Pythium blight (also referred to as grease spot or cottony blight) is a grass fungal infection that can affect the majority of the most common cool-season turfgrasses. There are several species of the disease and it tends to be most prevalent during hot, humid weather. Turfgrass is particularly susceptible when conditions combine high temperatures (between 80°F to 90°F), wet soil and minimal air movement over the turf.
The disease is destructive and can spread quickly, killing large areas of already established turf or seedings in a couple of days. Although a predominantly hot weather fungus, it can also take hold during cool wet weather periods 55°F to 65°F.
Pythium root and crown rot occur when root and crown tissue is attacked. This disease can strike at any time, whether the weather is hot, warm, or cool. Generally, though the disease thrives in wet, humid conditions.
Symptoms of Pythium Blight
During hot, humid weather, Pythium blight initially shows up as small, sunken, circular patches up to six inches in diameter and can spread into significantly bigger irregular areas, particularly during extended periods of humid, rainy, and/or cloudy weather at any time of year.
These symptoms are sometimes confused with dollar spot, so it is important to have an accurate diagnosis. The grass blades in affected regions are tangled, orange or dark grey in colour, and greasy to the touch. When the leaves are wet or the humidity is high, grey, cottony mycelium can be seen in the infected areas.
The disease spreads quickly often along drainage flows or mowing directions and can be traced using the right equipment. When conditions are favourable for the development of Pythium blight, these infections can affect extensive damage to a grass blade very quickly.
When Does Pythium Blight Occur?
For Pythium blight disease to take hold three factors need to be present. Firstly the pathogen needs to arrive or to be already dormant in your lawn. Secondly, the proper host is needed which unfortunately is most common turfgrasses Lastly the day/night temperatures to be greater than 55°F.
Pythium Blight Disease Cycle
This pathogen can lie dormant as resistant spores over the winter and other periods when disease development is hampered by climatic conditions. The pathogen can also be transported from one location to another wind, soil movement, the use of garden equipment such as mowers, human foot traffic or water. It also thrives in over-fertilized alkaline-rich soil where there are high levels of nitrogen and low levels of calcium.
Pythium fungi can also cause ‘seed decay’ in turfgrasses. The fungus may also invade grassroots and crowns. This results in much-reduced growth and turf thinning . This is called Pythium root and crown rot.
Grass areas that become infected become staging posts from which the fungus spreads. If the weather is hot and humid, this spread can be rapid. Many grass varieties are favoured by the disease due to high nitrogen fertility. Alkaline soil and soil deficient in calcium are also conducive to the disease.
How to Prevent and Treat Pythium Blight
Once you have a Pythium blight infection it can be hard to get rid of unless you catch it early, otherwise you may well require a fungicidal solution. However, as with many lawn fungus diseases prevention is the best solution and limits your risk of your lawn being infected. Prevention can be either cultural, chemical or a combination of both.
Pythium blight treatment – cultural management
Cultural practises can be used to foster an environment where Pythium species infection is limited. Irrigation is a critical cultural practise that must be monitored. Watering early in the day allows grass blades to dry, reducing the likelihood of the blades being wet overnight. Watering also has an effect on the relative humidity of turfgrass blade coverage (foliar canopy). This is the main reason why watering in the evening should be avoided, particularly on hot days.
Pythium blight management should also include making sure there is good surface and subsurface drainage when laying and establishing new turfgrass lawns and renovating existing lawn areas where water pools.
Other critical management practises include removing thatch to improve drainage, reducing drought and nutrient stresses on turf, and removing Pythium inoculum sources. Excess thatch should be removed from the top of the grass as it can trap moisture underneath. To remove thatch, scarifying and or vertical mowing can be used.
A well-balanced turfgrass nutrition system is also required for Pythium blight control. Excessive fertilisation, particularly nitrogen applications, during the hot months can worsen disease severity.
Pythium blight likes nitrogen-rich soil therefore nitrogen levels applied to turf should be carefully monitored with monthly applications being no more than around ½ a pound of nitrogen per 1,000 square feet when the weather is hot. When applying nitrogen-based fertiliser try to use slow-release products when Pythium blight is most likely to occur. It is also critical to avoid calcium deficiency and to keep soil neutral or slightly acidic.
Getting air to your lawn can make a big difference. By cutting back trees and shrubs that surround grass areas you will help to promote good airflow across the lawn, which can significantly aid in the drying of grass when it gets wet. Also, you can improve turfgrass root growth by aerating your lawn and grass areas, reducing soil compaction. Delay overseeding with cool-season turfgrass species until late summer or early fall, when nighttime temperatures have dropped to 18°C (65°F).
Finally, when mowing your lawn, avoid areas of wet grass when the temperature is in excess of 70°F. It is good practice to thoroughly clean lawnmowers and other equipment before using them on unaffected areas as these practises will help to limit Pythium blight’s spread.
Pythium blight chemical control
Cultural management may not be sufficient and if your lawn has been infected with Pythium blight then a chemical solution might be necessary.
There are various fungicides on the market that can effectively control and eliminate Pythium blight infection the most effective ones contain the following active ingredients: fosetyl-Al, chloroneb or metalaxyl-M. In addition, there are other commercial fungicides that can be applied but these can, in certain states or countries be restricted to professional use only.
It is worth noting that the overuse use of certain Pythium fungicides, particularly metalaxyl or mefenoxam (phenylamides) can result in the pathogen becoming resistant to them. It is advisable to rotate between various fungicides or use fungicides from different chemical groups over a period of time to stop resistance from occuring. You can also cycle between contact and systemic fungicides prevent any resistance from developing.
When looking to overseed you can buy pretreated seed is available for turfgrass establishment or for overseeding dormant warm-season grasses.
Having a proper lawn care program should include a program for dealing with the periods where your lawn is at the highest risk from Pythium blight and adjust your fungicide program to accommodate. In addition, having a program for monitoring for possible outbreaks of Pythium blight can help drastically reduce the damage that could subsequently be caused.
One aspect should also include monitoring air temperature and relative humidity (RH). The time of high disease risk from a Pythium blight infection is when nighttime temperature and relative humidity, when added together, are 150 or above. For example, if the nighttime temperature was around 70°F and humidity was greater than 90% then the total of 160 should be a warning of possible disease occurrence.
Keith has been involved in the gardening and landscaping industry fro the past 21 years. From a jobbing gardener to running his own landscaping services. He is now dedicated to bringing you the latest in gardening news.