Fusarium and microdochium in cereals

This complex of diseases, which can be seedborne, soilborne or trashborne, causes a variety of symptoms, including seedling blight, foot/crown rot and ear (head) blight and the production of mycotoxins. Learn about the pathogens and how to manage them.

Many species of fusarium affect cereals (wheat, barley, oats, rye and triticale), as well as grasses.

Fusarium avenaceum, F. culmorum, F. graminearum, F. poae, F. langsethiae, Microdochium nivale and M. majus

These fungi form a disease complex on seeds, seedlings and adult plants. The seed-borne pathogens Microdochium nivale and M. majus (formerly known collectively as Fusarium nivale) are also included in this group. M. nivale also causes snow mould.

Fusarium seedling blight

M. nivale is the primary pathogen in the group that causes seedling blight. Seedling blight causes pre- and post-emergence damping off. This can result in seedling death and poor establishment. Surviving seedlings may develop a brown lesion around soil level. This can develop into foot and root rot. Symptomless infections can also occur.

Fusarium foot rot/crown rot

Foot rot becomes obvious from late stem extension onwards. It results in dark-brown staining of the lower nodes. Long dark streaks may also appear at the stem base. On older plants, fusarium infection can produce a true foot rot, where the stem base becomes brown and rotten, resulting in lodging and whiteheads. This symptom is less common in the UK but can develop very dry seasons.

Fusarium ear (head) blight

Fusarium species cause a range of symptoms on the ear. Bleached ears often show above the point of infection around the milky ripe stage (GS 75). Later infections may result in infection of the grain without obvious bleaching of the ears. The presence of orange/pink fusarium spores may also be visible on infected spikelets. As the crop ripens, symptoms become less visible. At harvest, fusarium ear blight can result in shrivelled grains with a chalky white or pink appearance, although this is not always the case. There is little correlation between fusarium-damaged grains and mycotoxin occurrence. Therefore, the presence of ear blight symptoms is not a good indicator of mycotoxin risk.

Primary infection by fusarium is from infected seed, soil, crop debris and volunteers or host weed species. Spores – from seedling blight or foot rot lesions – that are splashed up the plant or move from leaf to leaf are the main source of ear blight infection. For some fusarium species, spores are also wind-spread, although this is not an important infection source. Ear blight infection occurs during flowering. It infects the grain and completes the life cycle.

Environmental conditions affect disease development and fusarium species have different temperature requirements. For example, M. nivale seedling blight is most severe under cool, wet soil conditions, whereas F. graminearum seedling blight is most severe under warmer, drier soil conditions. Warm, wet, humid conditions during flowering favours infection by fusarium species, causing ear blights and seed-borne infection. Further rainfall and humid conditions allow secondary infection to occur, allowing further fungal growth and mycotoxin production.

Most cereal crops develop fusarium symptoms each year. F. culmorum and F. graminearum are the most commonly found species in the UK that cause ear blights. Although infection by fusarium species can cause poor establishment and lower yields, the most important issue is the production of mycotoxins in the grain by some species (see mycotoxin section below). However, the presence of ear blight is not a good indicator of likely mycotoxin risk. Mycotoxins are present at lower levels in barley and oats compared to wheat. The overall risk of DON exceeding legal limits in wheat is low and in barley and oats is very low.

Wheat

F. graminearum and M. nivale cause the most significant seedling losses in UK wheat. However, crops usually compensate from the loss of a few plants through tillering. F. graminearum is more common in a maize-producing area, whereas M. nivale is more generally distributed. Severe foot rotting in wheat is very rare in the UK because badly infected seed is not used and seed treatments are effective, and losses are generally very small.

High levels of ear blight can occur, especially when conditions are conducive (e.g. wet) during flowering, but yield losses are rarely serious. Seed saved from these crops can suffer from poor establishment, unless the seed is treated with a product effective on fusarium.

The mycotoxins DON and ZON are frequently detected in wheat but average concentrations are usually below the legal limits. Limits are most frequently exceeded in wet harvest years.

Barley

Seedling blight in barley due to fusarium species is rare, but may occur where there are very high infection levels and seed is sown into cold seedbeds. Losses are generally not as high as those seen for wheat.

Early sown spring barley is at more risk of M. nivale seedling blight infection.

Ear blight and mycotoxin risk is also lower in barley than in wheat, but should be considered if barley is commonly grown in rotation with maize with minimum tillage. Developed for wheat, the AHDB fusarium mycotoxin risk assessment tool is also useful for assessing risk in barley.

DON, ZON, HT-2 and T-2 levels in barley have been routinely low with legal limits rarely exceeded.

Oats

Oats are more resistant to fusarium infection than wheat and barley, and it is difficult to see the symptoms in this crop. Symptoms can include premature plant death or bleaching of spikelets.

F. langsethiae is the predominant species that infects oats and produces the mycotoxins HT-2 and T-2. There is good evidence that at least 90% of mycotoxins are removed during dehulling. Previous Food Standards Agency surveys of fusarium mycotoxins in retail oat products have identified that exposure to these toxins from oat products in the UK diet is very low.

T-2 and HT-2

T-2 and HT-2 mycotoxins are produced by fusarium species that are favoured by drier conditions, such as F. langsethiae. Therefore, risk factors are different to those for DON/ZON. Currently (2019), there are no legal limits for T-2 and HT-2. In 2013, the European Commission published a Recommendation that included indicative levels for the combined concentration of T-2 and HT-2. The Recommendation states that Member States, in conjunction with industry, should continue to monitor these mycotoxins and, where they exceed the indicative level, conduct investigations to determine why the exceedances occurred and what mitigation can be used to avoid exceedances occurring in the future.

Table 2. EU indicative levels (I) for the combined concentration of HT-2 and t-2 (ppb) in unprocessed cereals

• Legal limits – maximum levels for specific mycotoxins in cereals and cereal products, as defined by European Commission regulations and applied at the point of sale
• Guideline limits – guidance as to the acceptability of feed and feedstuffs
• Indicative levels – guidance on when to investigate high levels to identify mitigating actions

The risk of fusarium diseases can be minimised throughout the season: from rotation planning, to deciding which field to harvest first.

Pink grains indicate possible fusarium infection

Ear blight infection can cause bleached ears