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

Abstract

Argentina is the world´s second largest exporter of garlic (Allium sativum) and blue mold rot is annually responsible for significant crop losses. Diseased plants appear stunted and chlorotic with withered leaves and a reduction in bulb size as well as bulbs often being covered with blue/green conidial masses. To identify the pathogen, symptomatic plants (Figure 1) were sampled in the field at La Consulta station (33°45´ S, 69°02´ W). Visible fungi were cultured and identified as P. allii. Representative strains IBT# 26466, 26467, 26511 and 26512 were accessioned into the IBT collection, BioCentrum-DTU. Identifications were confirmed by comparing RP-HPLC secondary metabolite profiles with those of standard and ex Type P. allii strains (IBT 3772, 4112 and 21503). To confirm pathogenicity sterilized garlic cloves inoculated with the P. allii strains (1.5 mm deep, 5 µL of a suspension with 5x106 con.mL-1) were planted in a randomized design (4 replications plus an un-inoculated control) in a field not previously cultivated with garlic. The survival rate of inoculated plants was 68% with an overall yield of 30% marketable bulbs. P. allii was re-isolated from symptomatic field plants. Micro-morphology and yellow exudate production (Onion, 1981), were previously used to identify P. viridicatum as a pathogen of garlic in Argentina (Gatica and Oriolani, 1984); however at that time P. allii was not yet characterized. P. allii is micro-morphologically similar to P. viridicatum and also produces yellow exudate (Vincent & Pitt, 1989). To compare pathogenic ability, P. viridicatum (IBT 16939 and IBT 15053) and P. allii strains were inoculated into sterilized garlic cloves and incubated for 12 d. P. viridicatum strains were not pathogenic meanwhile P. allii strains sporulated on the garlic cloves. P. allii has been reported as an aggressive pathogen of garlic in pathogenicity trials conducted in moisture chambers (Overy et al., 2005).