by 9.1 to 30.3% and root weight by 9.7 to 30.9%. Also, Pi exceeding 5,000 juveniles/pot resulted in severe decline symptoms.  Plant growth was not adversely affected as long as the resulting nematode infestation stayed below certain critical levels, i.e., 2,450 juveniles/lOO em} soil; 1,250 eggs, 3,700 juveniles, 590 females, or 5,540 total developmental stages/g root. These levels of infestation represent damage thresholds of this nematode on sour orange seedlings under growth chamber conditions.