Grass disease detection XII - Gray Leaf Spot

Aetiology of Gray Leaf Spot

Grey Stain, also known as Gray Leaf Spotis a fungal disease affecting a wide range of plants, including grasses such as rice, wheat and turf. The disease is characterised by the appearance of greyish or brownish spots on the leaves, which may enlarge and coalesce until they cover the entire leaf. In severe cases, the infection can lead to the death of the plant.

Causative agent of Gray Leaf Spot

The causal agent of grey spot in turfgrass is the fungus Pyricularia grisea. This ascomycete fungus produces conidia, which are asexual spores that can be carried by wind and rain, facilitating the spread of the disease. The conidia germinate and penetrate the leaves through natural openings or wounds, where the mycelium of the fungus develops.

The infection-conidiogenesis cycle is completed within a week under favourable conditions, and thousands of conidia can be released from a lesion in a single night, and conidia can be generated over the next 20 days and spread rapidly in the environment.

Symptoms of Gray Leaf Spot on Lawns

Lolium perenne y Festuca arundinacea

• Grey lesions on leaf margins. Greyish, twisted leaf tips.
• Yellow halo around lesions.
• Appearance of purple felt on infected leaves early in the morning.
• Drought-like greyish appearance in severely infected stands.
• Rapid collapse of affected stands.
• Yellow and thin grass in less severe conditions.

Pennisetum clandestinum y Stenotaphrum secundatum

• Oval lesions with purple or brown borders.
• Size of lesions up to 0.5 inches long.
• Yellow halo around lesions.
• Felted appearance of the lesions.

Symptoms are similar to those of water stress, and the mycelium of the fungus is different from that of the cottony type seen in other diseases such as Dollar Spot, Brown Patch o Pithium.

Most susceptible species

Both the Lolium perenne as the Festuca arundinacea are cool season grasses that prefer moderate temperatures. Pyricularia griseaThe fungus that causes Gray Leaf Spot thrives in similar temperature ranges. In addition, these grasses may lack natural resistance mechanisms against this particular fungus, making them more vulnerable.

Under moderately favourable conditions, large areas of young grass can die in a matter of weeks.

Gray Leaf Spot: Appearance and Favourable Conditions

Appearance

• Grey leaf spot can appear any time between mid-summer and late autumn.

Favourable conditions

• The disease is favoured by prolonged periods of heat and drought stress in late summer.
• However, it can also develop in Lolium perenne in cooler conditions in late autumn.

Factors Influencing Emergence

• Temperature: The optimum temperature for the development of grey spot is between 24°C and 35°C.
• Humidity: The disease develops best in conditions of high humidity. 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.
• Stress: Drought stress or nutritional deficiencies can weaken turf and make it more susceptible to disease. The use of humidity probes can help us to monitor this infection factor.
• Other factors: Excess nitrogen, lack of sunlight, leaf damage.

Disease Control

Resistance

• Cultivars of Lolium perenne vary in their susceptibility to grey spot, although none are considered resistant.
• It is recommended to seed or repair perennial ryegrass lawns damaged by grey patch with poa pratensis.
• The Festuca arundinacea is best repaired by dormant seeding.

Cultural Practices

• Avoid watering in the late afternoon or early evening, especially after bud break. Moisture on the leaf surface encourages disease.

Chemical Control Options

• The most effective fungicides are the strobilurins (QoI) such as Trifloxystrobin, Fluoxastrobin, Azoxystrobin y Pyraclostrobin. Fungicide selection should be guided by a sound resistance management strategy.

Analysis and diagnosis of Gray Leaf Spot or grey spot

The most effective way to diagnose Gray Leaf Spot is always our Gray Leaf Spot tests. qPCR fast where we can identify dozens of diseases with a single sample.

In addition, it is interesting to know the symptomatology in order to identify the disease in the field and with just a microscope to be able to analyse the most identifiable structures of this fungus for future infections.

Source: Turfgrass Disease Profiles, Purdue Extension BP-107-W, Richard Latin, Professor of Plant Pathology Justin Stewart, Graduate Research Assistant

Source: Gray leaf spot NC State Extension News. https://www.turffiles.ncsu.edu/diseases-in-turf/gray-leaf-spot-in-turf/

Source: Craze HA, Pillay N, Joubert F, Berger DK. Deep Learning Diagnostics of Gray Leaf Spot in Maize under Mixed Disease Field Conditions. Plants (Basel).