Allen, T.W., A. Martinez, and L.L. Burpee. 2004. Pythium blight of turfgrass. The Plant Health Instructor. DOI:10.1094/PHI-I-2004-0929-01
DISEASE: Pythium blight of turfgrass
PATHOGEN: Pythium aphanidermatum and Pythium ultimum
HOSTS: Warm- and cool-season turfgrasses (cool-season grasses are more susceptible).
AuthorsTom W. Allen, Alfredo Martinez, and Lee L. BurpeeUniversity of Georgia, Griffin Campus, Griffin, GA
Initial symptoms of Pythium blight in tall fescue (Courtesy L.L. Burpee)
Symptoms of Pythium blight are evident during warm, humid weather when turfgrass leaves are wet for at least 12 hours. The disease is particularly severe when daytime temperatures exceed 28°C (82°F) and night temperatures fail to fall below 20°C (68°F). Initial symptoms appear as dark green to purple water-soaked leaves that aggregate into circular or irregularly shaped patches in turfgrass swards (Figures 1-9). These patches can range in size from < 1 to > 20 cm (0.4 to > 8 in.) in diameter. No distinct lesions are formed on infected leaves, which often feel slimy or greasy when rubbed between two fingers. Patches of infected grass can enlarge and coalesce, causing severe damage to lawns, golf courses and athletic field turf (Figures 10-11). Pythium blight can also appear as streaks in areas where the fungal-like pathogen is spread by flowing water or mowing equipment (Figure 12). In addition to foliar blight, crown, stolon and root rot are frequently associated with Pythium blight.
Pythium blight is also called "cottony blight" which refers to the cotton candy-like growth of white, fluffy aerial mycelium growing from infected turfgrass foliage (Figures 13-15). The mycelium is most abundant when infected leaves are wet, e.g. in the morning after a night of rain or heavy dew. Aerial mycelium produced by Pythium spp. is similar in appearance to mycelium produced by Sclerotinia homoeocarpa, which causes dollar spot, and Rhizoctonia solani, the causal agent of brown patch. However, the latter pathogens produce foliar symptoms that are distinct from those produced by Pythium spp. Even though the aerial mycelium of the three pathogens is similar in appearance, microscopic observation of the hyphae reveals major differences. The most notable difference is the absence of septa in hyphae of Pythium spp. (Fig. 16A), whereas Rhizoctonia and Sclerotinia spp. are septate. In addition Rhizoctonia produces hyphae that branch at right and acute angles to the main hypha (Figure 16B). The branch hyphae appear slightly constricted at their origin, and a septum is present near the branch origin. Right angled branching is also a characteristic that distinguishes R. solani from S. homoeocarpa (Figure 16B and C). The diameter of S. homoeocarpa hyphae is generally larger than R. solani hyphae.
Members of the genus Pythium belong to a group of fungal-like organisms broadly known as the "water molds." Pythium spp. form hyphae that are devoid of septa and contain cellulose in their cell walls. Pythium spp. are not considered to be "true fungi," and recently molecular evidence has indicated a relationship with the red algae in the Kingdom Stramenopila. Species of Pythium are placed within the phylum Oomycota and are commonly referred to as the oomycetes.
Two species of Pythium are most notably associated with Pythium blight: P. aphanidermatum and P. ultimum. Some researchers list as many as 15 species of Pythium as causative agents of this disease. The most severe disease symptoms are caused by P. aphanidermatum . The relative abundance of P. aphanidermatum and P. ultimum from samples submitted to plant disease diagnostic clinics is not readily known. For the most part, when submitted disease samples are diagnosed as Pythium blight, the species of Pythium is not normally reported.
Asexual reproduction. Pythium spp. produce sack-like structures called sporangia which vary in shape from filamentous to spherical. Pythium aphanidermatum forms lobed sporangia (Figure 17) while P. ultimum forms spherical sporangia (Figure 18). When mature, a sporangium can germinate and produce a hypha or give rise to a vesicle. Asexual, motile spores called zoospores are formed in the vesicle and released when the vesicle wall ruptures. Zoospores use flagella to "swim" in free water in soil or on plant surfaces (see disease cycle). Zoospores encyst on potential infection sites, and then the cyst germinates to form a hyphal germ tube that infects host tissues. In addition to sporangia and zoospores, mycelium serves as inoculum by growing through the foliar canopy of turfgrasses and penetrating leaves directly.
