Uncinula macrospora

As an obligate biotroph, this pathogen relies exclusively on living host tissues to derive nutrients, forming specialized haustoria for absorption.

The fungus is characterized by the production of chasmothecia (formerly cleistothecia) equipped with distinctive, hooked appendages which allow them to attach to host surfaces.

Its mycelium typically spreads across the leaf surface, creating a delicate, web-like white growth that later develops into a denser powdery mass.

The species is primarily associated with the genus Ulmus (elms), acting as a significant phytopathogen within these forest and ornamental populations.

The primary hosts of Uncinula macrospora are various species of elms, particularly those susceptible to powdery mildew infections in North American climates.

The infection attacks the foliage and tender new shoots, significantly reducing the surface area available for photosynthesis and carbon assimilation.

Symptoms include curling, stunted growth, and premature chlorosis, which compromises the overall vitality and aesthetic appeal of the tree.

Chronic infestation weakens the plant's defense systems, making it more susceptible to secondary stressors such as drought, frost, or wood-boring insects.

In nursery settings, heavy infestation can cause severe stunted growth and complete failure of saplings to establish properly in the field.

The disease cycle begins in late spring, triggered by rising temperatures and humidity levels that favor spore germination.

Optimal conditions for pathogen development typically occur at temperatures between 18°C and 25°C, with intermittent periods of high humidity.

The secondary spread of the disease occurs throughout the summer as the fungus produces abundant conidia, which are easily dispersed by wind currents.

The fungus overwinters as chasmothecia, which are hardy fruiting bodies that survive on fallen leaves and debris throughout the winter months.

As temperatures increase in the following spring, these overwintering structures release ascospores, initiating the infection cycle anew.

Early infections appear as small, dusty white spots on either the adaxial or abaxial surface of the leaf, often initially going unnoticed.

As the colony expands, the mycelium covers larger areas, turning the leaf surface a distinct, chalky white color that is symptomatic of powdery mildew.

• Development of small, dark fruiting bodies (chasmothecia) by late summer.
• Leaf distortion, including curling and cupping of the foliage.
• Premature defoliation resulting in sparse canopy density.

Severe infestations cause the affected leaves to develop brown necrotic patches, leading to premature drying and leaf death.

Young twigs may show signs of abnormal growth or curvature, indicating systemic infection of the developing vegetative tissues.

Sanitation is the cornerstone of management; raking and removing fallen leaves in the autumn reduces the primary inoculum source for the following spring.

Promoting air circulation through strategic pruning of the tree canopy helps reduce ambient humidity, making the microclimate less favorable for fungal growth.

Fungicidal applications, particularly those containing systemic triazoles or strobilurins, can be effective if applied during the early stages of the disease.

Avoiding overhead irrigation or watering during the mid-morning helps minimize the humidity levels around the tree foliage.

Balanced fertilization programs that avoid excess nitrogen are recommended, as high nitrogen levels often stimulate excessive tender growth susceptible to infection.