Cherry powdery mildew

The fungus Podosphaera clandestina is a significant plant pathogen responsible for causing powdery mildew on stone fruits. As an obligate parasite, it relies exclusively on living host tissue to complete its life cycle. It belongs to the Erysiphaceae family and is recognized by the distinct white, powder-like mycelium that covers infected plant surfaces.

The primary hosts of this pathogen are sweet cherry (Prunus avium) and sour cherry (Prunus cerasus). In nursery settings, young saplings are particularly susceptible to severe outbreaks, which can stunt development and affect the structural integrity of the trees. The fungus predominantly targets young leaves, tender shoots, and occasionally fruits.

Symptoms typically begin with small, whitish, circular spots of mycelium, which eventually coalesce to cover entire leaf surfaces. As the infection progresses, the leaves become distorted, brittle, and may show signs of chlorosis. By late summer, small dark fruiting bodies known as cleistothecia appear on the mycelial mat, serving as the overwintering stage of the pathogen.

Environmental conditions play a critical role in the severity of Podosphaera clandestina. Warm temperatures ranging from 20°C to 25°C, combined with high relative humidity, create an ideal environment for spore germination and colonization. Poor orchard airflow, excessive density, and over-application of nitrogenous fertilizers further exacerbate the disease pressure.

The economic impact of the disease includes reduced photosynthetic capability, which limits the tree's ability to store carbohydrates, leads to winter injury, and results in premature fruit drop. To manage and prevent the spread of the disease, growers should focus on the following strategies:

• Proper pruning techniques to increase canopy airflow and light penetration.
• Sanitation, which includes removing and destroying infected shoots and fallen leaves.
• Balanced fertilization practices to avoid excessive vegetative growth.
• Timely application of chemical or biological fungicides during high-risk growth periods.