Fusarium Head Blight (FHB)

Pathogens responsible

All the fungi responsible for FHB are present in the soil. There are two groups of fungus involved; Fusarium and Microdochium. Fusarium species include F. culmorum, F. graminearum, F. avenaceum, F. poae and the recently discovered F. langsethiae. The Microdochium species include M. nivale and M. majus.  F. culmorum, F. graminearum are the main culprits in the development of mycotoxins including the highly toxic trichothecene mycotoxins, such as deoxynivalenol (DON) and zearalenone (ZON).

DON contamination of grain is most severe in wheat crops following grain maize. DON is considered the most important mycotoxin produced by F. graminearum and has been shown to cause vomiting in both animals and humans, often resulting in feed refusal in livestock. DON can also reduce starch and protein concentration in grain. ZON is a strong estrogenic compound which causes reproductive problems in animals, such as cattle, swine and poultry.

There are legal limits for fusarium mycotoxins deoxynivalenol (DON) and zearalenone (ZON) in wheat intended for human consumption and guidance limits for grain for feed.

Source of infection.

Crop residues from previous cereal crops are main source of infection. The spores are splashed up through the crop on to the ears. However, it is highly likely that conidia are transported by rain up through the canopy from leaf layer to leaf layer in a series of steps before finally being splashed on to the ear. Fusarium graminearum can also release spores in to the air in the spring especially if warm and humid and these airbourne spores can travel for miles.

Infection is classified into 5 types. Type 1 infection refers to the initial infection of a spikelet, Type 2 is when this initial infection spreads through the wheat ear to other spikelets.  Types 3 – 5 refer to effect on grain size, grain retention, yield tolerance and breakdown of mycotoxins.

Infection takes place as the mycelia grow along the surface of the spikelets and any anthers that are protruding from the floret. Once inside the floret the mycelium colonise the developing grain. During colonization of wheat heads, marked alterations in the host tissues occur, including degeneration of cytoplasm and cell organelles, and disintegration of host cells. This degradation suggests that FHB pathogens may secrete corresponding cell wall–degrading enzymes. Most infections occurred on the inner surfaces of lemma, glume, palea, and rachis, and not necessarily through anthers.

Factors influencing the development of FEB.

Severe epidemics of FEB occur when there is a large amount of inoculum, suitable weather for infection and host plants at a susceptible growth stage, i.e. at flowering. Regions with intense wheat and, or maize cultivation are likely to have greater concentrations of airborne inoculum.  The disease occurs wherever cereals are grown and is a major problem in the US, Australia, China and Europe.

The predominant FHB pathogen worldwide is  Fusarium graminearum. In cooler maritime areas of northwestern Europe, commonly occurring FHB species are F. culmorum, F. avenaceum, and Microdochium species. In field sampling across Europe, including the UK, it was not uncommon to have all six species (F. graminearum, F. culmorum, F. poae, F. avenaceum, and two Microdochium species) present at one site.

Cultivations: minimal cultivations increase the risk of infection as these leave crop residues on the surface as a source of inoculum.

Plant height: The Rht2 dwarfing gene has been used extensively in modern wheat breeding programmes to reduce the height of wheat. Work carried out at CSIRO in Australia compared isolines of wheat with and without the dwarfing gene.  Tall isolines all gave better type I resistance than their respective dwarf counterparts. One theory is that shorter plants tend to be more susceptible to FHB because the ears are closer to the soil surface, leading to more ready dispersal of spores from infected debris on the soil surface near the spikes. The researchers found that when they raised the dwarf isolines so that the ears were at the same height as the tall isolines the difference in type 1 resistance disappeared.

Weather:  

Mathematical models for predicting Fusarium outbreaks have used the following criteria.
 (1) ascospore spread: in the growth stages 39/41-61, at least one rain (≥4 mm), then one or more warm days (average temperature ≥16°C); (2) infection of the heads by ascospores: immediately after the spread of the ascospores in the growth stages 55-69, at least two days with rain ≥2 mm and temperature ≥17°C. (3) production of macroconidia on the upper leaves and infection of the heads: immediately after the spread of ascospores a moist (and cool) weather period; infection of the heads by high numbers of macroconidia even at low temperatures.

Weather at flowering

As ever, the over-riding issue this year will be the weather at flowering. "If conditions are right at flowering, infection is a big risk, almost irrespective of inoculum pressure. So keep an eye on weather forecasts. If you don't get a spray on in time, you'll never get infection out of the ear," says Fera's Dr Philip Jennings.

Mycotoxin producers

With all three pathogens threatening yield and quality, and F. graminearum and F. culmorum producing harmful mycotoxins, the need for robust control is clear he says. Last year's extremely testing conditions showed prothioconazole was again the product of choice, especially where Microdochium was present, since other options do not reduce it," notes Dr Jennings.

And last season also showed how crucial dose and timing is. He points out that many sprays were too late and even where follow up sprays were used, it would have only controlled later infections. "Full rate triazoles gave six days of protection, typically three days either side of the application date, which delivered good control if flowering started and finished in that period. But where flowering was extended, follow-up sprays would have been needed to prevent further infection.

"One glimmer of hope around this year's severely delayed crop development could be that crops rush through flowering very quickly. If growth stages are subject to a concertina effect, one well-timed ear spray should suffice. But rates need to be robust, he stresses.

Keep the rates up

Whilst reduced rates may not reduce the period of protection, they certainly reduce effectiveness, Dr Jennings adds. "When it comes to Fusarium I'd say don't play with rates. There's not much point using anything below half rate. Three-quarter rate isn't bad, but full rate is best, and that means a full-rate of triazole. Some formulations contain a reduced ratio of triazole, so even if the full product rate is used, you might still only apply half rate triazole, which really isn't enough," he warns.

Microdochium species are resistant to Strobilurins.