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Disease Management

Cultural management

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).

Chemical management

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.

Biological control

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.

Resistant species and cultivars

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).