As many barley growers will know to their cost, ramularia is on the rise and researchers worldwide are trying to work out why and what can be done to reduce its impact. Andrew Swallow reports.
Seed-borne inoculum, fungicide resistant strains, co-hosts and a possible plant breeding side-effect are emerging as key factors in the relentless rise of a barley disease rarely reported before the 1990s.
Ramularia collo-cygni, the fungus causing the disease generally referred to as ramularia or ramularia leaf spot (RLS), was first identified in Italy in the 1890s*. Recent retrospective DNA tests of archived seed samples from Rothamsted Research Station in the United Kingdom show it was also present there in the 19th century, though it wasn’t identified in crops at that time.
Only in the 1990s and 2000s did the disease become widely reported across Europe. New Zealand’s first case was reported in 1983, North America’s in 1960, and South America’s in 1925, in Chile. Argentina and Uruguay didn’t report a case of ramularia until 2002 and 2001 respectively.
Uruguay’s crops appear to be some of the hardest-hit by the disease. A scientific review paper* by Havis et al in 2015 notes yield loss estimates of up to 70% in epidemic years. While such losses haven’t been reported here, Plant & Food Research’s Soonie Chng says 30% yield reductions have been attributed to the disease, and some growers reckon they’ve seen worse.
Chng was a speaker at the Foundation of Arable Research’s winter conference and relayed findings from two years of assessing cultivar performance trials (CPT) in Canterbury for the disease.
“It’s often mistaken for physiological spotting,” she told delegates.
To confirm if the small, reddish-brown rectangular lesions with a yellow margin on the upper surface of the leaf are indeed ramularia, you need to look on the underside of the leaf with a hand-lens, Dr Chng said. If it is ramularia, there will be lines of white, snowflake-like spores.
Rapid loss of green leaf area follows emergence of such symptoms and crops typically senesce within a fortnight, curtailing grain-fill with consequent yield loss. Awns and developing grains are also affected.
Visual symptoms of the disease are usually seen shortly after flowering however it’s now known the fungus is present in affected plants much earlier, and in some cases is introduced by seed. An endophytic relationship with host plants has even been suggested.
“We occasionally see it on seedlings at stem extension,” Chng noted at the FAR conference.
Secondary infection also occurs, not only from spores released from barley growing from infected seed, but from infected co-hosts, which include wheat, rye, oats, ryegrass and couch grass (Agropyron repens). Havis et al also note the likelihood of green bridge spread, from volunteers and/or autumn-sown crops to spring crops.
Laboratory work has shown it takes at least two to four weeks from infection for visual symptoms to emerge, however, visual symptoms and consequent yield loss do not always occur, even in infected plants.
“The disease usually remains asymptomatic until after the crop flowers,” Chng said. “Then lesions seem to appear if the crop is under stress.”
Havis et al note prolonged leaf wetness and 5-15C temperatures favour spread and suggest leaf wetness at growth stage 30-31 is one of the key factors in disease development.
In the 2015/16 season Chng assessed irrigated CPT barley trials for ramularia at Temuka, Chertsey and Methven. There was no significant difference in symptom severity between treated and untreated trials, with 0-50% of leaf area affected, suggesting the triazole (DMI mode of action) and strobilurin (QoI mode of action) fungicides used were ineffective.
Subsequently, grain from the trials was tested for ramularia inoculum and the fungus was found to be in all lines.
“What was most interesting was the sprayed plots had higher [grain] inoculum concentration than the unsprayed plots, so the fungicides had reduced the other diseases, but not ramularia.”
In 2016/17 CPT barleys, when an SDHI plus DMI fungicide combination was used on the treated plots, there was a 32-84% reduction in ramularia symptoms suggesting the SDHI chemistry applied at GS49-55 (awns visible to ear emergence) was effective on the disease.
