Sawadaea bomiensis

The first sign of the disease is the appearance of a characteristic white, powdery coating on the upper or lower surfaces of maple leaves. This coating consists of fungal mycelium and a mass of pathogen spores.

Over time, these white patches may merge, covering a significant area of the leaf blade, giving the tree a sickly, flour-dusted appearance. Affected leaves often deform, curl, and turn yellow prematurely.

During the later stages of disease development, especially towards the end of summer or early autumn, small black dots can be seen within the white coating. These are the fruiting bodies of the fungus (cleistothecia), indicating the pathogen's transition to a dormant state.

Unlike some other species, Sawadaea bomiensis can cause localized necrosis of leaf tissues under the mycelial patches. This leads to the appearance of brown spots that eventually dry out, causing the leaf to drop prematurely.

• Appearance of a white powdery coating on leaves.
• Deformation and curling of leaf blades.
• Formation of black cleistothecia within the coating.
• Premature yellowing and leaf drop.
• Localized tissue necrosis.

Sawadaea bomiensis is a specialized ascomycete fungus belonging to the order Erysiphales. This pathogen is the causative agent of a specific form of powdery mildew that primarily affects tree species of the genus Acer (Maple).

In the fungus's life cycle, the formation of cleistothecia plays a critical role, allowing the pathogen to successfully overwinter on fallen leaves and bark of affected trees. In spring, under favorable conditions, ascospores are released, serving as the primary source of infection for young leaf tissues.

The fungus develops primarily as an epiphyte, penetrating only the epidermal cells of the leaf with its haustoria. This process disrupts normal photosynthesis and weakens the overall immune system of the host tree.

The morphology of Sawadaea bomiensis is distinguished by the specific structure of its cleistothecia appendages. This characteristic allows mycologists to precisely identify the species through microscopic analysis, distinguishing it from other powdery mildew species that occur on deciduous trees.

The pathogen spreads mainly through airborne conidia throughout the growing season, which ensures a high rate of crown colonization in conditions of high ambient humidity.

Moderate temperatures combined with high relative humidity are optimal for the infection to develop. The ideal temperature range for spore germination is +18...+25°C.

Dense maple stands where natural air circulation is restricted create a localized microclimate with high humidity, which is critical for spore germination and the development of the fungus mycelium.

Trees that are already stressed due to nutrient deficiencies or chronic water stress are highly vulnerable to infection by this fungal pathogen.

Periods of frequent dew and fog in the evening or morning hours facilitate the rapid spread of secondary conidia throughout the canopy, potentially leading to severe outbreaks within parks or nurseries.

A high volume of overwintering inoculum on fallen leaves from the previous year provides a base for primary infection of young shoots as soon as spring temperatures rise.

Sawadaea bomiensis significantly reduces the aesthetic value of park and garden maples, which is a major concern in landscaping and urban forestry management.

Intensive leaf infection disrupts the photosynthetic process, causing the tree to receive fewer essential nutrients, which consequently leads to stunted annual shoot growth.

Severe infection triggers premature leaf drop, causing the tree to enter the winter season in an unprepared state. This drastically lowers its winter hardiness and resilience against severe frosts.

If the infection cycle repeats annually, trees become progressively exhausted and more susceptible to secondary pests, such as bark beetles or wood-decay fungi that exploit weakened bark.

In nursery settings, the disease causes direct economic loss, as affected seedlings lose their commercial appeal and may die during transplanting due to a lack of stored nutrients in their tissues.

The primary control method involves thorough raking and disposal (by burning or deep composting) of all fallen leaves, where the fungus's fruiting bodies overwinter.

To prevent the spread of infection, it is recommended to carry out timely sanitary and thinning pruning of the canopy. This improves aeration and light penetration, creating unfavorable conditions for powdery mildew development.

Once the economic threshold of damage is reached, systemic fungicides from the triazole or strobilurin groups should be applied, as they effectively suppress mycelial growth on the leaf surface.

Maintaining high levels of plant care is essential, including proper irrigation and balanced fertilization (avoiding excessive nitrogen, which makes leaf tissues softer and more susceptible to pathogens).

Upon detection of initial symptoms, applications of sulfur-based contact fungicides are necessary. These effectively block spore germination and slow down the further spread of the pathogen throughout the tree canopy.