TYLENCHULUS SEMIPENETRANS BIOTYPE IN SIX CITRUS-PRODUCING PROVINCES OF THE REPUBLIC OF SOUTH AFRICA

Due to the existence of nematode biotypes, trifoliate orange (Poncirus trifoliate) and its hybrids had unintended consequences in the management of the citrus nematode (Tylenchulus semipenetrans) in South Africa. The challenge could be traced to equating assumptions to empirical evidence. After the discovery of T. semipenetrans, the South Africa biotype was purported to be the Mediterranean biotype, with most growers switching over to Poncirus rootstock on virgin soil in order to suppress the citrus nematode. Failure of citrus orchards on P. trifoliate rootstocks on virgin soils resulted in the initiation of a national study to determine the South African citrus nematode biotype using three differential hosts, viz. rough lemon (Citrus jambhiri), P. trifoliate and olive (Olea europaea). An orchard was randomly selected in each of the six citrus-producing provinces, viz. Eastern Cape, Kwa-Zulu Natal, Limpopo, Mpumalanga, North West and Western Cape. Three-month-old differential host seedlings were inoculated with ca. 10000 J2s, arranged in a randomised complete block design with 15 replicates. At harvest, 120 days after inoculation, isolates from each province had the reproductive factor (RF = Pf/Pi) of less than one on olive, but greater than one on rough lemon and trifoliate orange. Reproductive factors less than one suggested that T. semipenetrans failed to reproduce and develop on olive, whereas that of greater than one suggested that the isolates reproduced and developed on the other two hosts. In conclusion, the differential host tests suggested that the citrus nematode biotype in South African was the Poncirus biotype, implying that the P. trifoliate rootstock which had been recommended for use in the suppression of the Mediterranean biotype was unsuitable.