First report of Penicillium allii as a field pathogen of garlic (Allium sativum)

Blue mould disease in garlic is associated worldwide with various Penicillium species and has been attributed to significant annual crop losses in Argentina; the world's second largest exporter of garlic. To identify the responsible pathogenic Penicillium species, symptomatic plants were sampled in La Consulta station (33°45' S, 69°02' W) and damp chambered. Characteristic disease symptoms are stunted and chlorotic plants with withered leaves and reduced bulb size. Bulbs are often covered with blue/green conidial masses. Isolations were made from fungal colonies emerging on affected bulbs. Pure cultures (IBT 26466, 26467, 26511 and 26512; CMB collection, BioCentrum-DTU, Denmark) were initially identified by micro-morphology as Penicillium allii and identifications were confirmed by comparing RP-HPLC secondary metabolite profiles with those of type P. allii strains.

To confirm pathogenicity, sterilised garlic cloves were injured with a needle and inoculated with 5 μl of P. allii spore suspensions (adjusted to 5×10⁶ conidia per ml). Cloves were planted in a field not previously cultivated with garlic (4 replications and an untreated control). The original disease symptoms were produced on inoculated plants while control plants remained healthy. The survival rate of inoculated plants was 68%. P. allii was re-isolated from symptomatic field plants.

P. viridicatum was first reported as the causal agent of blue mould of garlic in Argentina (Gatica & Oriolani, 1984) before the characterisation of P. allii (Vincent & Pitt, 1989). P. allii is micro-morphologically similar to P. viridicatum and both species produce yellow exudates in pure culture. To compare pathogenic ability, standard P. viridicatum strains IBT 16939 and 15053 were inoculated into sterilised garlic cloves and incubated for 12 days. The P. viridicatum strains were not able to sporulate on the garlic cloves.

P. hirsutum was recently reported as a pathogen on garlic in Argentina (Cavagnaro et al., 2005). However, P. allii but not P. hirsutum has been reported as an aggressive pathogen of garlic in comparative pathogenicity trials conducted in moisture chambers (Overy et al., 2005). Our results suggest that P. allii, rather than P. hirsutum or P. viridicatum, is the pathogenic species responsible for garlic crop losses due to blue mould rot in Argentina. This is the first report confirming P. allii as a field pathogen of A. sativum.