Pythium in turf. 31

There are several Pythium sp that cause Pythium in turf. This turfgrass blog discusses pythium fungus in grass, what this is, and how to identify and control it. The species of Pythium fungus in grass can cause Pythium blight, root rot, root dysfunction, and damping off. 

You tend to see disease symptoms as irregular patches of discoloured, dead, or wilted turf. These are often prevalent in hot, humid weather.

Effective disease management involves the use of cultural practices and fungicides. Cultural options include:

• Water management.
• Drainage.
• Thatch reduction. 

In Australia tests only tend to diagnose the Pythium in grass as “Pythium sp“. This is despite the fact that you could be dealing with root rot, Pythium blight, root dysfunction, or damping off.

Each of these has their own characteristics, and their effective control varies depending on the causal pathogen.

This diagnosis as Pythium sp is oversimplifying this disease. To make the correct pythium fungicide choice¹Hampy, H.D., Van Ryzin, B.J., Butler, E.L., Kerns, J.P., Etiology and management of Pythium root rot in golf course putting greens, Int Turfgrass Soc Res J.2022;14:851–860 you must know which species is present.

There are two key reasons for this:

• Not all Pythium treatments and turf fungicides work against all species of pythium in turf. This is the case in both the home garden and professional turf markets.
• Incorrect disease ID is a waste of your time and money.

Table of the Main Species of Pythium in Turf.

Pythium in Turf.

Damping off.

Damping off is a Pythium disease in turf that occurs when grass is at the seedling stage. If conditions such as too much moisture, the right temperature, and poor light levels are present, damping off becomes more likely.

Pythium Root Rot (PRR).

A complication in the diagnosis of root rot is that there are two different root diseases.

Pythium Root Rot (PRR) and Pythium Root dysfunction (PRD) are both seen in the plant roots. However, there are distinct differences between these that affect how you manage them.

• They tend to occur in wet soils.
• Root Rot affects all turf varieties.
• They are more common in high organic matter content or poorly draining soils.
• These are commonly seen in close mown, intensively managed turf.
• Root death and a rotting smell are symptoms of this fungus in infected grass.
• The soil temperature impacts the organisms that cause this disease.
• This explains why you can get such variable results when you carry out chemical control.

Conditions That Favour Root Rot.

• PRR tends to develop on cool-season grass such as golf greens.
• Often you see it in the Summer when the soils are warm and wet.
• On warm-season turf, this disease is most severe when there is low light. This is often in the Autumn, Winter, and the Spring.
• PRR tends to occur where water pools. However, you can also see it on high areas after rain.
• Greens in shade where there is little air movement are the most susceptible to this. It is a key reason why many clubs use turf fans to improve air circulation, and promote surface drying.
• When soils stay warm and wet, you can continue to see the symptoms as new spots or areas that continue to expand,
• PRR tends to increase when sting or root-knot nematodes are present²Gu, M., Bermudagrass root rot complexes associated with plant parasitic nematodes and pythium species on golf courses in Florida, May 2019, Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy.

Cultural Control of Pythium Root Rot.

• Reduce plant stress.
• Improve light levels and air movement.
• Don’t apply too much N.
• Improve the soil drainage, and carry out regular soil aeration.
• Increase the height of cut where you can.
• Regularly topdress. This helps water move away from the surface.

Chemical Control of Pythium Root Rot

• Cyazofamid provides the best control of this Pythium in turfgrass.
• Other options are: Fosetyl-Aluminium, Mefonaxam, and Propamocarb.
• Etridiazole is effective³Kerns, J.P., Pythium in golf course putting greens, Golf Dom, November 23, 2021, but you only get a residual of three to four days.
• Make sure to rotate your actives.
• With cool-season grass greens in the Spring, aim to monitor the temperature. Once it reaches 18°C at a 50mm depth for four to five consecutive days, begin to use Cyazofamid at 2.5 L/Ha.
• A rotation of Cyazofamid, Solitaire, and Shrapnel on a two-week cycle works well in a PRR program. You must make sure you water these in with at least 3 mm of water.
• Preventative foliars of Grenadier also work well.

Pythium Root Dysfunction (PRD).

This is a very different pythium fungus in grass in comparison to PRR.

• You only see Root Dysfunction on bent golf greens.
• It doesn’t kill the plant roots which means it is more difficult to identify.
• The causal pathogen, P. volutum, is a root-rotting pathogen. It affects the ability of creeping bent to take up water. This means the turf looks wilted when the surface and soil temperatures are high.
• Traditional Pythium fungicides do not control Root Dysfunction.
• Instead you should use the QoIs (azoxystrobin, pyraclostrobin) and Cyazofamid as these are the most effective.
• A standalone Pyraclostrobin hasn’t a label in Australia, so your best option is Lexicon Intrinsic fungicide.
• Make preventative applications of Root Dysfunction fungicides in the Autumn and the Spring. Do this before you see any symptoms.

