New analysis debunks septoria control myths

The mild, wet winter has increased disease pressure this spring with rusts and septoria already being seen in wheat crops, but is 2014 going to be a rust year or will there be a septoria epidemic?


Having a better understanding of the factors that influence the severity of septoria could help growers tailor their disease programmes. And this was the aim of a detailed FERA analysis of its disease database, which has debunked several myths in the process.





Septoria tritici, caused by the fungus Mycosphaerella graminicola, is the most important wheat disease in the UK. Yield losses are in the 30-50% range, with HGCA estimating costs of £250/ha based on a grain price of £150/t and an average treated yield of 8.5t/ha.


Having the ability to forecast risk can prove valuable in disease control, as successfully seen with Smith periods for blight in potatoes. But previous attempts at predicting septoria, such as using rain splashometers, have had very limited success, suggesting a more complex process.


To examine what factors influence the severity of S. tritici in a given season, FERA plant pathologist Moray Taylor carried out an epidemic analysis of the agency’s extensive database collected as part of DEFRA’s annual wheat disease monitoring surveys. It extends to more than 300 farms a year from 1982 to 2013.


Also, data from FERA’s more detailed assessments of untreated crop plots as part of the Crop Monitor disease forecasting service was used to track infection in the leaf layers (leaves 1-4) from 15 sites around England with 5-6 key varieties and has been running since 2003.


Variety resistance


The first factor he examined was varietal resistance, as some varieties are more susceptible than others. Wheat varieties on the HGCA Recommended List are rated 1-9 for their resistance.



Septoria myths debunked



  • Septoria variety ratings of one to seven have no clear benefit on final disease
  • There is weak correlation between early signs (leaves five and six) and eventual severity (leaves one and two)
  • There is no carryover of risk from one season to the next
  • Delayed crop development did not play a major role in the 2012 epidemic, which was enhanced by difficulties in effective spray timing due to wet soils and variable leaf emergence. 2007 was a higher pressure year than 2012
  • Plants with more erect leaves or that are taller are not at reduced risk as there is weak correlation between early signs (leaves 5 and 6) and eventual severity (leaf 1 and 2)

“However, variety choice is not giving an indication of how the plant is going to cope with the disease pressure.


“We saw a lot of year to year variation between varieties rated as ‘resistant’, indicating season was a greater factor than variety [see graph on the right of p10],” says Dr Taylor.


Bayer CropScience cereal fungicides manager Alison Daniels points out the potential shortcomings of the RL rating system. “Basing it on two years data of may not be very reliable, as shown by the high inter-season variability in septoria levels on varieties, independent of their resistance levels in these large field data sets,” she says.


“The best that top rated varietal resistance appears to offer is perhaps a little timing flexibility at T2.”


In the current Recommended List, there is only one at 7 with the rest at 6 or less. Dr Taylor says the resistance of the current high yielding varieties is quite average.


Early signs


The second risk factor is the amount of early disease. There is a belief that if you see a lot of disease early in the season on leaves 5 and 6, you are more likely to see a lot of it on the flag leaf and leaf 2, he says. These leaves are crucial as they contribute about 65% to yield.


“However, the correlation between early signs and eventual severity is weak, with a wide spread of data points (see graph on the left of p10).


“There are some points where there was no disease seen on leaves 5 and 6 and we still saw a significant amount on the flag leaf. The opposite was also true, with lots of disease being seen on leaves 5 and 6, but none was found on the flag leaf in untreated plots.”


Bayer CropScience cereals development manager Andrew Flind says it confirms his observations in the field that the absence of septoria on early leaves does not necessarily mean there will not be an epidemic.


“There is so much variability because of climate with no two seasons the same. In 2011, it was drier and a rusty year, so there was a focus on yellow rust. The following year’s disease strategy went wrong as rust programmes based on the previous year failed to control septoria.”


There is some linkage between bottom and middle leaf layers, as they are closer together for the pathogen to spread upwards. But it is not just down to rain splash, adds Dr Daniels.


“In 2012, inoculum levels were very high and we had cold nights and warm days and saw spores spread with the dew. There was inoculum transfer without any rain.”


