Peronospora belbahrii causing downy mildew disease on Agastache in the UK: a new host and location for the pathogen

Abstract

The Food andEnvironment Research Agency, Sand Hutton, York, YO41 1LZ, UKAgastache is a genus of 30 species of aromatic perennials in the familyLamiaceae. In June and July 2009, cultivars of Agastache sp. ‘TangerineDreams’, A. mexicana ‘Red Fortune’, A. aurantiaca ‘Apricot Sprite’ andA. cana ‘Purple Pygmy’ at Wisley gardens were infected by downy mil-dew.Thesymptomswerechloroticleafspotseventuallyturningbrown.Adowny mildew-like growth was observable underneath the leaf surfaces.Microscopic examination revealed conidiophores typical of Peronos-pora.Conidiophoreswerestraight,monopodially branched (typically 4–6 times), 137–325 lm in length. Ultimate branchlets were sinuous andobtuse, the longer measuring 12–20 lm, the shorter 5–8 lm. Conidiawere olive brown, broadly ellipsoidal to subglobose and measured 22–30 · 15–25 lm.Nooosporeswerefound.TheITSofisolatesfoundoncultivarsofA.mexicana‘RedFortune’andAgastachesp.‘TangerineDreams’(GenBankAccessionNos.GQ390794and GQ390795) showed 99% homology to a new Peronospora speciesfound on coleus and basil recently named as Peronospora belbahrii(Thines et al., 2009). Specimens were deposited at Kew (K(M)163651andK(M)163652).Tosupportthemolecularidentification,conidiawererubbed from infected Agastache leaves onto leaves of four differenthealthy coleus cultivars (‘Winsome’, ‘Mrs Pilkington’, ‘Roy Pedley and‘DurhamGala’).Afterinoculation, aplastic bagwasplaced onplantsfor48 hrs. The plants were misted with water twice a day and left outdoorswhere the temperature ranged from 14 Cto27 C. Controls were notinoculated. After 14 days, the same downy mildew was found on cvs.‘Winsome’and‘DurhamGala’whilsttheotherplantsremainedhealthy.This is the first record of P. belbahrii on Agastache and in the UK. Theonly other downy mildew species recorded on Agastache is P. lophantibut this species has been found to be conspecific with P. lamii (Shin &Choi, 2006). Peronospora belbahrii is distinct from P. lamii (Belbahriet al.,2005). It was firstreported from Africain1933,then much later in1993 in the USA and in 1999 in Europe where it is causing epidemics(Thines et al., 2009).The fungus poses serious threats to production ofcoleus and other Lamiacae grown commercially. Statutory action hasnowbeentakenagainstthispathogen.References