Pathogenicity of Puccinia porri on Allium in South Africa

Allium crops are commonly grown in South Africa and harvested as either fresh produce for the domestic and export markets or as seed. Apart from occasional outbreaks on garlic, rust is problematic as a cosmetic disease with unappealing uredinia regularly observed on freshly packed produce of bunching onion and leek in supermarkets. Spore morphology and phylogenetic analysis of five rust samples collected from A. fistulosum (bunching onion) confirmed the causal organism as Puccinia porri. Garlic and bunching onion varieties were mostly susceptible to P. porri, whereas leek varieties were either susceptible or segregating in their response, with bulb onions being resistant. Microscopy of early infection structures showed appressorium formation, stomatal penetration, and a substomatal structure which differentiated into infection hyphae and haustorium mother cells. At microscopy level differences in host response became visible from 48 h post-inoculation onwards with prehaustorial and early hypersensitivity observed as resistance mechanisms in onions.