Season


Another potential risk factor is carryover from season to season, as seen with other diseases like light leaf spot in oilseed rape.


The amount of pod disease in the previous summer is a key factor when forecasting light leaf spot and some may assume it is similar for septoria.


Dr Taylor started by comparing regions, as there may be differences between the key wheat growing areas in the east and the south west where wheat is more of a break crop.


“However, there were no relationships between the level in the current year and the previous seven in any of the years,” he says.


He believes this reflects how the crop is grown in close rotations, with a continuous cycle of inoculum. But he adds that there are still gaps in knowledge on the disease’s lifecycle.


“These include airborne acospore movement and survival and the fact there is little data quantifying this part of the lifecycle. Also what are the alternate host plants for S. tritici and how is disease moving around and affecting plants.”


Another seasonal effect is crop growth and one perception was that delayed crop development played a major role in the 2012 epidemic. This was thought to be because the crop did not grow away from the infection on lower leaves.


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But looking at the number of days to key growth stages, 2012 started earlier than average, but by growth stage 39, it was behind. “In the end, 2012 was not that unusual in terms of time to growth stage,” says Dr Taylor.


Growth habitat and the architecture of plants may also affect the severity of disease with more erect-type leaves being smaller targets for the disease.


“An erect flag leaf will grow away from the disease and possibly reduce risk.”


Again, the FERA data showed no trend of growth habitat on eventual disease rating in untreated plots, says Dr Taylor. “Season was having a bigger effect on how they grow, with crop and leaf architecture having little impact on disease level.”


But Dr Daniels adds that leaf rub transfer can be a factor, particularly in short crops.


“Last year we saw direct rubbing of lower infected leaves against the flag leaf as it emerged, despite leaf 2 having been kept clean of disease.


“If you spray too early, you can leave emerging foliage unprotected and these will become infected under high inoculum pressure.”


Dr Taylor also looked at straw length, as opting for taller-type crops may reduce risk, with a greater distance between the lower leaves and leaves 1 and 2.


“But the influence of architecture and growth habitat on septoria risk was weak, again suggesting that the spread is not simply a matter of rain splash and is more complex, possibly due to ascospores.”


Septoria graphs 



Septoria is the key disease threat in the west and one independent agronomist believes that correct spray timings including a T0 will be key to getting on top of it this season.

“Currently levels are moderate to high and disease is being seen in the bottom of crops,” says Antony Wade, of Hillhampton Technical Services, who covers Herefordshire, Shropshire and Worcestershire. “It will depend on March and April rainfall on how it progresses up the plant.”

The T0 application will help to prevent disease building up. “We also need to keep timings tight in a higher pressure year.”

“In 2012, the last high pressure year, it proved difficult with the poor weather at key spray timings. Hopefully, this season will be easier to get sprays on and keep to three week intervals, to hit leaves as they emerge.”

Starting with T0, Mr Wade plans to go with chlorothalonil plus a good dose of azole such as tebuconazole, especially on rust susceptible varieties. Then three weeks later at T1, he will again go for a robust three quarter dose of azole plus chlorothalonil and top up with a strobilurin if yellow rust is still prevalent.

“However, if there is a lot of septoria then I would opt for a SDHI at T1 for the extra eradicant activity such as Adexar (fluxapyroxad + epoxiconazole), particularly if timings get stretched. “We can justify the extra cost as we have got a lot more crops with higher potential this year.”

Mr Wade is also considering an SDHI at T1 for second wheats and light land instead of the strobilurin, as eyespot risk is higher and Vertisan (penthiopyrad) use has been shown by researchers to benefit rooting and some indication of activity on eyespot.

“The key T2 timing will be an SDHI as I am convinced on its benefits and cost justification, so 80-90% of my wheat area will receive Aviator (bixafen + prothioconazole). If pressure is high, I may go to the full 1.25litres/ha rate.”



One of the main dangers of the pathogen is that you only see visible symptoms in the last third of its lifecycle, as it has a long latent period.

“The danger is that you see a green unaffected leaf and assume it’s clean, but the disease is already present inside the leaf and requires curative fungicides,” warns Dr Daniels.

Once inside the leaf, the growth period of the fungus (the latent period) can be prolonged before it enters its killing stage.