However, ramularia resistant to SDHIs was confirmed in Germany earlier this year so resistance is a real concern for control programmes here, she warns. In Europe, chlorothalonil provides an alternative mode of action however it is not approved for use on barley here.
Inoculum tests on grain from the 2016/17 CPT plots are still underway.
Triazoxide + tebuconazole seed treatment (as in Raxil S) has been shown to reduce ramularia in susceptible varieties, but not always (Havis et al). SDHI plus triticonazole seed treatments also reduced ramularia DNA in seed and seedlings but use of such treatments would require “careful stewardship” due to the risk of fungicide-resistant strains emerging, they said.
Chng told <itals>Country-Wide<end itals> that seed treatments, including the SDHI Systiva (fluxapyroxad), haven’t been seen to offer ramularia control in the field here.
In due course it’s hoped Chng’s work, which is funded by Plant & Food Research, FAR and the seed companies with barleys in CPT, will lead to cultivar resistance ratings.
“We need at least another year of assessments to be more certain and we need to take into account seed-borne inoculum,” Chng said after the FAR conference.
*Havis et al, Ramularia collo-cygni – An Emerging Pathogen of Barley Crops, Phytopathology Vol 105, No. 7, 2015.
FAR’s Rob Craigie says part of the problem with producing resistance ratings for ramularia is allowing for the growth stage of the crop.
“The breeders say that because it comes in as the crop senesces, early maturing varieties will show up with more ramularia on a given day but we can’t say that means it is more-susceptible than a later-maturing variety.”
Fungicide controls should be based on SDHI chemistry, supported with a triazole.
“We know prothioconazole is the stronger triazole on barley but it depends which SDHI you’re using and what it is formulated with. Adexar (fluxapyroxad + epoxiconazole) has been a good product and the results have been up there in trials.”
Given the late emergence of the disease, fungicide at awns-emerging timing (GS49) appears to be the key timing, though Craigie notes the similarity with septoria’s spread.
“Like septoria, we know ramularia’s encouraged by wet weather at stem extension so a season like last year can really lick that infection along.”
As for chlorothalonil, given the concerns about toxins in residues of crops treated with the fungicide, he’s not heard of any company interested in seeking to get their product approved for use on barley here.
“And I wouldn’t be surprised if at some stage it gets taken off the market in the EU.”
Mildew resistance implicated
So what triggered the rise of ramularia from a rarely noticed fungus to one of the world’s worst barley pathogens? Havis et al’s review notes Danish studies of 75 cultivars early in ramularia’s rise, and subsequent studies, found ramularia-susceptibility correlated with mildew-resistance conferred by recessive alleles of the mlo genes. However, the strength of the effect varies with environment, location and genetic background.
Sadras & Calderini report mildew resistance conferred by mlo genes was first observed in mutagenised barleys**. Triumph was one such variety which became hugely popular in both New Zealand and Europe in the late 1970s, however in 1983 it suffered a spectacular breakdown to powdery mildew. Breeders have since sought more-stable forms of mildew-resistance but recessive mlo alleles remain in many varieties today.
Havis et al note there’s variation in susceptibility to ramularia even in cultivars not affected by mlo indicating resistance to ramularia is governed by many genes, ie: polygenic. That’s also supported by observations that ramularia-resistance is quantitative, not an on/off trait.
As such, cultivars selected for ramularia-resistance are less likely to suffer a catastrophic breakdown to disease, as can happen with single-gene resistance such as mlo.
They also note resistance to the fungus, and a cultivar’s likelihood of developing symptoms, are “at least partly under separate genetic control.” Hence selection for both may be important: true resistance helping reduce seed inoculum; symptomatic resistance reducing yield loss.
** Sadras & Calderini: Crop Physiology, Applications for Genetic Improvement and Agronomy, Academic Press, 2009.
Ramularia – what we know:
- Caused by fungal pathogen Ramularia collo-cygni.
- Seed and wind dispersal.
- Multiple hosts including wheat, ryegrass and couch, but barley worst affected.
