Seasonal Activity of Turfgrass Pathogens in the Midwest
Richard Latin, Department of Botany and Plant Pathology
Fortunately, for those of us who worry over unhealthy turf, all turfgrass pathogens are not active at the same time. Pathogen activity is largely temperature dependent and it seems as though each season has its own compliment of turf diseases. In most cases, pathogen activity coincides with disease outbreaks. However, in some cases (especially with root diseases), infection and colonization of plant tissues occurs long before to the appearance of recognizable symptoms.
Understanding when turf pathogens are active allows turf managers to anticipate disease outbreaks and schedule fungicide applications at the most appropriate times.
The accompanying chart and narrative provide estimates of when important turf pathogens are active in the lower Midwest. The length of the colored bars in the chart approximate the months of activity for individual turf pathogens. Thick bars define periods of greatest likelihood of pathogen activity. Thin bars identify periods of light activity.
For specific diseases, more information on conditions that favor infection, symptom descriptions, and control recommendations are given in the Turfgrass Disease Profiles accessed through www.agry.purdue.edu/turf/
There remains some uncertainty regarding the environmental conditions that favor anthracnose infection and development. The disease has two phases, a basal rot phase and a foliar phase. The foliar phase is evident on plants suffering from a variety of summer stresses. According to some studies, infection will occur during hot humid conditions. The basal rot phase has been identified during the cool wet conditions of spring, as well as warm wet conditions in summer. Plants under stress (putting greens maintained at dangerously low heights of cut and low levels of nitrogen fertilizer) are most prone to infection and damage associated with anthracnose.
Brown patch is a disease of summer. Extended dew periods (12 hours or more) and elevated temperatures (average temperatures of 65ºF or greater during the wet period), favor infection and spread of the pathogen.
Brown ring patch
Brown ring patch is largely a disease of annual bluegrass putting greens (there are a few reports of symptoms on creeping bentgrass). Outbreaks usually coincide with periods of mild weather with ample precipitation in mid-late spring. Brown ring patch may be confused with yellow patch, a disease caused by a related fungal pathogen that occurs under cooler temperature The brown ring patch pathogen was identified as Waitea circinata var. circinata in 2005. In some cases it is referred to as “Waitea patch.”
The dollar spot fungus is active during extended dew periods when temperatures range from 50ºF – 70ºF. The dollar spot infection process occurs throughout the growing season, from early May through the end of October.
Gray leaf spot
Outbreaks of gray leaf spot normally occur during mid – late summer. Pathogen activity is favored by long dew periods, warm evenings and heavy rains (or frequent irrigation). In southern Indiana, disease development has been confirmed as early as the first week in July. In northern Indiana, outbreaks have occurred as early as mid-August. Initial outbreaks often appear shortly after remnants of Gulf Coast hurricanes bring heavy rains into the Ohio Valley. Disease activity beyond the first few weeks of autumn is not unusual.
Gray snow mold (Typhula blight)
The gray snow mold fungi are active in a very narrow range of low temperatures (31ºF – 36ºF). Also, like almost all pathogenic fungi, they require ample moisture for establishment and spread. That explains why the most severe outbreaks of gray snow mold occur during prolonged periods of snow cover. Fungicide applications in late fall (after the turf goes dormant and before the first lasting snow fall) are essential on sites with a history of the disease. Gray snow mold will not spread once snow melts in the spring.
Leaf spot / Melting out
Until the latter part of the 1900’s, leaf spot and melting out were classified as a single disease and referred to as “Helminthosporium leaf spot.” Modern classification divides the Helminthosporium pathogens into numerous groups (genera) including Bipolaris, Exserohilum, Drechslera, and Marielliottia. They cause leaf spotting diseases on various cool- and warm-season turf. Melting out caused by Dreschslera spp. typically appears in April and May during cold, rainy weather. Leaf spot diseases caused by Bipolaris sorokiniana (on cool season turf) and Bipolaris cynodontis (on bermudagrass) occur during summer and require long wet periods, elevated evening temperatures, and ample precipitation for establishment and spread.
Necrotic ring spot
Necrotic ring spot is a root disease that is caused by a pathogen that is active in warm wet soils. Infection and colonization of roots normally occurs when soil temperatures range from 60ºF – 75ºF. Field patterns (rings, frogeyes, or arcs) first become evident in mid-summer, but they may also be apparent after dry periods in spring, when plants with infected roots grow slowly compared to healthy plants.
