In February 2012, hydroponically grown sweet basil plants were submitted to Crop Health Services (CHS), Agriculture Victoria, for diagnosis of possible causes of brown necrotic patches on leaves, necrotic segments on stems, and shoot tip necrosis. Fungal isolation detected the fungi Verticillium tricorpus and Corynespora cassiicola from the necrosis-affected stem segments. The identity of C. cassiicola was confirmed using ITS sequence data, and that of V. tricorpus was confirmed using both ITS and elongation factor sequence data. The sequences were compared to reference sequences deposited in GenBank (http://www.ncbi.nlm.nih.gov) and were found to share the highest identities, either with V. tricorpus (99% identity, 100% coverage), or C. cassiicola (100% identity, 100% coverage), respectively. V. tricorpus has been reported from several hosts before (Taylor 1968), but is generally considered a saprophyte. However, C. cassiicola was previously reported to be pathogenic in a wide range of host plants, including O. basilicum, worldwide (Smith 2008).
Nematode extractions detected Aphelenchoides fragariae (foliar nematode) from the affected leaves and high levels of Helicotylenchus dihystera (spiral nematode) from the growth medium (21698 nematodes / 200 mL). Nematode identifications were confirmed by morphology, by comparison with reference specimens in the Victorian Plant Pathogen Herbarium (VPRI). A. fragariae is well documented for causing damage to leaves on many host plants (Siddiqi 1975). The presence of H. dihystera at such high numbers in the growth medium explained the considerable root damage observed, as H. dihystera is known to feed externally on roots of many host plants (Siddiqi 1972).
The detections of the fungi V. tricorpus and C. cassiicola, and the nematodes, A. fragariae and H. dihystera, from O. basilicum are all first records from this host in Australia. The sweet basil crop was unmarketable and consequently destroyed.