- Spread favoured by wet leaves.
- Green bridge spread from volunteers and early crops to later plantings.
- Latent period similar to Septoria tritici but may remain asymptomatic.
- Symptom exhibition stress related.
- Yield loss up to 70% seen overseas: 30% plus reported here.
- SDHI fungicides effective but resistance risk high.
While ramularia-resistance ratings are not yet part of FAR’s Cultivar Evaluation system (see main story) cultivar-resistance to ramularia has been on the United Kingdom’s Recommended List for spring barleys since 2013.
Topping the 2017 UK Recommended List for yield and equal top for ramularia resistance is RGT Planet, which also features in FAR’s recently released 2016/2017 Spring Sown What & Barley Cultivar Evaluation booklet, as does Scholar, another variety with eight out of nine rating for ramularia resistance in the UK.
Other cultivars popular in New Zealand can be found in Recommended List archives. For example, the 2013 list scored Sanette eight out of nine for ramularia resistance, the highest of the 23 cultivars listed, while Quench and Garner scored five. The lowest score was three.
Plant & Food Research’s Soonie Chng (see main story) warns cultivar resistance ratings overseas won’t necessarily apply here because the strains of disease present may be different.
For example, the wheat variety Claire was rated highly resistant to stripe rust in the UK until 2013, but in NZ a strain of stripe rust developed that overcame the cultivar’s resistance in 2005, rendering it highly susceptible to the disease thereafter, she explains.
What is Ramularia
Ramularia is the leaf-spotting disease caused by the fungus Ramularia collo-cygni. The collo-cygni part of the name means “swan neck”, referring to the distinctive shape of the spore, Chng explained to the FAR conference.
The fungus is a member of the Mycosphaerellaceae family of fungi which includes well-known wheat pathogen Zymoseptoria tritici (the new name for Septoria tritici). Like septoria, ramularia has a long latent phase, typically two-four weeks, when the fungus is active inside leaves but no symptoms are visible externally. Whether or not symptoms become visible after that appears to depend on environmental stress and other factors.
Fungicide resistance rising in European ramularia
Senior Plant Pathologist at Scotland’s Rural College, SRUC, Dr Neil Havis, says fungicide resistance is the main development in ramularia since the 2015 review he co-authored in Phytopathology.
“Mutations against SDHI’s were first picked up in Germany in 2015 but it appears they have spread and testing of 2016 isolates by Bayer in Germany and Denmark showed a significant shift in sensitivity to the SDHI’s and also the triazoles,” he says.
Havis says Bayer’s tests on ramularia isolates from England and Ireland also indicate fungicide-sensitivity shifts, while SRUC’s testing in Scotland shows a gradual decline in triazole efficacy, and a more significant shift in sensitivity to the SDHIs.
“In the UK we have been stressing to farmers the importance of including chlorothalonil in their T2 sprays.”
In light of chlorothalonil not being registered for use on barley in New Zealand it could be important to know the resistance status of the pathogen in NZ, he suggests.
In terms of cultivar resistance to the disease, for the first time later this year official ratings for winter cultivars will be included in the UK Recommended List. Spring cultivar ramularia resistance ratings have been included since 2013.
However, Havis notes varietal resistance to ramularia can break down.
“Optic started as high/medium resistance but was down to a rating of four [out of nine] before it was taken off the Recommended List.”
Fundamental research into the fungus is ongoing in Europe. The genome sequence has already been published and a Danish government-funded study is looking at ramularia’s interaction with host plants at a molecular level during symptom development. Meanwhile, SRUC is working to identify which ramularia genes are involved in toxin biosynthesis.
“The hope is that it might give leads to breeders on markers for breeding and agrochemical companies for potential antifungal targets,” Havis says.
Ramularia is normally more severe in the north of England and Scotland but this year there has been a lot further south where winter barley is more common. Havis suggests unusual spring weather and reduced activity of fungicides could be to blame.