Huanglongbing (Greening) Detection in South Africa

Abstract

Thirty leaf samples collected from citrus orchards in South Africa in December 1993 were tested by DNA-DNA hybridization with probes In 2.6 and As 1.7 specific for Libero-bacter asiaticum and L. africanum, respectively. All samples showing Huanglongbing (HLB) (greening) symptoms gave positive hybridizations with probe As 1.7. None reacted with In 2.6. Leaves of a citrus relative, Toddalia lanceolata, on which leaf distortions caused by larvae of the HLB vector, Ti-ioza erytreae, were seen, also gave a positive hybridization with As 1.7. Four year-old Nules Clementine trees showing stunting and small yellow leaves but no mottle gave strong hybridization signals with As 1.7. The presence of the HLB Liberobacter in these trees was confirmed by electron microscopy. Forty-two samples collected in June 1994 were also analyzed by DNA-DNA hybridization and PCR. Among the 25 trees showing mottle or yellow vein symptoms, 20 were positive and five were negative with the two techniques. The 17 samples from symptom-less trees gave negative results by PCR and hybridization. In recent years, various serologi-bing (HLB) (greening) liberobacter cal and molecular reagents for the species occurring in Africa (2), have detection of "Candidatus Libero-been produced. These include mono-bacter africanum", the Huanglong-clonal antibodies (1, 4), DNA probe Fig. 1. Dot blot hybridization at high stringency, of probe As 1.7 (left) and In 2.6 (right) with the DNAs extracted from citrus leaves of field trees in South Africa (see text). GI: DNA from healthy, greenhouse grown citrus leaves; G2, left and G3 right: DNA extracted from greenhouse grown citrus leaves infected with L. africanum (South Africa); G3, left and G2, right: DNA extracted from greenhouse grown citrus leaves infected with L. asiaticum (India).