Graminicolous fungi, including Bipolaris, Curvularia, and Exserohilum, are economically significant plant pathogens, causing diseases in cool- and warm-season turfgrasses. With shared asexual life cycles and the ability to infect all plant parts, these fungi pose a threat to leaves, stems, crowns, and roots.

Causal agents

Bipolaris, Curvularia, and Exserohilum spp. often coexist as disease complexes.

Primary Hosts

In terms of host specificity, Bipolaris and Exserohilum spp. primarily infect warm-season grasses. However, there are exceptions such as B. sorokiniana and B. gigantea, which can also infect a wide range of cool-season and warm-season grasses. On the other hand, Curvularia spp. have a broader host range and can infect both cool-season and warm-season grasses. In most cases, the presence of Curvularia is not indicative of disease as this species is often an opportunistic saprophyte.

Disease Cycle

These fungi primarily infect the foliage of warm-season grasses during cool, wet periods, typically from autumn through spring.

Crown and root rots caused by Bipolaris cynodontis are more prevalent during warm, dry periods in the summer.

Bipolaris sorokiniana causes leaf spot symptoms on warm-season grasses under moderate temperatures. However, as temperatures exceed 30°C (86°F), leaf blight becomes more prominent and can lead to plant death above 35°C (95°F).

Epidemiology

The epidemiology of bipolaris leaf spot diseases, caused by Bipolaris and Exserohilum spp., is influenced by various environmental factors throughout the year. These fungal diseases tend to thrive during warm and wet periods from autumn through spring, creating favorable conditions for infection. The moisture from rainfall or irrigation, combined with elevated temperatures, provides an ideal environment for the fungi to penetrate and infect the turfgrass leaves.

In contrast, warm and dry periods during the summer contribute to the prevalence of crown and root rots caused by Bipolaris cynodontis. These conditions favor the development of fungal infections in the crown and root tissues, leading to the deterioration and decline of the affected turfgrass. The dryness and heat stress the grass, making it more susceptible to the pathogens.

Temperature also plays a crucial role in the manifestation of leaf spot symptoms caused by Bipolaris sorokiniana. Moderate temperatures create favorable conditions for the development of leaf spot, characterized by small, oval lesions on the turfgrass leaves. However, when temperatures rise, particularly during hot summer periods, the disease can progress to leaf blight, causing extensive damage and even plant death.

Understanding the epidemiology of bipolaris leaf spot diseases helps turfgrass managers in implementing effective disease management strategies. By considering the climatic conditions and the specific pathogens involved, they can optimize cultural practices, such as irrigation and fertilization, and timely application of fungicides to minimize disease development and maintain healthy turfgrass.

Symptoms

Symptoms of bipolaris leaf spot diseases caused by Bipolaris and Exserohilum spp. are characterized by distinct patterns of infection and damage on the turfgrass. Initially, the disease manifests as small leaf spots on older tissue, while younger tissue may exhibit pinpoint leaf flecking. These spots progress downward from the leaves to the basal leaf sheaths, stems, crowns, and upper roots, gradually spreading and intensifying the infection.

Bipolaris cynodontis primarily infects Axonopus and Cynodon spp., while Exserohilum rostratum targets a broader range of grass species including Agrostis, Cynodon, Bromus, Pennisetum, Stenotaphrum, and Zoysia spp. As the disease progresses, severely infected leaves develop a light tan to straw-colored appearance and may ultimately die, leading to a noticeable loss of turf density or “melting-out” symptoms. The affected plants form irregular patches that resemble symptoms of drought stress, further compromising the overall aesthetics and health of the turfgrass.

The extent of damage is particularly severe when the pathogens attack the crowns, stolons, or rhizomes of the turfgrass. These vital plant structures play a crucial role in growth and propagation, and their infection can significantly impact the turf’s ability to regenerate and maintain healthy growth. The disease’s progression in these areas can result in substantial thinning, weakened structural integrity, and even death of the affected plants.

Credit: PACE Turf

Signs

Bipolaris and Exserohilum spp. produce conidia on conidiophores. The size, shape and color overlap among the species, so molecular identification is often necessary. Conidia are fusoid (tapering at both ends) and germ tubes generally develop on the terminal cells, hence the name Bipolaris for ‘bipolar” germination.

Management

The management of diseases caused by Bipolaris and Exserohilum spp. in bermudagrass requires a combination of cultural and chemical approaches. Cultural practices, including vertical mowing, can inadvertently increase the susceptibility of ultradwarf bermudagrasses to leaf spot, necessitating the use of fungicide applications in conjunction with intense cultural management. In terms of chemical management, there are several fungicide chemistries available that have proven efficacy in controlling diseases caused by Bipolaris and Exserohilum spp.

Cultural

Cultural management practices, such as vertical mowing, may increase the susceptibility of ultradwarf bermudagrasses to leaf spot. Some seeded bermudagrass cultivars have shown host tolerance to these diseases, and utilizing resistant turfgrass varieties can be an effective management strategy.

Chemical

Effective fungicides for controlling diseases caused by Bipolaris and Exserohilum spp. is limited. Generally, the fungicides suggested for Pyrenophora diseases (which we also discuss in this course) are usually recommended. For crown and root rot control, fungicides should be watered-in immediately after application.