However successful your fungicide strategy so far, disease carry-over and the unknown of what the weather will do during wheat’s critical flowering period means fusarium risk remains high. Martin Rickatson seeks some advice from Fera’s fusarium expert Phil Jennings.
Think back to 2012. Not a pleasant memory for most cereal growers, but one in which, while the dull, wet harvest stands out, the last part of the spraying season had perhaps a more lasting impact.
When what had been a relatively dry spring broke to become a pretty miserable June, the conditions were perfect for development of fusarium ear blight, with the weather turning not just damp but cold. That led to wheat flowering periods which extended from the usual one week to as much as three in many crops, with T2 fungicides running out of steam while many T3s were delayed way past their ideal timing, and some did not go on at all.
While there is little which can be done to predict or mitigate against this sort of weather, sound agronomy can influence the level of infection, says Dr Phil Jennings, principal plant pathologist at Fera. Putting insurance in place by selecting the most effective product for the T3 timing, and getting that timing as spot-on as possible, can do a lot to guard against both yield loss and mycotoxin development.
“While there are seven main fusarium species, it is microdochium nivale, fusarium culmorum and fusarium graminearum which are the key concerns. Microdochium produces no known toxins, but infects individual spikelets and can affect crop yields by up to 13% by preventing nutrient and water flow to the uninfected spikelets above it. Culmorum and graminearum, meanwhile, are potential producers of the mycotoxins deoxynivalenol (DON) and zearalone (ZON) and are much bigger hitters of yield – up to 30% – because they can bleach the whole ear.
“That said, it is possible for an ear to be subject to multiple spikelet infections by microdochium, adding up to create part or whole white ears, as many saw in 2012.”
Dr Jennings says the best method of differentiating between a fusarium infection which is the result of microdochium infection and one which is caused by culmorum or graminearum – the latter is the most significant – is plants infected with the latter will have a rachis or main stem which is also bleached. With microdochium infection, there may be bleached spikelets all the way up the ear, but the rachis will still be green.
“It’s important to note the different causes of the two key mycotoxin types, DON and ZON. While they are produced by the same fusarium species, culmorum and graminearum, the former is associated with infection at flowering, and levels increase in a wet July. It’s produced in the chaff, but because it’s soluble in water, late-July rain helps transfer it from the chaff to the grain, concentrating it as the grain dries and producing white, shrivelled grains.
“ZON, meanwhile, develops during wet, delayed harvests. It is thought such conditions result in the grain embryos starting to germinate, and fusarium then grows around the grains, utilising the sugars produced by the embryo. This growth produces the pink grains characteristic of ZON infection.”
While seasons 2013 and 2014 did not see fusarium levels quite so high as those of 2012, they were nonetheless significant, and every year since 2007 the annually recorded incidences of the disease have been on far higher levels than previously noted in samples submitted to Fera’s Defra-funded national disease survey, says Dr Jennings. Before then, fusarium incidences tended to be dominated by less impactful pathogens. The growing dominance of graminearum over the culmorum species over the past dozen years, and the general overall rise in fusarium cases, may well be due to the steady increase in min-till crop establishment systems, and the sharp jump in the maize area for fodder or AD, he says.
“From 2000, when we first started isolating samples for fusarium ear blight, up until 2007, most samples showed no symptoms. Pre-2002, culmorum was the predominant mycotoxin-causing species. Since that time, graminearum has become more dominant, probably due to min-till establishment meaning the structures which hold the spores (fruiting bodies) are left on the soil surface where they have the light and warmth necessary to mature.
“The likelihood of fusarium development in any one cropping year is favoured firstly by a warm, dry autumn which, in terms of the key pathogens, is conducive to the establishment of culmorum and graminearum inoculum on old crop debris, and the development of microdochium nivale at the stem base of new plants.
“If it’s warm and dry early in the spring, pathogen development in their respective areas is further encouraged, while a showery May later enhances development of the graminearum pathogen’s fruiting bodies on debris.”
But although earlier sprays can help to regulate inoculum levels, it’s the early summer flowering period, in the run-up to flowering and the T3 spray, when the wheat crop is most susceptible to infection, says Dr Jennings.
“The primary species in any one season can vary according to that season’s particular weather conditions, and the level and type of pathogen development is related to the species involved and the weather.
“While dry conditions are needed for development, wetter, more humid ones are required for infection, but the type of precipitation has a bearing on the fusarium pathogen which may infect the crop. The macroconidia spores which are produced by fusarium culmorum are transferred by ‘splashy’ rain because they aren’t wind-borne, while the ascospores which cause fusarium graminearum and microdochium nivale are more reliant on wind transfer and heavy dews.”
He advises wheat growers to carry out an assessment of the risk their crops are at according to variety susceptibility, the previous crop, the cultivation and drilling methods used to establish the current crop, and the T0-T2 and planned T3 fungicide strategy.
“Previous cropping which left behind a lot of trash has a significant role as a carry-over host, and minimum or no-till establishment systems exacerbate the risk this poses, with both fusarium graminearum and microdochium nivale much more evident after direct drilling and reduced cultivation systems than when land has been ploughed.”
Product choice, rate and timing will all have a bearing on the efficacy of fungicidal control, with different azoles having different effects on fusarium ear blight pathogens, Dr Jennings points out.
“It’s important to choose an active which controls as wide as possible a range of the different fusarium pathogens, which in terms of azoles means the only real option is prothioconazole. Before prothioconazole, microdochium and fusarium species had to be controlled in different ways, using a strobilurin for the former and azoles for the latter. The introduction of prothioconazole has meant farmers have a single product option which works against both microdochium and the fusarium species, which is doubly important because of the full-scale resistance which microdochium has developed to strobs.”
Application timing is as critical as product choice in terms of T3 spray efficacy, with the crop most susceptible during flowering, Dr Jennings says.
“Prothioconazole should give you the coverage required for at least seven days, covering the normal flowering period. A mid-flowering (GS65) application will give best results, with three to four days’ kickback and the same period of future protection. Something like tebuconazole will only give a five-day total window. And if you spray early, then the plant may be unprotected for the latter part of its flowering period.
“The other benefit of prothioconazole is that, if it has been used earlier in the programme, at T1 and/or T2, there will be a knock-on benefit in terms of reduction in early season inoculum of graminearum and culmorum. But robust rates are essential – check not just the product, but its active load. According to Bayer CropScience, ‘robust’ in terms of Proline means 0.55 litres/hectare, which equates to 150g/ha of prothioconazole.”