Disease control – Keep bright not white

 

So far this season, conditions have been favourable for fusarium inoculum to build up. If you’re playing T3 by ear, then CPM finds out what you need to know about this quality-robbing disease complex.

Although last year there wasn’t much FEB in crops, that doesn’t mean inoculum will have decreased.

By Lucy de la Pasture

So far, the weather patterns this season are looking like they may have favoured the build up of fusarium ear blight (FEB) pathogens, says Dr Phil Jennings, plant pathologist at Fera.

 

“Although last year there wasn’t much FEB in crops, that doesn’t necessarily mean inoculum will have decreased,” he highlights.

Dry weather in the autumn and early spring can result in the establishment of Microdochium species at the stem base, and Fusarium culmorum and Fusarium graminearum inoculum on old crop debris. Further warm, dry weather during March and April also aids pathogen development. So far, the 2018-19 season has ticked these boxes and the worst-case scenario will be a showery May, as this enhances development of F. graminearum fruiting bodies on trash.

 

FEB inoculum build up is being favoured by the increase in min-till and no-till practices, which enables the inoculum to develop in the trash left from the previous crop, explains Phil. “Inoculum is a key component for FEB. Once you have high levels of disease one year, then there’s a base for infection in the subsequent season.”

 

In 2002 there was a shift in the population of FEB pathogens, with F. graminearum displacing F. culmorum as the dominant deoxynivalenol producing pathogen. Although inoculum from both pathogens found on crop debris in the soil, F. graminearum builds up in other trashy crops, such as maize and potatoes, and even in game cover crops, he points out.

 

“In theory it’s not really a good idea to plant a game cover crop right next to a cereal crop, particularly if it contains maize. And this is backed up by Fera survey data, which found the crop was heavily infected with fusarium in the 10m adjacent to the headland cover crop,” he comments.

 

Phil believes that it’s more than the recent increase in maize area that’s behind this swing in the FEB species and multiple factors are involved. “Differences in the type of spores produced by the two species have played a part. F. graminearum produces perithecia containing wind-borne ascospores which can move easily over long distances, whereas F. culmorum produces conidia, which are spread by rain splash, so don’t move as far.

Good control of FEB has become important because of the mycotoxins which the Fusarium species produce – notably deoxynivalenol (DON) and zearalenone (ZON) in wheat, which are produced by both F. culmorum and F. graminearum.

 

The Microdochium species also form a part of the FEB complex but are considered less important because they don’t produce mycotoxins and are associated with relatively lower yield penalties.

 

With five true fusarium species and two species of Microdochium capable of infecting wheat ears, identifying which one you have can be tricky. Three of the fusarium species – F. avenaceum, F. poae and F. langsethiae – only have a minimal impact on yield and quality, and are likely to be controlled by most azoles, he points out.

 

“The bigger issue is whether you have F. graminearum, F. culmorum or Microdochium spp. With F. graminearum and F. culmorum, the spine of the ear or rachis will be bleached. Microdochium spp may bleach many or all of the spikelets, but not the rachis,” he explains.

 

With few FEB-resistant varieties available, control is dependent on foliar fungicides. But timing is critical to get control, highlights Phil.

 

“Prothioconazole is the best spray option but even here the window is narrow. Just ahead of mid-flower (GS63) is the best application timing, giving protection for the flowering period and a bit of kick-back. However, the protective activity is less with secondary azoles.”

 

“GS59 is too early. Pollen stimulates germination of Fusarium spores, so when a fungicide is applied before flowering begins all you’re getting is forward protection of up to 4 days with prothioconazole and up to 3 days with tebuconazole. That’s not enough for FEB control all the way through anthesis.

 

“At GS63 you are getting some forward and a smaller amount of backward protection, which makes better use of the period of up to seven days that the fungicide is effective against FEB,” he says.

How about using an SDHI at T3?

 

With their septoria activity, the new generation SDHI fungicides naturally suit the T1 and T2 slots in wheat disease control programmes. But their use as an ear spray was cited as a possibility when the first arrived in the form of Aviator (prothioconazole+ bixafen).

 

That idea has gained ground over recent years, with YEN data highlighting the importance of early biomass and keeping the crop green to fuel ear numbers upwards of 700/m². The arrival of highly resilient varieties to septoria such as KWS Extase and LG Sundance strengthens the case, making an azole plus CTL mix at the T1 timing a realistic option, in some cases.

 

With interest growing, Bayer invited its Xpro Farmer Club members to trial the idea for themselves as part of the company’s ‘Judge for Yourself’ initiative. Using split field trials, farmers took Bayer’s standard programme and switched Aviator from its T1 slot to T3, with a Proline (prothioconazole) plus CTL mix taking its place at T1. The programme started with a CTL plus tebuconazole at T0, with Ascra (prothioconazole+ bixafen+ fluopyram) as the flag leaf spray.

 

One farmer keen to test the idea was Andrew Cook of Lawn Hall Farm, Dunmow Essex. He grows over 400ha of milling wheats and put aside two 5ha plots in two fields of Crusoe.

 

In a season where septoria and brown rust were the primary threats, Bayer’s Sam Harvey reveals the result from the two trial plots could be considered a draw – yield, specific weight and moisture levels being broadly similar.

 

He feels the result is understandable. “For these diseases, prothioconazole and bixafen is a great combination – together they offer effective control of both diseases.

 

“In the first part of spring there was a reasonable amount of septoria around, but with the warm weather it dried out and then the brown rust really took off in May. It just happened that last year the season suited a product like Aviator at either T1 or T3 timings.”

 

Sam points out that one season is insufficient to draw any conclusions, but the results certainly suggest that if conditions were similar this season, it’s a close call about which strategy to use. But if septoria threatens, Sam would be reluctant to move away from Aviator as the T1 option.

 

“Prothioconazole remains our most septoria-active azole and in trials we still see effective control using it with CTL at T1. But with our maritime climate, in most seasons septoria is winter wheat’s number one disease threat and you just cannot afford to let it get away.

 

“So when septoria threatens we have to use our most active products. The addition of bixafen not only boosts protective properties but also provides some kick-back against established infection,” he concludes.

 

Prof Fiona Burnett of Scotland’s Rural College understands the need to extend crop greening and protection in the summer but thinks the balance of risks still favours using SDHIs at T1 in preference to the T3 slot.

 

“2018 was an unusual year and what little early season disease there was present was dried up by the hot summer and, even in a year like that, there was little to show an advantage of moving the SDHI to the T3 slot.

 

“In a more normal year, the importance of protecting early biomass, getting a strong start on septoria and protecting against stem base disease does tend to favour the T1 slot. The main strength of the SDHIs is not the ear diseases and while green leaf retention is a feature, there are probably other ways to achieve this – for example with biostimulants, foliar feeds or even strobilurins – without over exposing this key fungicide group,” she comments.

 

“Thinking of resistance management, whilst we’ve no data to back this up, it maybe isn’t ideal to extend the period that SDHIs are used in the UK.’