- Lead scientist
- Dr Béatrice Henricot
- Partners
- Dr Roger Cook, Plant Pathologist consultant
- Start date
- 2001
- End date
- 2006
- Keywords
powdery mildew, Catalpa, Acanthus, Erysiphe, Neoerysiphe, Oidium
- Benefits to gardeners
The project will improve our means of identification of invasive and important powdery mildews affecting garden plants. It will also improve our knowledge on the host range of powdery mildews which can be used for deterring the pathogen.
- The problem
In Europe, there are five species of powdery mildews recorded on the genus Catalpa. These are Erysiphe elevata, E. catalpae, Oidium hiratae, a Neoerysiphe sp and Leveillula taurica. Of these only E. catalpae and O. hiratae were known in Britain prior to 2002.
E. elevata was previously known only in North America but has recently received much attention following its sudden appearance in Europe in 2002.
E. catalpae was described from a 1957 collection on C. bignonioides in Armenia. In England, a powdery mildew, probably E. catalpae, was seen on C. bignonioides in Essex, 1984, but it was not until 1998 that chasmothecia (sexual structures) were found and therefore the species was confirmed as E. catalpae.
O. hiratae was described from C. bignonioides with an isotype. It was based on a 1978 collection from Royal Botanic Gardens Kew but the species was not identified at the time due to absence of chasmothecia.
A Neoerysiphe sp. was recently reported on Catalpa bignonioides in Switzerland in 2004. It was a unique finding but no detailed description of the anamorph (asexual stage) was given and the species remains unconfirmed.
Leveillula taurica is a polyphagous, largely Mediterranean species with very few vouchered British records, none from Catalpa.
- Approach
Three out of the five known powdery mildews in Europe were identified during the survey by morphological characteristics and DNA analysis. These were E. elevata, E. catalpae and Neoerysiphe galeopsidis.
The powdery mildew species fall into two anamorph types. The first one includes all the species producing conidia that mature singly and belong to the anamorph Oidium subgen. Pseudoidium. In this group belongs E. elevata and E. catalpae. The second anamorph type (euoidium types) to which N. galeopsidis and Oidium hiratae belong includes species with catenate conidia that mature in a chain.
The Oidium subgen. Pseudoidium species
- Erysiphe elevata was the predominant species in the South of England, found on C. bignonioides and its hybrids C. x erubescens and x Chitalpa tashkentensis. It was a new record for the UK. One hybrid of C. bignonioides, x Chitalpa tashkentensis ‘Pink Dawn’ was a new host record for this mildew.
- Erysiphe catalpae found only at Harlow Carr on a single young tree of C. bignonioides ‘Aurea’.
The catenate Oidium species
- Neoerysiphe galeopsidis. This is the first powdery mildew observed in the year during the survey (in June) and also a new record of this species on Catalpa. It was found on C. bignonioides, its hybrids C. x erubescens and C. speciosa and by the end of the growing season it was over run by E. elevata.
- O. hiratae. This species was not found during the survey.
No powdery mildew was found on three species of Chinese origin: C. bungei, C. fargesii or C. ovata.
The molecular results and the morphological characteristics of the Neoerysiphe sp. isolated from Catalpa confirmed for the first time its identity as Neoerysiphe galeopsidis. N. galeopsidis is now the first species of the genus proved to affect members of more than one family. Indeed, the genus Neoerysiphe is composed of five species recorded on approx. 300 herbaceous hosts. N. galii affects only Rubiaceae, N. cumminsiana only Asteraceae, N. chelones only Chelone glabra (Scrophulariaceae) and N. geranii only Geraniaceae. Until recently the host range of N. galeopsidis was thought to be restricted to Lamiaceae. This is the first positive record of this species on a host belonging to the Bignoniaceae, a family in the same order (Lamiales), as Lamiaceae. Now, its host range is becoming much wider. There is also a recent first report of it affecting Althaea rosea in the family Malvaceae in China and more recently we confirmed the species on Acanthus (Acanthaceae).
The DNA analysis showed ITS sequences were identical in specimens of E. elevata from England, France, Hungary and USA, reinforcing the suggestion that E. elevata is a recent introduction to Europe. Its preference for American rather than Chinese catalpas suggests that it originated in the USA. Compared to some other recently introduced powdery mildews, e.g., E. azaleae and E. symphoricarpi, E. elevata has spread rapidly across Europe. E. elevata appears to be more aggressive than N. galeopsidis as it over-ran infections initially caused by that species.
- Further information
Read more on powdery mildews
To read the studies on the pathogenicity of Neoerysiphe galeopsidis on Catalpa, Acanthus and Lamium
To read the new disease reports
- References
Cook R T A, Henricot B, Henrici A and Beales P (2006). Morphological and phylogenetic comparisons amongst powdery mildews on Catalpa in UK. Mycological Research 110, 672-685.
Cook R T A, Henricot B and Beales P (2006). First record of Neoerysiphe galeopsidis on Acanthus spinosis in the UK. Plant Pathology 55, 575
For reports and more infomation
Cook R T A, Henricot B and Kiss L (2004). First record of Erysiphe elevata on Catalpa bignonioides in the UK. Plant Pathology 53, 807.
For more information