Ramularia leaf spot

Ramularia collo-cygni

Order - Mycosphaerellacae
Family - Ramularisphaerella

Other Names:

Ophiocladium hordei, Ovularia hordei, Ramularia hordeicola.
Recent work indicates that it may be a Mycosphaerella species.

Description:

A late season fungal disease of barley causing variable brown rectangular spots with a yellow halo on the upper leaves.
The UK uses the 5 'R's for identification of symptoms.
Ringed with yellow margin
Rectangular
Restricted by leaf veins - unlike spot type and net type net blotch.
Reddish-brown colouration 
Right through the leaf - unlike physiological leaf spot.
A fungal body called an asteromella may occur on stubble.

Species Affected:

Barley, grasses, maize.

Biology:

Life Cycle:

Infected barley seed germinates normally and Ramularia is a symptomless endophyte until tillering or later. Plants may also be infected by spores from barley volunteers or grasses or possibly from stubble. Symptoms can occur on lower leaves around tillering if they become stressed and start to die back. Symptoms are rare on newly emerged and healthy leaves. When the crop starts to flower, symptoms start to appear on the upper leaves and especially those exposed to higher levels of sunlight. Well fertilised crops show fewer symptoms than those suffering nitrogen deficiency. Flowering often triggers the development of symptoms as nutrients are diverted from the leaves for grain formation. Spores may form on leaves and are released 24-48 hours after a period of prolonged leaf wetness of several hours. The spores enter the leaves though stomata to produce the secondary infection of the leaves, heads or awns. A second type of spore called an asteromella develops on stubble late in the season.
The lesions are caused when the fungus produces rubellin toxins in response to plant stress or flowering. Rubellin is activated by light leading to more symptoms on leaves exposed to the sun.
It appears as tiny pepper spots on leaves and similar to physiological leaf spots. Ramularia spots will be on both surfaces on the leaf whereas the physiological spots are usually only on the leaf surface exposed to high light levels. Ramularia spots develop into the typical rectangular brown patches about 2 mm long by 0.5 mm wide with straight longitudinal sides defined by the leaf ridges and jagged tops and bottoms. There is a darker brown spot in the centre which is the original “pepper spot”. The brown lesion is surrounded by a yellow halo then normal green leaf. The characteristic of the Ramularia lesion is that it is easily seen on both the upper and lower surface of the leaf and has straight edges defined by the leaf ridges. As time progress the leaf turns yellow and starts to die, usually from the tip. On the underside of the dead leaf, lines of translucent spores may be seen emerging from stomata. These are not associated with the brown lesions and may be seen with a hand lens. Occasionally a red coloration develops around the brown lesions late in the season.
On leaf sheaves and awns the lesions are similar but smaller and remain after senescence.
It is uncommon to see lesions on seed but it is an important source of infection.
Seed borne, stubble borne, secondary spread occurs with in-crop spore production. This spread is helped by leaf wetness. The duration of leaf wetness in spring predicts the severity of ramularia leaf spot after flowering.
There are large environmental effects on the severity of the disease e.g. light and water logging.
There is a high risk of fungicide resistance, due to large populations and not mutation.
(Oxley et al., 2011)

Origin and History:

The first record was in 1893 in Italy but it has only been recognised as a significant disease since the early 2000's

Distribution:

Australia, Europe, North and South America and New Zealand

Significance:

May cause up to 30% loss of barley grain yield and affects grain quality.
Estimated to cost the barley industry $18 m/year in the UK.

Management and Control:

Use clean seed. Test it for diseases using molecular tests. Use seed that has less than 1pg of DNA per 100 ng of total DNA.
Control barley volunteers, grasses and maize.
There are no early visual symptoms as it has a latent, symptomless phase.
Control is required before symptoms develop. Once symptoms have started to develop no fungicides will provide control.
Early treatments at tillering reduce the epidemic but don't stop late symptoms.
The best timing for a single treatment appears to be at flag leaf emergence to when the awns are starting to show.
Chlorothalonil, triazole and SDHI fungicides have provided good control.
Mixture of fungicides will help reduce the risk of developing resistance.
Prothioconazole and chlorothalonil have provided the best control either alone or in various mixtures.
Some varieties of barley are more tolerant than others but there are no major genes which is a breeding challenge.

Related and Similar Species:

Physiological leaf spot has small pepper spots with a yellow halo on the light exposed side of the leaf and are not rectangular.
Net blotch has similar coloured lesions but they are not restricted by the leaf ridges at the edges of the lesion. Spot type net blotch is difficult to separate from ramularia leaf spot.
Halo spot has yellow spots with brown halos rather than brown spots with yellow halos.
Septoria has oval shaped lesions and are more common on the lower leaves rather than the upper leaves.

References:

Oxley, S., Havis, N., Evans, A., Waterhouse, S., Tonguc, L., 2011. A Guide to the Recognition and Understanding of Ramularia an other Leaf Spots of Barley. SAC and BASF The Chemical Company.


Acknowledgments:

Collated by HerbiGuide. Phone 08 98444064 or www.HerbiGuide.com.au for more information.