Detection of grass diseases XI - Southern blight

The disease known as Southern Blight affects C4 grasses such as Bermudagrass and Zoysiagrass. It is related to turfgrasses stressed by excess moisture in combination with high temperatures, which favours the growth of fungi of the genus Sclerotium.

Aetiology of Southern blight:

The Southern blight o Southern blight is a fungal disease that can affect greens, tees, fairways and football pitches. The disease is characterised by the appearance of brown or orange spots that can spread rapidly and cause the death of large areas of turf.

Causal agent of Southern blight:

The causative agent of the Southern blight is the fungus Sclerotium rolfsii. This deuteromycete fungus produces sclerotia, which are survival structures that can remain in the soil for years. The sclerotia germinate and produce mycelium, which infects plants.

Symptoms of Southern blight:

General appearance:

• It affects circular areas of turf, with diameters up to 2.7 metres.
• In some cases, plants in the centre of affected areas may remain alive.
• Partial circles or crescent-shaped areas may be observed.
• The grass turns reddish-brown as it dies.
• Infected plants are completely necrotic.

Mycelium and sclerotia:

• As the fungus progresses, there is a white mycelium abundant on the lawn.
• They are formed sclerotia white at first, turning to light or dark brown at the base of the stems.
• These sclerotia are small, hard and resting, and resemble mustard seeds. Their presence is of great help in detecting the disease.

Most susceptible species:

- Bermuda (Cynodon dactylon): The species most susceptible to Southern blight.

- Zoysia Lawn (Zoysia spp.): Also susceptible, especially fine-leaved varieties.

- Ryegrass (Lolium spp.): Susceptible, especially in high humidity conditions.

- Poa pratensisSusceptible, especially in shaded areas.

- Festuca arundinaceaLess susceptible than other species, but may still be affected.

Conditions conducive to infection:

• Warm or hot climate: Air temperatures above 24°C. Optimum conditions for disease development are air temperatures between 29°C and 35°C.
• High humidity: High ambient humidity or excessive irrigation leading to high moisture in the thatch layer. The use of humidity probes can help us to monitor this infection factor.
• Abundant presence of thatch: The accumulated thacht creates an environment conducive to fungal growth.
• Poor turf infiltration: The accumulation of water in areas with poor infiltration increases the chances of infection. We can know this parameter in detail with a infiltration test.

Cultural control:

Prevention:

• Reduce thatch:
  • Chop to remove soil plugs with accumulated thatch.
  • Cut vertically to help break down the remaining straw.

• Avoid excessive moisture:
  • Do not overwater the lawn.
  • Ensure good soil drainage to avoid waterlogging.

• Adequate fertilisation:
  • Use soil acidifying fertilisers as the main source of nitrogen. Examples: ammonium sulphate and SCU (urea sulphate and ammonium chloride).

• Preventive disease control:
  • If the disease has been a problem in the past, apply a systemic fungicide preventively according to the manufacturer's recommendations.

Curative treatment:

If disease is already present, mix an effective contact fungicide and a systemic fungicide in the application tank.

The most commonly used fungicides for the control of Southern blight are:

o Azoxystrobin

o Iprodione

o Propiconazole

o Tebuconazole

Analysis and diagnosis Southern blight:

The most effective way to diagnose Southern blight is always our qPCR fast where we can identify dozens of diseases with a single sample.

It is important to be able to identify the disease in the field and with just a microscope to analyse the most identifiable structures of this fungus.

Source: Chatzaki, A., Papadaki, A.A., Krasagakis, N. et al. First report of southern blight caused by Athelia rolfsii on hemp in Greece. J Plant Pathol 104, 871-872 (2022).

Source: Suwanchaikasem P, Nie S, Selby-Pham J, Walker R, Boughton BA, Idnurm A. Hormonal and proteomic analyses of southern blight disease caused by Athelia rolfsii and root chitosan priming on Cannabis sativa in an in vitro hydroponic system. Plant Direct. 2023 Sep 8