First Report of Black Spot Caused by Boeremia exigua var. exigua on Field Pea in Australia

Abstract

Black spot is a major disease of field pea (Pisum sativum L.) production across southern Australia. Known causal agents in Australia include one or more of Mycosphaerella pinodes (Berk. & Bloxam) Vestergr., Phoma medicaginis var. pinodella (L.K. Jones), Ascochyta pisi Lib., or P. koolunga (Davidson, Hartley, Priest, Krysinska-Kaczmarek, Herdina, McKay & Scott) (2), but other pathogens may also be associated with black spot symptoms. Black spot generally occurs on most plants and in most pea fields in Western Australia (W.A.), and during earlier winter/spring surveys of blackspot pathogens, some isolates were tentatively allocated to P. medicaginis var. pinodella despite different cultural characteristics on potato dextrose agar (PDA). Recently, single-spore isolations of a single culture each from an infested pea crop at Medina, Moora, and Mt. Barker in W.A. were made onto PDA. A PCR-based assay with TW81 and AB28 primers was used to amplify from the ITS-5.8S rDNA region. Purified DNA products were sequenced for the three isolates and then BLASTn was used to compare sequences with those in GenBank. Our sequences (GenBank Accession Nos. JN37743, JN377439, and JN377438) had 100% nucleotide identity with P. exigua Desm. var. exigua accessions (GI13385450, GI169894028, and GI189163921), an earlier synonym of what is now known as Boeremia exigua var. exigua ([Desm.] Aveskamp, Gruyter & Verkley) (1). Davidson et al. (2) used the same primers to identify P. koolunga, but none of our isolates were P. koolunga. A suspension of 107 conidia ml–1 of each representative isolate was inoculated onto foliage of 15-day-old field pea cv. Dundale plants and maintained at >90% relative humidity for 72 h postinoculation. Control plants inoculated with just water remained symptomless. Brown lesions were evident by 8 to 10 days postinoculation and mostly 1 to 3 mm in diameter. B. exigua var. exigua was readily reisolated from infected leaves. Isolates have been lodged in the W.A. Culture Collection Herbarium maintained at the Department of Agriculture and Food W.A. (Accession Nos. WAC13500, WAC13502, and WAC13501 from Medina, Moora, and Mt. Barker, respectively). Outside Australia, its synonym P. exigua var. exigua is a known pathogen of field pea (4), other legumes including common bean (Phaseolus vulgaris L.) (4) and soybean (Glycine max [L.] Merr.) (3), and is known to produce phytotoxic cytochalasins. In eastern Australia, P. exigua var. exigua has been reported on common bean (1930s and 1950s), phasey bean (Macroptilium lathyroides [L.] Urb.) and siratro (M. atropurpureum (DC.) Urb.) (1950s and 1960s), mung bean (Vigna radiata [L.] Wilczek.) (1960s), ramie (Boehmeria nivea [L.] Gaudich.) (1939), potato (Solanum tuberosum L.) (1980s), and pyrethrum (Tanacetum cinerariifolium [Trevir.] Schultz Bip.) (2004 and 2007) (Australian Plant Pest Database). To our knowledge, this the first report of B. exigua var. exigua on field pea in Australia, and because of its potential to be a significant pathogen on field pea, warrants further evaluation. References: (1) M. M. Aveskamp et al. Stud. Mycol. 65:1, 2010. (2) J. A. Davidson et al. Mycologia 101:120, 2009. (3) L. Irinyi et al. Mycol. Res. 113:249, 2009. (4) J. Marcinkowska. Biul. Inst. Hod. Aklim. Rosl. 190:169, 1994.