Sexual reproduction. In addition to asexual reproduction, most Pythium spp. reproduce sexually. Sexual reproductive structures include an oogonium and a club-shaped antheridium (see disease cycle). The two structures can form from a common strand of hyphae, or from two different hyphal strands. When the two structures come in contact, a fertilization tube enters the oogonium from the antheridium, and the antheridial nucleus is transferred to the egg within the oogonium (Figure 19). The antheridial and oogonial nuclei unite and form a zygote. The wall of the egg cell then thickens creating an oospore which contains the zygote. The oospore (Figures 20-21) can survive periods of drying and remain viable for up to 12 years.
Pythium aphanidermatum and P. ultimum survive as oospores in turfgrass root zones and thatch, and as vegetative mycelium in turfgrass leaves and roots. When environmental conditions are not conducive for disease development, survival as sporangia, zoospores and mycelium is considered to be short-lived. Under favorable conditions for disease development mycelium can resume growth and infect a grass plant in 1-2 hours. Infection of turfgrass leaves occurs by direct penetration from vegetative hyphae or zoospores. Zoospores, which are released from vesicles produced by sporangia, require free water to move and infect other turfgrass plants. Pythium blight can also spread to other adjacent susceptible grass plants by mycelial growth. Infested material in the form of soil and thatch can spread the disease when transported to other areas. Movement of Pythium propagules can also occur on infested mower clippings, as well as dispersal of zoospores on equipment when conditions are wet.
Pythium blight can occur during periods of cool (13 - 18°C / 55 - 64°F), wet weather, but the disease is most severe during periods of hot (30-35°C / 86 - 95°F), humid, rainy or cloudy weather. Pythium aphanidermatum is most actively pathogenic when temperatures are between 30° and 35°C (86 - 95°F) and nighttime temperatures remain above 21°C (70°F). The highest frequency of infection occurs during periods of high relative humidity (>90%) and prolonged leaf wetness.
The susceptibility of some turfgrasses to Pythium sp. can be enhanced by specific environmental factors. For example, high soil salinity predisposes creeping bentgrass to infection by P. aphanidermatum. Soil salinity is normally a problem in arid environments where salt levels fluctuate throughout the year based on water quality and the frequency and duration of irrigation events. During periods of high salinity, Pythium blight can occur at air temperatures and atmospheric humidities that are lower than those normally thought to be favorable for disease development. In addition to soil salinity, drought stress may enhance turfgrass susceptibility to Pythium infection. For example, creeping bentgrass is more susceptible to attack from P. ultimum when grown at low soil moisture (-1.5 MPa) than at field capacity (-0.033 MPa).
Turfgrass site factors that favor the development of standing water enhance the spread of Pythium spp. Fertility can also have an impact on Pythium blight. High levels of nitrogen fertilization enhance the severity of Pythium blight by creating a lush turf canopy. Variations in calcium nutrition, particularly a calcium deficiency, can predispose plants to Pythium blight.
Cultural practices can be used to promote an environment where infection by Pythium species is limited. Irrigation is an important cultural practice to monitor. Watering early in the day will allow grass blades to dry and thus decrease the probability of leaves remaining wet overnight. Irrigation also has an impact on the relative humidity within the foliar canopy of turfgrasses. This is the major reason that late afternoon and evening watering should be avoided on warm days. Providing for good surface and subsurface drainage when establishing new turfgrass sites, and renovating areas where water can pool in established turfgrass areas is another important step in Pythium blight management. Removing thatch can improve drainage, reduce drought and nutrient stress on turf and remove sources of Pythium inoculum. Thatch should be removed if it is greater than 0.6-1.2 cm (0.25-0.5 in.) in depth, depending on height of cut. Thatch is removed by vertical mowing and topdressing with sand or soil. A balanced system of turfgrass nutrition is also a key to controlling Pythium blight. Excessive fertility during hot months, particularly nitrogen, can exacerbate disease pressure. Levels of nitrogen applied to turf should be monitored, and monthly applications of less than 25 kg of nitrogen per ha (0.5 pounds of nitrogen per 1,000 square feet) during periods of hot weather are recommended. When applying nitrogen-based fertilizers use slow-release sources or "spoon-feed" turf with light foliar applications of fertilizer during spring or summer when occurrence of Pythium blight is most likely. Avoid calcium deficiency and maintain a slightly acidic soil pH. Promote good air flow across golf greens and other areas of turf by pruning trees and shrubs to promote light penetration and increase air movement to dry. When mowing, avoid areas of wet turf when the temperature is > 21°C (70°F), as this will help minimize the spread of the pathogen. Wash mowing equipment before entering unaffected areas. Also, alleviate soil compaction, in order to improve turfgrass root growth. When overseeding with cool-season turfgrass species, delay the process until late summer or early fall when the nighttime temperatures have cooled to < 18°C (65°F).
