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

Agastacheis a genus of 30 species of aromatic perennials in the family Lamiaceae. In June and July 2009, plants of this genus (Agastache sp. cv.‘Tangerine Dreams’, A. mexicana cv. ‘Red Fortune’, A. aurantiaca cv. ‘Apricot Sprite’, A. cana cv. ‘Purple Pygmy’) at Wisley gardens (Surrey) were infected by downy mildew. The symptoms were chlorotic leaf spots eventually turning brown (Fig. 1a). A downy mildew-like growth was observable underneath the leaf surfaces (Fig. 1b). Microscopic examination revealed conidiophores typical of Peronospora. Conidiophores were straight, monopodially branched (typically 4-6 times), 137-325 μm in length. Ultimate branchlets were sinuous and obtuse, the longer measuring 12-20 μm, the shorter 5-8 μm. Conidia were olive brown, broadly ellipsoidal to subglobose and measured 22-30 x 15-25 μm (Fig. 2). No oospores were found.

The ITS of isolates found on cultivars of A. mexicana (‘Red Fortune’) and Agastache sp. (‘Tangerine Dreams’) (GenBank Accession Nos. GQ390794 and GQ390795) showed 99% homology to a new Peronospora species found on coleus and basil recently named as Peronospora belbahrii (Thines et al., 2009). Specimens were deposited at Kew (K(M)163651 and K(M)163652). To support the molecular identification, conidia were rubbed from infected Agastache leaves onto leaves of four different healthy coleus cultivars (‘Winsome’, ‘Mrs Pilkington’, ‘Roy Pedley and ‘Durham Gala’). After inoculation, a plastic bag was placed on plants for 48 hrs. The plants were misted with water twice a day and left outdoors where the temperature ranged from 14^°C to 27^°C. Controls were not inoculated. After 14 days, the same downy mildew was found on cvs. ‘Winsome’ and ‘Durham Gala’ whilst the other plants remained healthy.

To our knowledge, this is the first record of P. belbahriion Agastache and in the UK. The only other downy mildew species recorded on Agastache is P. lophanti but this species has been found to be conspecific with P. lamii (Shin and Choi, 2006). Peronospora belbahrii is distinct from P. lamii (Belbahri et al., 2005). It was first reported from Africa in 1933, then much later in 1993 in the USA and in 1999 in Europe where it is causing epidemics (Thines et al., 2009).The fungus poses serious threats to production of coleus and other Lamiacae grown commercially. Statutory action has now been taken against this pathogen.