Symptoms of Pythium Root Dysfunction.

• This Pythium fungus in grass only occurs on creeping bent.
• It tends to be worst on new golf greens, and especially those with root zones of > 85 % sand⁴Kerns, J.P., Soika, M.D., and Tredway, L.P., 2009. Preventative control of Pythium root dysfunction in creeping bentgrass putting greens and sensitivity of Pythium volutum to fungicides. Plant Disease. 93(12): p. 1275-1280..
• Root Dysfunction is more common on well-drained young bent greens that are less than 5 years old.
• Roots become short, have a lack of root hairs, and develop bulbous root tips.
• As they have no root hairs, sand does not stick to the roots. When you shake a soil plug most of the sand will easily fall off the roots.
• Roots are only slightly more tan or buff than normal roots.
• You first start to see symptoms on exposed golf greens under drought stress.
• The disease develops in high areas first. It look like hot spots or drought stress. Even after you hand water these areas they will not improve.
• You often see the symptoms in the Summer months. However, it also develops in the Autumn, Winter, and Spring in warm, windy, and/or dry spells.

Cultural Control of Pythium Root Dysfunction.

• Use more resistant cultivars of bentgrass⁵Kerns, J.P., Biology and management of Pythium root dysfunction in North Carolina.   North Carolina State University ProQuest Dissertations Publishing,  2008. 3329206.to reduce chemical use for this pythium fungus in grass.
• Improve the drainage and aerate to reduce any soil compaction.
• Reduce thatch.
• Increase the height of cut above 3 mm. Keep the turf at this height until it recovers.
• Try and use a walk-behind mower.
• Reduce how often you mow.
• Irrigate based on what the turf needs rather than a schedule. Try to hand water badly affected areas, and use soil wetting agents to improve water movement away from the surface.
• Don’t mow when the turf is wet. This will reduce the chances of more damage.
• Supply the turf with enough N. High N encourages this disease, while too little N will slow recovery.
• Oversow as soon as you can..

Pythium Blight.

The final Pythium in turfgrass you can often see in the early morning as white, fluffy mycelium that grow from infected grass.

• These mycelium are often mistaken for dollar spot and brown patch but it is in fact Pythium blight.
• This disease can kill turf in 24 to 48 hours and the key to its control is prevention.
• Different species can cause blights at low or high temperatures.
• It needs high humidity or leaf wetness for the disease to develop⁶https://www.golfcourseindustry.com/news/turftip-050415-golembiewski-pythium-expert/.
• High temperatures plus periods of high humidity or wet weather favour this disease.

Cultural Control of Pythium Blight.

• Water early in the day to allow the grass blades to dry. This reduces the chances of leaves staying wet overnight.
• Any practice that reduces leaf wetness helps in the control of this disease.
• Ensure the site drains well.
• Reduce thatch.
• Do not over fertilise with N. This is very important in the hot Summer months.
• Avoid low soil Ca.
• Maintain an acid soil pH.
• Promote good airflow across golf greens⁷Koh, K.J., et al. “Shade and airflow restriction effects on creeping bentgrass golf greens.” Crop Science, vol. 43, no. 6, Nov.-Dec. 2003, pp. 2182.
• Reduce shade.
• Don’t mow wet areas when the temperature is > 21°C.
• Wash mowers before they enter unaffected areas.
• Aerate to encourage root growth.
• Delay any over seeding with cool season turfgrass for as long as you can. You should aim to seed in the late Summer or the early Autumn when the night temperatures are < 18°C.

Chemical Control of Pythium Blight.

• Pythium blight is the most common pythium fungus in grass.
• You can often see it along drain lines and when air movement is restricted in shade areas.
• The best way to manage Pythium blight is with a preventative program.
• Base a program on Fosetyl and/or Cyazofamid.
• If it does appear, Fosetyl doesn’t work as a curative.
• Cyazofamid is the best option.
• Metalaxyl is less expensive. However, turf recovery takes a longer time and you can see scars for several days after you use it.
• Curative applications only protect plants that aren’t already infected.
• These fungicide chemical groups work against Pythium blight: M3, Group 4, Group 14, Group 11, Group 21, Group 28, and Group 33.
• The repeat use of some Pythium fungicides, like metalaxyl (phenylamides), may select for resistant populations of P. aphanidermatum.
• It is important to rotate with chemicals from other groups.
• If you alternate between systemic and contact fungicides it may also delay resistance developing of pythium in turf.