Isolation and Identification of Fusarium spp., the Causal Agents of Onion (Allium cepa) Basal Rot in Northeastern Israel.

Ben Kalman,Shaul Graph,Yael Meller Harel,Rafael Perl-Treves,Dekel Abraham,Ofir Degani

doi:10.3390/biology9040069

Abstract

Over the past decade, there have been accumulating reports from farmers and field extension personnel on the increasing incidence and spread of onion (Allium cepa) bulb basal rot in northern Israel. The disease is caused mainly by Fusarium species. Rotting onion bulbs were sampled from fields in the Golan Heights in northeastern Israel during the summers of 2017 and 2018. Tissue from the sampled onion bulbs was used for the isolation and identification of the infecting fungal species using colony and microscopic morphology characterization. Final confirmation of the pathogens was performed with PCR amplification and sequencing using fungi-specific and Fusarium species-specific primers. Four Fusarium spp. isolates were identified in onion bulbs samples collected from the contaminated field: F. proliferatum, F. oxysporum f. sp. cepae, and two species less familiar as causative agents of this disease, F. acutatum and F. anthophilium. Phylogenetic analysis revealed that these species subdivided into two populations, a northern group isolated from white (Riverside cv.) onion bulbs, and a southern group isolated from red (565/505 cv.) bulbs. Pathogenicity tests conducted with seedlings and bulbs under moist conditions proved that all species could cause the disease symptoms, but with different degrees of virulence. Inoculating seeds with spore suspensions of the four species, in vitro, significantly reduced seedlings’ germination rate, hypocotyl elongation, and fresh biomass. Mature onion bulbs infected with the fungal isolates produced typical rot symptoms 14 days post-inoculation, and the fungus from each infected bulb was re-isolated and identified to satisfy Koch’s postulates. The onion bulb assay also reflected the degree of sensitivity of different onion cultivars to the disease. This work is the first confirmed report of the direct and primary cause of Fusarium onion basal rot disease in northeastern Israel. These findings are a necessary step towards uncovering the mycoflora of the diseased onion plants and developing a preventive program that would reduce the disease damage.

Highlights

Onion (Allium cepa L., a member of the Amaryllidaceae family, known as the bulb onion or common onion) is one of the most commonly grown and consumed vegetable crops in the world and is an important crop in Israel, grown during the winter–spring and harvested during the hot summer.In 2013, onion bulb production in Israel covered an area of ca 2500 ha throughout the country [1].In 2017, the commercial production of onion bulbs for the local markets reached ca. 47,000 tons (9% of the bulk fresh vegetables according to data from the Israel Ministry of Agriculture and RuralDevelopment)
Wilting of onion plants with rot in the roots and basal plate of the bulb has been related mostly to Fusarium oxysporum f. sp. cepae [4], but formae speciales (f.sp., a group of isolates that are pathogenic to particular host plants) of Fusarium solani are common in the roots of onions and cause plant wilting [5]
The tissue was cut into small segments, and each piece was surface-sterilized by dipping it in 70% alcohol for a few seconds, followed by immersion in 1% sodium hypochlorite for 1 min and washing twice in sterile double-distilled water (DDW)

Summary

Introduction

Onion (Allium cepa L., a member of the Amaryllidaceae family, known as the bulb onion or common onion) is one of the most commonly grown and consumed vegetable crops in the world and is an important crop in Israel, grown during the winter–spring and harvested during the hot summer.In 2013, onion bulb production in Israel covered an area of ca 2500 ha throughout the country [1].In 2017, the commercial production of onion bulbs for the local markets reached ca. 47,000 tons (9% of the bulk fresh vegetables according to data from the Israel Ministry of Agriculture and RuralDevelopment). Species of the fungus genus Fusarium are widely distributed in soils in all climatic zones around the globe, associate with a vast diversity of plants, and cause severe plant diseases in crops [2]. Some members of this genus are responsible for diseases that have a significant economic impact in several onion-growing countries [3]. Cepae [4], but formae speciales (f.sp., a group of isolates that are pathogenic to particular host plants) of Fusarium solani are common in the roots of onions and cause plant wilting [5]. Fusarium proliferatum was more recently reported as the predominant fungal species isolated from the roots and bulbs of onion and garlic plants [6]

Objectives

Methods

Results

Discussion

Conclusion