Pink snow mold / Microdochium patch
Pink snow mold and Microdochium patch actually represent 2 different phases of the same disease, caused by the same pathogen (Microdochium nivale). In the pink snow mold phase, the pathogen spreads by radial expansion of mycelium under snow cover. The symptoms resemble the well-defined circular patches associated with gray snow mold. In the Microdochium patch phase, the pathogen produces similar patches, but also may spread (especially on putting greens) via conidia (spores) that are rain-splashed or carried with surface water to create new infections. Temperatures that favor infection and spread can range from 32ºF – 50ºF. This pathogen often is very active through mid-spring.
Powdery mildew infection occurs during mild weather (evening temperatures ranging from 40ºF – 55ºF) in areas with heavy – moderate shade. Frequent rains or extended dew periods are not essential for infection.
Hot wet weather is ideal for outbreaks of Pythium blight. Long dew periods, elevated evening temperatures (daily minimum temperatures of 68ºF or greater), and heavy rains during the heat of the summer favor Pythium development.
Red leaf spot
Red leaf spot is a disease that primarily occurs on creeping bentgrass putting greens. Damage caused by red leaf spot is largely cosmetic, but moderate–severe outbreaks may disturb the smoothness of affected playing surfaces. The disease occurs most often on older varieties of creeping bentgrass. The pathogen Drechslera erythrospila is related to leaf spot and melting out pathogens and produces similar symptoms. Red leaf spot tends to occur in late spring, when evening temperatures range between 45ºF and 55ºF, and when long dew periods and ample precipitation are prevalent.
Red thread and pink patch
These are similar diseases caused by closely related fungal pathogens (Laetisaria fuciformis and Limnomyces roseipellis for red thread and pink patch, respectively) that are active during the same environmental regimes. Infection occurs during rainy weather or after extended periods of leaf wetness (greater than 12 hours) when temperatures range from 45ºF – 60ºF. Symptoms are most often observed in mid-late spring, but outbreaks occasionally occur in the fall.
Rhizoctonia large patch
Rhizoctonia large patch (RLP) of zoysiagrass, also called zoysia patch, is the most signiﬁcant infectious disease of the zoysiagrass (Zoysia japonica and Z. matiella) species. Disease development is favored by moderate temperatures (50°–70° F) and long dew periods in spring and fall. Large patch symptoms are normally evident in fall and spring as zoysiagrass enters and emerges from winter dormancy. Rapidly growing turf during the summer heat can mask the effects of underlying infection and drastically diminish symptom expression.
The leaf rust pathogen may be active over a broad range of temperatures. Disease development can occur from late summer through mid autumn. Like many other fungal pathogens, long dew periods favor infection. Rust outbreaks normally occur on slow growing turf. Rust development during spring may be evident in heavily shaded areas.
Smut infection occurs within a narrow range of temperatures (evening temperatures around 50ºF) during spring. The pathogen remains with infected plants until plants eventually die during periods of heat and drought stress. Large amounts of black spores that erupt from infected plants during summer ensure the pathogen’s over-winter survival, but do not create new infections. Infection spores (called basidiospores) are very fragile and infect turf only when two different mating types are present in the spring. The highly specialized circumstances under which infections occur explain why smut diseases are not very common on turfgrasses.
Summer patch is another important disease that affects roots of Kentucky bluegrass and annual bluegrass. The summer patch fungus does not become active until soil temperatures (usually measured at the 3 inch depth during the heat of the day) remain above 65 F for several consecutive days. In north central Indiana, this usually may occur from mid-May to early June. The fungus is active throughout the summer. Symptom expression coincides with extended periods of hot, droughty weather, usually beginning in late July.
Take all patch
Take all patch is caused by a root infecting fungus (Gaeumannomyces graminis f.sp. avenae) that is active in cool wet soils (50ºF – 65ºF). In north-central Indiana, activity occurs mostly during the months of April and May, with some additional activity as soil temperatures cool in September and October. Symptoms may be expressed throughout the summer, especially in dry conditions because infection-impaired roots are unable to support normal turf growth periods of drought stress.
This disease also is called cool season brown patch because it is caused by a species of Rhizoctonia (Rhizoctonia cerealis) that is very closely related to the fungus that causes the brown patch disease in mid-summer. Extended dew periods with low temperatures (40ºF – 55ºF) favor pathogen activity. Symptoms are most often observed in early-mid spring, but also occur in October and November.