First Report of Fusarium proliferatum Causing Rot on Garlic and Onion in Argentina.

Abstract

In October 2001 and January 2002, in onion fields (Allium cepa L. cv Valencianita) in the Provinces of San Juan (SJ) and Mendoza (MZ), Argentina, plants were observed with chlorosis, dry leaf tips, and bulbs showing discoloration and rot. During the summer of 2002, a tan rot with white mycelium in rot cavities was also observed in stored garlic bulbs (Allium sativum) in MZ. Four monosporic cultures obtained with a micro punch adapted microscope (three from onion CSJ1, CMZ1, CMZ2 and one from garlic AMZ1) were characterized by morphology on PDA and carnation leaf agar (2). The isolates were deposited in the fungal collection of the Plant Mycosis Laboratory of the Integrated Unit Balcarce. The isolates produced abundant aerial white mycelium and a violet to vinaceous pigmentation. Club-shaped microconidia were abundant, in chains on both mono- and polyphialides. Slender, thin-walled and relatively straight macroconidia were produced only under black light and were mostly 3-septate. Chlamydospores were absent. The isolates were identified as Fusarium proliferatum. Crosses to confirm mating populations and to identify mating types were made in triplicate on carrot agar (3) with standard tester strains D-04853 (MATD-2) and D-04854 (MATD-1) as female parents and the field isolates as male parents. Crosses were examined weekly and were scored positive only if perithecia were seen oozing a cirrhus of ascospores. The identities of these isolates were confirmed as showing positive crosses with standard tester strains of Gibberella intermedia. Pathogenicity tests were conducted with healthy 45-day-old onion seedlings (cv. Valcatorce INTA). The roots of the onion seedlings were soaked in a conidial suspension (5 × 106 conidia/ml) of each isolate (CSJ1, CMZ1, CMZ2) for 2 h; the control was soaked in sterile water (SW). Seedlings were transplanted to pots in a sterile mixture of soil and sand (v/v). Five plants were used for each of 3 replications. The plants were placed in a greenhouse and irrigated with SW. After 3 weeks, symptoms were evaluated. All inoculated plants exhibited symptoms similar to those observed in the bulbs from which the pathogen was isolated and a brown rot appeared on the basal plate of the onion, later becoming dark brown. In garlic, the inoculation consisted of a wound 4.5 mm deep and 2 mm wide in superficially sterilized garlic cloves (cv. Nieve INTA). Inside the cavity, a drop (50 μl) was placed from a suspension of 5 × 106 conidia/ml (AMZ1), then covered with a drop of paraffin. Controls used SW. The garlic cloves were incubated in hermetically sealed trays at 22 ± 3°C in darkness for 3 weeks (1). Garlic showed tan rot and white mycelium in the wound. F. proliferatum was reisolated from inoculated onion seedlings and garlic cloves. The controls did not exhibit symptoms nor were any fungi recovered when tissue was excised from the inoculation points and plated on agar. F. proliferatum was previously reported in Argentina on asparagus (4) with symptoms similar to those of onion and garlic. To our knowledge, this is the first report of F. proliferatum attacking onion and garlic in Argentina. This pathogen has the potential risk of mycotoxin accumulation in contaminated bulbs.