Powdery Mildew

Erysiphe spp., Podosphaera spp., Leveillula spp., Oidium spp., Sphaerotheca spp. and Mycosphaerella spp.

1. Identification

Powdery mildew is a fungal disease common to numerous forest, ornamental and agricultural species, characterised by the formation of a powdery greyish‑white coating on leaves, shoots and, at times, fruits. The disease is caused by a group of obligately parasitic ascomycete fungi belonging to various genera, including Erysiphe, Podosphaera, Leveillula, Oidium, Sphaerotheca and Mycosphaerella. These fungi develop superficial mycelium and conidial structures that give them the typical “white powder” appearance.

2. Causal agent

Ascomycete fungi belonging to various genera of the family Erysiphaceae, notably Erysiphe spp. (originally described by Hedwig in 1789 and taxonomically reorganised by various authors throughout the 20th century). These fungi are obligate ectoparasites, forming superficial mycelium and haustoria that penetrate the epidermal cells to absorb nutrients.

• General characteristics: these include abundant production of conidia in chains, superficial mycelium visible to the naked eye, intracellular haustoria, a high capacity for aerial dissemination and a preference for conditions of moderate relative humidity and mild temperatures.
• Most frequently cited species: Erysiphe alphitoides (common on oaks), Erysiphe necator (on grapevine), Podosphaera leucotricha (on pome fruits), Leveillula taurica (on solanaceous plants), among others. Among European forest species, Erysiphe alphitoides is the most relevant.

3. Main hosts

• Pedunculate oak (Quercus robur)
• Sessile oak (Quercus petraea)
• Cork oak (Quercus suber) and holm oak (Quercus ilex) – variable susceptibility
• Sweet chestnut (Castanea sativa)
• Various ornamental and shrub species
• Young plants and tender shoots are particularly susceptible

4. Symptoms

• Leaves: presence of a powdery greyish‑white coating on the surface, deformations, marginal curling, chloroses and localised necroses. In severe infections, premature leaf drop occurs.
• Shoots: reduced growth, short internodes, deformations and, in serious cases, death of the young shoots.
• Fruits and reproductive structures: may show whitish spots, deformations and reduced quality.
• Other signs: presence of cleistothecia (dark globose structures) at the end of the season, containing ascospores responsible for overwintering survival.

5. Disease cycle

• The fungi survive as cleistothecia on fallen leaves or as mycelium in dormant buds.
• In spring, they release conidia or ascospores that infect young tissues.
• The disease progresses rapidly under conditions of moderate humidity and temperatures between 15–25 °C.
• Dissemination occurs mainly by wind, allowing rapid colonisation of new hosts.
• At the end of the season, cleistothecia form that ensure survival until the following spring.

6. Favourable conditions

• Mild temperatures (15–25 °C)
• Moderate relative humidity (40–70%), with no need for free water
• Partial shading and poor air circulation
• High plant density or closed canopies
• Young shoots and tender tissues are more susceptible

7. Disease management

• Cultural: these include the removal of infected leaves, improving air circulation through thinning or pruning, reducing excessive shading, and promoting balanced plant vigour to reduce susceptibility.
• Silvicultural: in forest stands, managing density and selecting less susceptible plant material can reduce incidence. Removing heavily infected shoots can be useful in nurseries.
• Prevention: this involves avoiding sprinkler irrigation over the canopy, reducing water and nutritional stress, and monitoring young shoots during spring.

Bibliographic references

• EPPO (European and Mediterranean Plant Protection Organization). PM 7/100 Erysiphe alphitoides. EPPO Diagnostic Protocol.
• European Forest Institute (EFI). Technical reports on powdery mildew in Quercus species.
• Fonseca, F., & Rodrigues, J. (2018). Powdery Mildews on European Forest Trees. European Journal of Plant Pathology.
• Braun, U., & Cook, R. T. A. (2012). Taxonomic Manual of the Erysiphales (Powdery Mildews). CBS Biodiversity Series.
• Jankovský, L., & Palovčíková, D. (2013). Erysiphe alphitoides and Powdery Mildew of Oaks in Europe. Forest Pathology.