Powdery mildew is a significant disease that affects susceptible cultivars of Poa and Festuca spp. This fungal infection is most commonly observed when these grass species are exposed to prolonged periods of low-intensity light. Shady locations or areas with poor air circulation provide favorable conditions for powdery mildew development.

Causal agents

Blumeria graminis (formerly Erysiphe graminis) fungus

Primary Hosts

Poa, especially Kentucky bluegrass), and Festuca spp.

Disease Cycle

The pathogen responsible for powdery mildew exhibits a survival strategy that involves different structures and stages. During winter, it primarily survives as mycelium within infected plants, allowing it to persist until favorable conditions return. In regions with harsh winters, the pathogen also survives as ascospores within cleistothecia, which are embedded in infected plants or debris. As spring arrives, the pathogen starts producing conidia, which serve as the primary source of inoculum. These airborne conidia are capable of germinating quickly and infecting susceptible hosts. Within a week, new conidia can be generated. As infected leaves mature or prematurely die, cleistothecia begin to develop, and ascospores are typically released after rainfall in the autumn, contributing to the pathogen’s life cycle and potential for disease spread.

Epidemiology

Powdery mildew exhibits specific patterns of epidemiology that contribute to its occurrence and severity. The disease is commonly observed during periods of cool, humid, and cloudy weather, which are prevalent in the spring and autumn seasons. These environmental conditions create a favorable environment for the development and spread of powdery mildew. 

Shaded areas with limited air circulation are particularly susceptible to powdery mildew outbreaks. The reduced air movement in these areas creates a microclimate that is conducive to the growth and spread of the pathogen. Lack of sunlight and stagnant air provide optimal conditions for the powdery mildew fungus to thrive and infect susceptible plants.

One factor that significantly influences the severity of powdery mildew is the nitrogen fertility of the turfgrass. Kentucky bluegrass (Poa pratensis), a commonly affected turfgrass species, is particularly susceptible to powdery mildew when it is heavily fertilized with nitrogen. Excessive nitrogen fertilization promotes lush and succulent growth in the turfgrass, making it more vulnerable to fungal infection. Proper nitrogen management, avoiding excessive nitrogen application, can help minimize the risk and severity of powdery mildew.

Symptoms

Powdery mildew of turfgrass is characterized by distinct symptoms that can be observed on the leaves. One of the prominent signs of powdery mildew is a powdery, white to grayish fungal growth on the surface of the affected leaves. This growth gives the appearance as if the leaves are dusted with lime or flour.

The infection is typically more pronounced on older leaves compared to younger leaves. As the disease progresses, heavily infected leaves start to show signs of yellowing. This yellowing is often followed by the development of tan or brown patches as the infected leaves begin to die.

The affected leaves may exhibit a diffuse, scorched, or ragged appearance due to the presence of necrotic areas interspersed with symptomless areas. These necrotic patches contribute to the overall decline and deterioration of the affected turfgrass.

Credit: Doug Linde/Delaware Valley College

Signs

Signs of powdery mildew include the presence of mycelium and cleistothecia. Powdery mildew initially appears as isolated colonies of fine, whitish mycelium on the leaves and leaf sheaths of turfgrass. These colonies quickly expand and develop into gray-to-white wefts of powdery or dusty mycelium that cover the surface of the leaves. The mycelium gives the affected areas a distinct powdery appearance, resembling a thin layer of dust or powder.

In addition to mycelium, powdery mildew can also produce cleistothecia, which are small, round, closed fruiting bodies. These structures are often formed within the mycelium or on the surface of infected plant tissues. Cleistothecia are typically spherical and range in color from light gray to brown. They can be observed as small, raised structures on the surface of the leaves.

Both the mycelium and cleistothecia of powdery mildew are important diagnostic features that help confirm the presence of the disease. The powdery mycelium and the presence of cleistothecia on the affected leaves are distinctive signs of powdery mildew infestation in turfgrass.

Credit: Doug Linde/Delaware Valley College

Management

Cultural and chemical management strategies are key components in effectively managing powdery mildew in turfgrass. Cultural practices focus on creating an environment that discourages disease development and promotes healthy turf growth. Selecting shade-tolerant cultivars during turfgrass establishment can significantly reduce the susceptibility to powdery mildew. By choosing cultivars that have demonstrated resistance to the disease, turfgrass managers can proactively mitigate its impact.

In severe cases, chemical management may be necessary. Fungicide treatments can be employed to control powdery mildew outbreaks.

Cultural

Cultural control practices play a vital role in managing powdery mildew in turfgrass. When establishing turf, it is recommended to choose shade-tolerant cultivars that have demonstrated resistance to powdery mildew. By selecting these cultivars, turfgrass managers can proactively reduce the susceptibility of the turf to the disease.

To create an environment less favorable for powdery mildew development, increasing sunlight penetration and improving air circulation is crucial. This can be achieved by selectively pruning ornamental shrubs and trees that may create excessive shade over the turfgrass. By allowing more sunlight to reach the turf and promoting better airflow, the conditions become less conducive for powdery mildew growth.

Maintaining a relatively higher mowing height is another cultural practice that can help manage powdery mildew. Additionally, proper irrigation practices should be followed, providing water as needed without excessive watering that can promote lush growth and susceptibility to powdery mildew.

Promoting overall turf health and growth at appropriate times is also important. However, it is essential to avoid excessive lushness, as succulent and actively growing turf is more susceptible to powdery mildew. Balancing nutrient applications and avoiding excessive nitrogen fertilization can help maintain a healthy growth rate without creating conditions favorable for disease development.

By implementing these cultural control options, turfgrass managers can effectively reduce the impact of powdery mildew and promote a healthier and more resilient turfgrass stand.

Chemical

Limited fungicide trials have been conducted on powdery mildew, but select DMI fungicides a highly effective:

• propiconazole (Banner Maxx, Propiconazole Pro, Spectator)
• triademifon (Bayleton)
• Myclobutanil (Eagle, Golden Eagle, ArmorTech Myclo)