Review of oak mildew, with particular reference to mature and veteran trees in Britain

Abstract

The origins, impact and management implications of powdery mildew of oak in Europe are reviewed in the light of over 100 years of research. The main fungus involved since the 1900s is Erisyphe alphitoides, which evidently originated in tropical Asia, perhaps on mango. It is often accompanied by a very similar species, Erisyphe quercicola, also of tropical or subtropical origin. An apparent increase in the severity of outbreaks of the disease in recent decades could be due to the concomitant occurrence of sexual reproduction in E. alphitoides, which used to be very rare. This may have favoured the evolution of increasingly aggressive genotypes. Also, the sexually produced spores are a significant additional source of host infection. Synergy between E. alphitoides and E. quercicola is another possible factor in the apparent increase in disease severity. Climate change might also have played a part and is likely to do so further. Moreover, mildew impairs the regulation of moisture loss and thus aggravates drought stress in hot summers. Extreme summer heat would, however, inhibit mildew. There is evidence that mildew can cause significant decline in host vitality and vigour, leading to increased susceptibility to other “pests” or pathogens, including those linked with the chronic and acute forms of oak decline. The potential for selecting or breeding oak trees with mildew resistance is discussed in the light of existing variation between and within oak species. Options for chemical, biological and cultural control are also discussed.