Oak powdery mildew – update on what’s all the fuss about?

Elsa Field wrote a very popular Observatree blog in 2017 about powdery mildews revealing an unlikely origin for the most common species of Oak powdery mildew Erysiphe alphitoides.  Therefore, it felt timely to provide an update.  Dr Roger Cook, a global expert on taxonomy of powdery mildews, has been working with colleagues around the world to unpick this complex story. I’m indebted to Roger, my former boss, for sharing his knowledge and helping me provide an update.

Powdery mildews are described as obligate plant pathogens, as they are “obligated” for growth, reproduction and survival to a particular living host.  So although symptoms of the white powdery growth are very similar on a range of plants, they are probably different fungal species. For example, sycamore powdery mildew is restricted to Acer species and cannot infect other broadleaf trees (Figure 1).

Powdery mildews can be very damaging plant diseases and are commonly seen on oak leaves and shoots at this time of year (Figure 2).  On August 1st, we not only celebrate Yorkshire Day, but also Lammas Day, an old Christian festival, to celebrate the start of the wheat harvest.  The term ‘lammas growth’ is used to describe the second flush or summer shoots of trees and in particular oaks in midsummer.  This soft, young growth is particularly susceptible to powdery mildew.

The powdery mildew situation on oaks is very complex, there are several different fungal species that could be present.  For many years, oak powdery mildew in the UK has been attributed to only Erysiphe alphitoides (previously known as Microsphaera alphitoides). However detailed morphological examination including scanning electron microscope, pioneered by Dr Cook, and molecular analysis has demonstrated a different picture.

Work by Pastirčáková et al. (2021) [Erysiphe hypophylla, a second powdery mildew (Erysiphales) on oaks in Britain - ScienceDirect] carried out analysis of samples taken from England (Figure 3). As expected, Erysiphe alphitoides was the dominant species, but surprisingly, there were no records of Erysiphe quercicola.  E. quercicola is common on the Continent and for many years was not distinguished from E. alphitoides. The absence of another mildew (Phyllactinia), only present in southern Europe, was expected.  However, a new mildew Erysiphe hypophylla was present; it was first described in Moscow in 1950 and has been spreading across Europe.  Based on worldwide distributions of these four species, E. alphitoides likes temperate conditions, E. hypophylla likes cool conditions whilst E. quercicola and Phyllactinia like warmth.  So if you are out surveying you are most likely to see E. alphitoides as this produces a conspicuous, white powdery mildew mainly on upper leaf surfaces and leaf distortion (Figure 2).  You will have to look very closely to see the other UK species, E. hypophylla.  As the name would suggest, it only occurs on the lower leaf surface [hypo] where it sporulates very sparsely and inconspicuously and does not cause leaf distortion (Figure 4). If you look even more closely on both fungi, you may see tiny black dots about the size of a pin head (called chasmothecia). With a hand lens, you can observe their intricately branched appendages. These will catch the wind, wafting chasmothecia onto bark and twigs on which they cling. In the spring they burst, releasing spores to re-infect new oak shoots. Given the recent summer heat and ongoing climate change, the distribution in the UK may need to be reconsidered as temperatures rise.  

I am indebted to Dr Roger Cook for his expertise and advice over the past 35 years and in writing this blog and to my former colleague and Observatree volunteer Dr Alan Inman for advice.