Horse chestnut powdery mildew

Horse chestnut powdery mildew is caused by the microscopic fungus Uncinuliella flexuosa, belonging to the kingdom Fungi, phylum Ascomycota. This is a highly specialized phytopathogen that predominantly affects species of the Aesculus genus.

The fungus lives as a parasite, developing on the surface of leaves as a characteristic white powdery coating. Unlike other powdery mildew species, this pathogen possesses specific appendages on its cleistothecia.

The fungus's mycelium is ectoparasitic: it resides on the leaf epidermis, penetrating host cells only with specialized structures called haustoria. This allows the fungus to extract nutrients directly from the plant.

The pathogen's life cycle includes both asexual reproduction (conidial stage) and sexual reproduction (formation of cleistothecia). The fungus overwinters as mycelium in buds or on fallen leaves as fruiting bodies.

The pathogen spreads rapidly under conditions of high humidity and moderate temperatures. It was first recorded in Europe in the late 20th century, becoming a serious threat to urban park landscapes.

The primary host is the common horse chestnut (Aesculus hippocastanum). The infection can strike trees of all ages, from young saplings to mature, century-old specimens.

Damage manifests as premature yellowing and withering of the leaf blade. Affected leaves lose their photosynthetic capacity, which significantly weakens the tree before the winter season.

Severe infection leads to premature leaf fall as early as mid-summer. This disrupts physiological processes and may trigger secondary bud burst in autumn, which is lethal for the plant.

The reduction in the tree's immunity due to the fungus makes it vulnerable to other pests, such as the horse-chestnut leaf miner. Collectively, these factors can lead to the complete death of the plant.

The aesthetic value of park areas drops sharply. Infected trees appear stunted, their crowns become sparse, which negatively impacts the landscape design and environmental quality.

Initial signs of infection appear in the second half of spring when stable warm weather settles in. During this period, the germination of spores that overwintered in the buds begins.

The peak of disease development occurs in the second half of June and July. At this time, air humidity is often high, and night temperatures remain warm enough for active mycelium growth.

Conidia are dispersed by wind, raindrops, and insects. The infection spreads rapidly, colonizing new leaves and shoots of the current year.

By August, tiny dark spots appear on the leaves — these are cleistothecia. This represents the sexual reproductive stage, which prepares the fungus to survive the unfavorable winter period.

The development process of the pathogen ceases only with the arrival of deep autumn frosts. Fallen leaves contaminated with cleistothecia serve as the main source of infection for the following year.

The first and most noticeable sign is the appearance of a powdery white coating on the upper, and sometimes lower, side of the leaves. Initially, these are individual white spots that quickly coalesce.

Over time, the white coating becomes denser and takes on a grayish or brownish hue. Leaves begin to deform, curl, and lose their natural green color.

As the disease progresses, tiny black dots — the fruiting bodies of the fungus (cleistothecia) — become visible on the coating. This indicates the transition of the fungus to the final stage of its life cycle.

Infected parts of the leaf die off, turning brown, creating an "all-burnt" effect. Leaf margins often dry out and become brittle.

A coating may also be observed on young, incompletely formed shoots. They stop growing, become twisted, and often die during the winter due to poor tissue preparation for freezing temperatures.

The most important agronomic measure is the thorough collection and destruction of fallen leaves in autumn. This is where the primary reservoir of infection is concentrated, capable of reinfecting the tree in spring.

To protect plantings, fungicides with systemic and contact action are applied. Sulfur-based preparations, as well as modern triazoles and strobilurins, are highly effective.

The first application should be performed as soon as the first signs of disease are observed. Subsequent treatments should follow at 10–14 day intervals, depending on the intensity of the infection and weather conditions.

Maintaining a balanced mineral nutrition is recommended. Excessive nitrogen fertilizer promotes rapid growth of young tissues, which are most susceptible to infection, so it is important to supplement with potassium and phosphorus.

In urban landscaping, it is crucial to support the overall vitality of the trees: timely watering, soil aeration, and sanitary pruning of dry branches help the plant better resist the pathogen.