Die-back of rooibos (Aspalathus linearis) causes substantial losses in commercial Aspalathus plantations in South Africa. In the past, the disease has been attributed to Phomopsis phaseoli (teleomorph: Diaporthe phaseolorum). Isolates obtained from diseased plants, however, were highly variable with regard to morphology and pathogenicity. The aim of the present study was thus to identify the Phomopsis species associated with die-back of rooibos. Isolates were subjected to DNA sequence comparisons of the internal transcribed spacer region (ITS1, 5.8S, ITS2) and partial sequences of the translation elongation factor-1 alpha gene. Furthermore, isolates were also compared in glasshouse inoculation trials on 8-mo-old potted plants to evaluate their pathogenicity. Five species were identified, of which D. aspalathi (formerly identified as D. phaseolorum or D. phaseolorum var. meridionalis) proved to be the most virulent, followed by D. ambigua, Phomopsis theicola, one species of Libertella and Phomopsis, respectively, and a newly described species, P. cuppatea. A description is also provided for D. ambigua based on a newly designated epitype specimen.

Fusarium solani, the causal fungus of root and crown rot of citrus, produces naphthazarin toxins that were shown to be toxic to citrus plants. It was found that citrus trees in South Africa had varying levels of naphthazarins in their xylem. Significantly higher concentrations of these toxins were detected in trees with blight symptoms than in symptomless trees. Citrus seedlings grown in hydroponic culture containing isomarticin showed inhibition of growth, shortening of roots and accumulation of zinc in the trunk wood. Zinc deficiency symptoms were also observed on certain rootstocks. A procedural ranking of rootstocks indicated that trees with observed tolerance to blight in the field generally had higher tolerance to isomarticin in hydroponic culture, whereas trees susceptible to blight were more sensitive to the toxin. These findings indicate that Fusarium-produced isomarticin is involved in the induction of some of the symptoms associated with citrus blight syndrome.