“This is especially the case in crops well supplied with nutrients, growing well and not under any stress. The disease can survive inside an apparently healthy leaf by nibbling off the outside of the cells without killing them,” she warns.

“That is why you can go into a crop on a Friday and see no symptoms and then just the following week you have coalescing lesions. In addition, it only takes one spore to cause full leaf infection as it is such a well-adapted pathogen,” she says.

Armed with the FERA findings, what should growers do to manage the disease this season? The best approach is to be proactive and go in hard with a “belt and braces” septoria programme and assume it is present.

There are three key components. First, minimise inoculum at T0; second, target the disease at T1 and T2; third, hit the pathogen at each stage with diverse modes of action.

Minimising inoculum

The T0 is effectively an insurance policy to help manage levels of septoria inoculum by reducing spore development on the lower leaves. It helps buy time if T1s are delayed.

“Your T0 septoria focus is chlorothalonil, where its protectant activity is best exploited. At the same time, fast moving rust azoles like cyproconazole and tebuconazole are ideal to get on top of yellow rust early, says Dr Daniels.

She points to the 2014 NIAB TAG recommendations for T0 strategy where they suggest “using a septoria azole at T0 such as epoxiconazole or prothioconazole may lead to early selection pressure on the developing septoria population, compromising performance in later sprays.”

T1 and T2: Target disease

Moving to T1, the principal aim is to protect leaf 3 and keep it clean to reduce inoculum transfer to the yield-driving flag leaf and leaf 2.

Then T2 is about protecting the flag leaf during its critical yield-building stage. The key to successful septoria programmes for these two crucial sprays is to provide a backbone of a 75% dose of either epoxiconazole or prothioconazole.

Then it is a question of adding in effective non-cross resistant septoria fungicides which act at the same time on the latent mycelium inside the leaf, says Dr Daniels.

“Without the necessary formulation technology to help align other modes of action with the azole, or through the right choice of partners that have similar movement profiles it is not easy to achieve and is often overlooked.

“The only effective non cross-resistant partner for epoxiconazole or prothioconazole is a leaf-mobile new generation SDHI, and not all fit this bill. This is why we’ve developed a new formulation system which aligns prothioconazole with bixafen inside the leaf – so one partner isn’t left stranded,” she says.

Dr Daniels claims the question of whether or not to add chlorothalonil is all about balancing selection pressure with risk management.

“Any co-formulation with a poor ratio runs the risk of increasing selection pressure. Where actives aren’t co-aligned, adding in chlorothalonil may be necessary but there’s no point in including mixed modes of action in tank mixes if they run any risk of reducing the curative activity of the principal fungicides, or are weakly active on septoria.”

One or two SDHIs

“Last year, we looked at two different T1 programmes, the difference being conventional azole chemistry plus chlorothalonil or an SDHI with the azole,” says Mr Flind.

“Our trials saw no real differences. Sometimes you can see a benefit, but most of time prothioconazole plus chlorothalonil gave just as good control as prothioconazole with bixafen.

“Therefore, in most situations it is not necessary to apply two SDHIs and we should be using them as little as possible to reduce selection pressure for resistance.”

But there are some situations where you can get benefit with an SDHI, for example when a T0 was missed and you need extra curative action or in an unusually high pressure septoria season, he says.

However, Dr Daniels adds if there is a significant amount of latent infection, and you want to maximise azole penetration into the leaf, chlorothalonil can reduce penetration, so is not always appropriate.

Recommendations

So taking this into account, the recommended approach is:


  • T0 – chlorothalonil + tebuconazole or cyproconazole (save epoxiconazole or prothioconazole for main sprays)
  • T1 – 75% septoria azole (such as 150g/ha prothioconazole or 94g/ha epoxiconazole) + chlorothalonil in average pressure season or in difficult septoria situations, 75% azole + SDHI (bixafen, fluxapyroxad, penthiopyrad)
  • T2 – 75% septoria azole + SDHI (bixafen or fluxapyroxad)

In conclusion, the difficulty in forecasting disease severity coupled with the pathogen’s ability to infect crops without exhibiting symptoms means growers need to take a protective approach and assume crops will become infected.


Spray-timing-graph