Sawadaea bomiensis

Sawadaea bomiensis is a highly specialized fungal pathogen belonging to the order Erysiphales, responsible for causing powdery mildew on various species of maple trees. As an obligate parasite, this fungus derives its nutrition exclusively from the living host plant by penetrating epidermal cells through specialized structures known as haustoria, which anchor the fungus to the plant tissue.

The primary hosts for this pathogen are members of the genus Acer (maple). The disease is commonly observed on Norway maple, field maple, and several other ornamental maple species. It frequently affects trees located in natural forests, as well as those in urban landscapes, private gardens, and commercial nurseries, where it can cause significant aesthetic damage.

The initial symptoms appear as a distinctive white or grayish, dusty coating on the surface of the leaves, consisting of mycelium, conidiophores, and conidia. As the infection progresses, this coating becomes denser and more persistent. By the end of the growing season, small, dark, spherical structures called cleistothecia develop within the mycelial mat, serving as the overwintering stage of the fungus.

The development and rapid dissemination of Sawadaea bomiensis are favored by moderately warm and humid conditions, which are characteristic of mid-to-late summer. Spores are easily dispersed by wind currents, enabling the disease to spread rapidly through tree stands. Poor air circulation within dense canopies and high plant density create a microclimate that is ideal for spore germination and colony expansion.

The impact of this disease is primarily associated with the disruption of photosynthesis, leading to leaf chlorosis, premature senescence, and early leaf drop. Chronic infection weakens the trees, making them more susceptible to environmental stressors and reducing their overall vigor. Recommended management practices include:

• Sanitation, including the collection and destruction of fallen leaves to remove overwintering inoculum.
• Pruning of dense canopies to improve air circulation and reduce humidity levels.
• Application of systemic fungicides, such as triazoles or strobilurins, during the early onset of symptoms.
• Use of biological control agents, such as Bacillus subtilis, to mitigate the spread of the pathogen.