Oak powdery mildew – what’s all the fuss?’, Elsa Field, Oxford University

It’s one of the most common pathogens on oaks in Europe and, in late summer (July-August), it’s hard to miss oak powdery mildew on the leaves of oak trees throughout the UK. You’ll notice a whitish “powder” coating the leaves, in some cases causing them to become highly distorted. This is the fungal mycelium, the asexual part of the fungus, which grows on living leaf tissue and hungrily taps into the sugars that the plant has busily captured during photosynthesis.

There is no cure for oak powdery mildew, but nurseries routinely have to spray fungicides to prevent the pathogen from stunting the growth of young trees. This tree disease is a biotrophic pathogen meaning it can only grow on living tissue so doesn’t kill the host directly. However, it will reduce its growth by coating a large proportion of the tree’s powerhouses – its leaves – with mycelia, thus preventing them from photosynthesising. Evidence from Europe suggests that, in combination with other stressors including insect pests, oak powdery mildew can have a significant impact on tree growth.  In some cases, leading to the death of the tree.

Oak powdery mildews have been observed in the UK since at least the 19th century and it’s plausible that at least one species of the pathogen is native to these shores. However, scientists recently revealed an unlikely origin for the most common species of oak powdery mildew, Erysiphe alphitoides – found to be exactly genetically identical to Oidium mangiferae, a powdery mildew on mango.  This suggests an origin for the species in tropical Asia and that somewhere along the line the species “jumped” to grow on oak, as well as mango! Oak powdery mildew may therefore be an early example of a plant disease brought in by international trade.  Something which is unfortunately, increasingly common.

In the future, as the impacts of climate change are increasingly felt, it’s hard to predict whether oak powdery mildew infections will become more or less severe. Because the fungus is dependent on vigorous, healthy trees for its growth and survival, if trees become more drought stressed under conditions of climate change this may lead to less powdery mildew growth. On the other hand, previous epidemics of powdery mildew have been associated with milder winter temperatures, although why this is remains unclear. Indeed, anecdotal observations from British foresters suggest that oak powdery mildew is becoming more severe, the causes of which could include climatic factors.

In combination with other emerging plant health problems, including acute oak decline (a mysterious syndrome characterised by stem bleeds and beetle attacks) oaks are increasingly under threat.

My work at Oxford University, in collaboration with Forest Research and the Centre for Ecology and Hydrology, aims to increase our understanding of the exact climatic conditions that favour oak powdery mildew development. We are testing oak trees grown from seed, sourced from different climatic zones, to see if they are more or less susceptible to disease. The ultimate aim is to be able to provide planting recommendations to woodland managers that can help them improve the resilience of woodlands to disease as the climate changes.

 

 

14 Comments
  1. I noticed this about 12 years ago on young oaks in Kilcooley Wood, Bangor Co. Down. I am pleased to report that it has not affected the growth of the trees as they are all now about 20ft tall. I did notice that it tended to disappear after the trees got a good soaking during a wet summer that caused them to grow more than in previous years.

    • Thanks for your observations, Peter!

      Rain tends to wash away mildew on leaves, so this would explain why it tended to disappear following a wet summer.

      Elsa

  2. if as you say the 2 types of Mildew are “found to be exactly genetically identical” why have they different Latin names or is that a work in progress?

  3. Hi Elsa. Thanks for sharing this awesome post. It will be really helpful. great post.

  4. Very interesting read thank you. It’s always good to stay well read on new and increasing diseases.

  5. The best idea to remove the tree successfully by using the different kind of tree cutting machines ,which help you to remove the tree easily in a short time without any damage of a garden ,you must give a water around a tree to soft a place for a cutting of tree easily and also discuss the idea of cutting a tree with the professional tree removal due to the change in atmosphere or Renovation Issues ,
    Thanks

  6. Notice that this disease doesn’t seem to affect Turkey Oak (or at least not so much) which might be one reason why it is spreading….

  7. Hi Elsa

    We have 3 specimen Oaks planted 16 years ago in a grass field . They are magnificent trees now perhaps 25 feet high with a fine shape ; I noticed the presence of mildew this summer. I wonder if this might affect their growth and health ?

    Thank you ; you are engaged in a fascinating area of research

    • Hi Simon,

      Glad to hear you have such wonderful oak trees! Unfortunately, most studies of mildew have been done on very young trees, so the answer is – we don’t really know! Powdery mildew is not thought to have such a strong impact on the growth of more mature oaks acting on its own, but this will of course vary with the level of infection and with what other stressors the trees are exposed to (e.g. drought, insect herbivory). Can I ask whereabouts in the country you are, and do you know what species of oak they are?

      Best wishes,

      Elsa

  8. Hi Elsa,
    I plant acorns each year in pots to eventually give away to the National Forest but this year my 1st and 2nd year trees have powdery mildew on them and even after washing the leaves with water it remains. Will the mildew come back next year even if I clear all the fallen leaves away and possibly change the soil in their pots?
    The pots are now separated by 30cm from each other; is this enough and should they be in the open rather than in a secluded yard area by the house?
    Regards,
    Kevin

    • Hi Kevin,

      I am happy to hear you are growing trees! Your actions may indeed reduce next year’s infection (the fungus overwinters in leaf litter) but mildew is wind dispersed and very common so it is unlikely that your trees will be free from infection, especially as they are kept close together. Moving them to a more open space where wind currents can disperse more freely may also help to reduce the build up of inoculum around individual trees.

      For more tips on how to reduce mildew on very young trees I would try contacting your nearest nursery – they will be very familiar with dealing with mildew!

      Hope this helps,

      Regards,

      Elsa

  9. Hi Elsa. Is it the same sp.of powdery mildew that effects Dogwood? One of the woods I manage has been severely infected but the oaks don’t look too bad. Wet and warm, good blight weather! Cheers. Steve

    • Hi Steve,
      I don’t know much about Dogwood powdery mildew but it is definitely not the same species, although dogwood is affected by at least one Erysiphe species (same genus as oak powdery mildew!).
      Elsa

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