ABSTRACT
In December 2006, symptoms typical of iris yellow spot caused by Iris yellow spot virus (IYSV; genus Tospovirus, family Bunyaviridae) were observed on scapes (seed stalks) in an onion (Allium cepa L.) seed crop in the Klein Karoo of the Western Cape Province, South Africa. Symptoms included diamond-shaped chlorotic or necrotic lesions on the scapes, some of which had 'green-islands' with nested diamond-shaped lesions, as well as indistinct, circular to irregular, chlorotic or necrotic lesions of various sizes. At the time symptoms were observed, approximately 5% of the scapes had lodged as a result of extensive lesions resembling those caused by IYSV. The crop was 2 to 3 weeks from harvest. Symptomatic tissue from two plants (two samples from one plant and four samples from the other plant) was tested for IYSV by reverse-transcriptase (RT)-PCR. Total RNA was extracted from symptomatic scape tissue with the SV Total RNA Isolation System (Promega, Madison, WI) according to the manufacturer's instructions. First strand cDNA was synthesized with the RevertAid H Minus First Strand cDNA Synthesis kit (Fermentas Inc., Hanover, MD), followed by PCR amplification with primers IYSV-For (TGG YGG AGA TGY RGA TGT GGT) and IYSV-Rev (ATT YTT GGG TTT AGA AGA CTC ACC), which amplify the nucleocapsid (NP) gene of IYSV. An amplicon of expected size (approximately 750 bp) was observed for each of the symptomatic plants assayed and was sequenced. Comparison of the sequence (GenBank Accession No. EF579801) with GenBank sequences revealed 95% sequence identity with the NP gene of IYSV GenBank Accession No. EF419888, with eight amino acid differences. The known geographic distribution of IYSV in onion bulb or seed crops has increased rapidly in recent years in many areas of the world (1). To our knowledge, this is the first confirmation of IYSV in South Africa. Approximately 6,100 ha of onion bulb crops are grown annually in South Africa in the Western Cape, Kwazulu Natal, Limpopo, and Northern Cape provinces, and 600 ha of onion seed crops are grown primarily in the semi-arid regions of the Western Cape. Examination of an additional 10 onion seed crops in the Klein Karoo during January 2007 revealed the presence of iris yellow spot in three more crops at approximately 5% incidence in each crop. The four symptomatic crops had all been planted as bulb-to-seed crops, using vernalized bulbs produced on the same farm. This suggests that IYSV may have been disseminated into the seed crops on the vernalized bulbs, either as infected bulb tissue or in viruliferous thrips on the bulbs. Reference: (1) D. H. Gent et al. Plant Dis. 90:1468, 2006.
ABSTRACT
Fusarium oxysporum f. sp. cepae and an unidentified Fusarium species have been reported to cause bulb rot of onion (Allium cepa L.) in Washington (1). In August and September 2002, a salmon-pink discoloration was observed on the outer three to four layers of dry scales of approximately 20% of white onion bulbs of cv. Cometa F1, in each of two 20-acre fields in the Columbia Basin of central Washington. Isolations from the discolored areas of the dry scales onto water agar and potato dextrose agar (PDA) yielded fungal colonies characteristic of F. proliferatum (3). The isolates formed long, V-shaped chains of microconidia on polyphialides. Pathogenicity of the isolates of F. proliferatum was tested on white onion bulbs purchased at a local grocery store. The outermost dry scales of each bulb were removed, and the bulb was inoculated by one of three methods: (i) a 5-mm3 section of the fleshy scales was removed using a scalpel, the wound was filled with a 3-mm2 plug of PDA colonized by F. proliferatum, the plug was covered with the section of scale that had been removed, and the inoculation site was covered with Parafilm; (ii) the basal plate of the bulb was dipped into a suspension of 106 microconidia per ml; or (iii) the basal plate was dipped into the spore suspension after wounding by inserting a dissecting needle into the bulb to a depth of 1 cm. A noninoculated bulb provided a control treatment. Bulbs were incubated in a moist chamber at 13°C and examined for discoloration of the outer scales and development of bulb rot. After 2 weeks, salmon-pink discoloration of the outer scales was observed at the inoculation site for both methods of dip inoculation, but not for the plug inoculation method. After 3 weeks, water-soaked, tan to golden, shrunken, soft tissue was observed on the remainder of each dip inoculated bulb, but symptoms of basal rot did not develop. Symptoms were similar to those reported in Idaho for a bulb rot of stored onions caused by F. proliferatum (2). One of the nonwounded inoculated bulbs did not develop a bulb rot, but pinkish discoloration was observed beneath the outer scales and in the neck. F. proliferatum was reisolated from the inoculated bulb tissues. The discoloration observed on the white onions raised concern about the potential for infection to develop into bulb rot in storage. However, thorough curing of the bulbs immediately upon storage restricted infection to the outer dry scales. Similar symptoms were observed at harvest on the bulbs of other white onions in a cultivar trial located near Quincy, WA, although symptoms were not observed on yellow or red cultivars in the trial. The same symptoms were later observed on approximately 70% of bulbs harvested from a 32-acre fresh-market crop of the cv. Sterling in the Columbia Basin. These symptomatic bulbs were rejected for the fresh market. To our knowledge, this is the first report of infection of onion bulbs by F. proliferatum in Washington, which in 2001, had the third largest acreage of onions in the United States after California and Oregon (USDA National Agricultural Statistics Service). References: (1) D. F. Farr et al. Fungi on Plants and Plant Products in the United States. The American Phytopathological Society, St. Paul, MN, 1989. (2) S. K. Mohan et al. (Abstr.) Phytopathology 87:S67, 1997. (3) P. E. Nelson et al. Fusarium species: An Illustrated Manual for Identification. The Pennsylvania State University Press, University Park, 1983.