Fungicides in the following classes are effective for the control of Pythium blight: aromatic hydrocarbons, carbamates, dithiocarbamates, phenylamides, phosphonates, and quinone outside inhibitors (QoI, includes the strobilurins). Instituting a preventative fungicide program is recommended in areas that have a history of Pythium blight. The repeated use of some Pythium fungicides, particularly metalaxyl or mefenoxam (phenylamides), may select for resistant populations of P. aphanidermatum. Fungicides from different chemical groups should be alternated or combined in a control program to limit development of resistant populations of Pythium spp. Alternating between systemic and contact fungicides may delay resistance development. Fungicide-treated seed is available for turfgrass establishment or for overseeding dormant warm-season grasses.
Forecasting the times of highest disease risk should be a component of any preventative fungicide program. Instituting a disease-monitoring system can limit losses caused by Pythium blight. Pythium blight has been forecast by monitoring air temperature and relative humidity (RH). The Nutter-Shane model predicts high disease risk when the maximum air temperature is > 30° (86°F), and RH is > 90% for a 14-hour period during which time the ambient temperature remains > 20°C (68°F). As a general rule, if average nighttime temperature plus average relative humidity equal 150, then the risk for Pythium blight is high. However, some researchers state that this model might overpredict the occurrence of Pythium blight, forecasting disease occurrence when it subsequently does not develop.
Attempts have been made to use bacteria (e.g. Enterobacter cloacae and Pseudomonas spp.) and fungi (e.g. Trichoderma hamatum and other Trichoderma spp.) to suppress Pythium blight. There is currently only one commercial biological control material, TurfMate™, containing Trichoderma harzianum, registered as a preventive agent for the control of Pythium blight on turfgrass.
In general, warm-season turfgrass species are less susceptible to P. aphanidermatum and P. ultimum than cool-season grasses. Among the cool-season grasses, annual bluegrass, perennial ryegrass and tall fescue are highly susceptible whereas the fine-leaf fescues and Kentucky bluegrasses are less susceptible. A few cultivars of Kentucky bluegrass, perennial ryegrass and tall fescue have exhibited moderate resistance to P. aphanidermatum. Cool-season grasses, used for winter overseeding of hybrid bermudagrasses, are highly susceptible to Pythium spp. This presents a significant problem to golf course managers in the southeastern U.S. Most of the improved cultivars of bermudagrass are not highly susceptible to P. aphanidermatum or P. ultimum. Consult local turfgrass extension personnel for information on disease resistance for your area. Information on the level of susceptibility of specific turfgrass species can also be obtained from the National Turfgrass Evaluation Program (www.ntep.org).
Pythium blight, also referred to as "spot blight", "grease spot", or "cottony blight", was first reported in the 1930s, but the disease was not recognized as a problem in some parts of the U.S. until 1954. Pythium blight was initially thought to be a disease of only golf course turf, however, the disease is now recognized as a problem on lawns and athletic fields as well. The disease has been detected on warm- and cool-season turfgrass species in the United States, Canada, Germany, France, and Japan.
The first fungicides used for the control of Pythium diseases included inorganic mercury, captan, dichlone, cycloheximide, and organic mercury compounds. These chemicals only provided limited control of Pythium blight. In 1979, metalaxyl was the first systemic fungicide registered in the U.S for the specific purpose of controlling diseases caused by Pythium, Phytophthora spp., and downy mildew pathogens (oomycetes). This included the specific use for controlling Pythium blight on turfgrasses. In 1983, metalaxyl-resistant populations of P. aphanidermatum were detected in creeping bentgrass samples in Pennsylvania. The samples were from golf courses that had used metalaxyl extensively to control Pythium blight over a period of three years. More recently, isolates of P. ultimum from greenhouse-grown ornamentals and turfgrass plants have been found to exhibit resistance to mefenoxam (another fungicide in the same chemical group) in laboratory assays.
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