Pyrenophora (formerly Drechslera) diseases are fungal diseases that primarily affect turfgrass species. They can cause significant damage to the root system, crowns, rhizomes, stems and leaves of the grass. The diseases are caused by different species of Pyrenophora fungi, which were previously classified under the generic name Drechslera.
Causal agents
Pyrenophora spp. fungi:
- P. poae: Causes leaf spot melting-out in Lolium and Poa spp.
- P. dictyoides: Causes leaf spot and net blotch in Festuca, Lolium, Poa, and Zoysia spp.
- P. lolii: Causes leaf spot and brown blight in Festuca, Lolium, and Poa spp.
- P. grahamii: Causes leaf streak in Agrostis and Lolium spp.
- P. catenaria: Causes leaf blight and crown rot in Agrostis, Festuca, and Lolium spp.
- P. erythrospila: Causes red leaf spot in Agrostis spp.
Primary Hosts
- Lolium and Poa spp. (P. poae)
- Festuca, Lolium, Poa, and Zoysia spp. (P. dictyoides)
- Festuca, Lolium, and Poa spp. (P. lolii)
- Agrostis and Lolium spp. (P. grahamii)
- Agrostis, Festuca, and Lolium spp. (P. catenaria)
- Agrostis spp. (P. erythrospila)
Disease Cycle
Pyrenophora fungi survive unfavorable conditions as conidia and dormant mycelium in infected plant tissues and debris. The conidia can be spread to healthy leaves or leaf sheaths through various means such as wind, rain, irrigation water, equipment, and human or animal activities. Infection occurs when leaf surfaces are moist, and temperatures are suitable for fungal growth. The fungi can also infect lower stems, roots, and rhizomes, leading to various rotting symptoms.
Epidemiology
Pyrenophora diseases exhibit certain epidemiological patterns. They are most prevalent during cool, wet, and overcast periods, particularly in spring and autumn. Red leaf spot caused by Pyrenophora erythrospila, however, can occur in warmer weather. Environmental factors such as high humidity, poor air movement, shading, and the presence of thatch and leaf clippings can contribute to disease development.
Symptoms
Symptoms of Pyrenophora diseases vary depending on the turfgrass species and the specific Pyrenophora species involved. Common symptoms include the appearance of circular to oval spots or lesions on leaves, which may turn necrotic and develop a brown or reddish-brown coloration. Yellowing, dieback from the tips, and thinning of the turf can also occur.
Credit: John Kaminski/Penn State University
Signs
Signs of the diseases include the presence of fungal mycelium, conidiophores, and conidia on the affected plant tissues.
Credit: Pace Turf/Peter Dernoeden University of Maryland
Management
Cultural and chemical control measures are available for managing Pyrenophora diseases. Cultural practices focus on promoting overall turf health and creating unfavorable conditions for disease development. This includes proper fertilization, irrigation, mowing, thatch management, and enhancing light penetration and airflow. On the other hand, chemical control involves the use of fungicides to suppress disease symptoms and prevent spore production. By combining these approaches, turf managers can effectively manage Pyrenophora diseases and maintain healthy and resilient turfgrass.
Cultural
Cultural practices play a crucial role in managing Pyrenophora diseases. These practices include:
- maintaining moderate plant growth through appropriate fertilization
- ensuring proper watering practices to avoid overstimulation or drought stress
- mowing turfgrass at recommended heights to minimize stress and promote healthy growth
- avoiding excessive use of growth-regulating chemicals
- managing thatch accumulation
- promoting adequate light penetration and airflow to prevent high humidity and disease development
Chemical
In the management of leaf spot and melting out in turfgrass, several fungicides are recommended. These include the following:
- propiconazole (Banner MAXX II)
- chlorothalonil (Daconil Ultrex, Manicure, Echo, etc)
- fluazinam (Secure, Rotator, Soteria)
- fluoxastrobin (Fame)
- iprodione (Chipco 26GT, Raven, etc.)
- mancozeb (Fore, Pentathlon, etc.)
- pyraclostrobin (Insignia)
Additionally, there are combination products that can be used effectively. These include azoxystrobin + propiconazole (Headway, Goliath), fluazinam + tebuconazole (Traction), and chlorothalonil + acibenzolar-S-methyl (Daconil Action), among others.
Some fungicides work on only certain phases of this disease or may not be effective on certain species. For instance, red leaf spot (caused by P. erythrospila) can be controlled by azoxystrobin, but may be enhanced by flutolanil. Additionally, DMI fungicides may cause phytotoxicity to select cool and warm-season species when applied during the summer months.