Fusarium diseases of cereals
About this project
Several species of fungi grouped together in the genus Fusarium affect cereals, causing a number of different diseases and also the production of toxins (mycotoxins) which are harmful to humans and animals. The review summarises the information available on both subjects, mainly from UK sources, and defines areas where information is lacking.
The species of Fusarium which cause diseases are F. avenaceum. F. culmorum, F. graminearum, F. nivale and F. poae. All five species cause diseases of the ear and can affect the grain. They (and some others), under suitable conditions, also produce mycotoxins. F. poae causes only a disease of the ear but the other four species also cause diseases of seedlings and stem bases (foot rots, often referred to as brown foot rot). F. nivale can cause a leaf disease, including a disease called snow mould.
Fusarium diseases have been reported since the latter part of the nineteenth century. Ear blight, which is conspicuous as bleached spikelets, has attracted attention periodically. Seedling diseases can result in a thinning of the crop and are usually associated with the use of untreated seed. Foot rots are only rarely severe enough to cause whiteheads. More often the symptoms are lesions on the lowest nodes and internodes and there are reports (not well authenticated) that such infections cause a weakening of the stem which may lead to premature ripening or lodging when the crop is mature.
Infections of seedlings can result in pre- or post-emergence death or may be limited to lesions on the leaf sheaths of plants which survive. Infections by F. avenaceum, F. culmorum and F. graminearum are most severe in warm, dry soils and by F. nivale in cool, dry soils. There are important interactions between the main factors affecting seedling infections, which are soil moisture, soil temperature and spore load on the seed. All the pathogens can be seed-borne and soil-borne. Severe disease is usually associated with high spore loads on the seed (except for F. nivale under otherwise favourable conditions) so that severe seedling disease in the field is usually associated with the use of infected seed which has not been treated with a fungicide. It occurs mainly in winter wheat which is more susceptible than barley.
F. nivale, which is best adapted to the environmental conditions in the autumn, is the common cause of seedling disease. A severe thinning of the stand can lead to loss of yield but, in normal growing systems, such losses are rare.
In some soils F. nivale causes little or no disease and this has been associated with the presence of other fungi such as Gliocladium roseum and Trichoderma viride. However, neither of these fungi has provided a very good control of disease when applied artifically to seed or soil.
The seed-borne Fusarium spp. are only partially controlled by organomercury because the chemical does not eradicate deep-seated infections. The triazole fungicides, triadimenol (in "Bay tan") and flutriafol (in "Ferrax"), give a similar level of control to organomercury. Both "Bay tan" and "Ferrax" contain an MBC-fungicide to improve the control of Fusarium spp., but one of the common seed-borne pathogens, F. nivale, is now mainly resistant to MBC fungicides so that this improved control is unlikely to be maintained.
Nevertheless, the level of control given by current seed treatments is sufficient to prevent serious losses of seedlings.
Plants which survive but are infected by Fusarium provide the sources for spread to other parts of the plant. The incidence and severity of these diseases is related to the amount of the pathogen in the crop and the weather.
F. nivale causes snow mould - a disease where leaves, and sometimes the crowns, can be killed under snow cover. Snow mould occurs mainly on winter barley in Scotland and is often found together with snow rot (caused by Tvphula incarnata) which is much the more damaging disease at that stage. F. nivale also causes a brown leaf spot which is sometimes prominent on the upper leaves of wheat and especially of triticale but at present is not sufficiently severe to cause loss of yield.
ADAS surveys in recent years have shown that a mean 22-38 per cent of the stem bases of winter wheat and winter barley crops have lesions caused by Fusarium spp., though less than 1 per cent have severe lesions causing a breaking or a softening of the stem tissues. There are ad hoc reports from the field of Fusarium foot rots causing severe damage especially late in the season, but there are no objective accounts of the symptoms, the specific causes or the damage done.
There is a need for a thorough investigation of Fusarium foot rots including an assessment of the damage they cause.
There is little information on the interactions of Fusarium stem base diseases and others such as eyespot and sharp eyespot.
In a few trials a spray of the fungicide prochloraz, applied in the spring for the control of eyespot, has reduced the incidence of Fusarium foot rots by about 33 per cent. However, there is no evidence that prochloraz or any other fungicide, as used at present, will consistently give a satisfactory control of the disease. Also there is no information on varietal resistances, if any exist, to Fusarium foot rots.
The earliest signs of ear infections in wheat are water-soaked spots on the glumes. The disease can progress to affect whole spikelets which soon become bleached in contrast to the healthy green spikelets. If the infection progresses into the rachis then all parts above die and also become bleached. All five species, F. avenaceum, F. culmorum, F. graminearum, F. nivale and F. poae, have produced these symptoms following artificial inoculation. F. culmorum is said to be the most common cause in field crops but no objective surveys have been done. Symptoms caused by F. poae and F. nivale are sometimes limited to lesions with brown margins on the glumes. The disease is favoured by wet weather and a prolonged ripening period. Infections occur mainly in the flowering period and are much stimulated by the presence of pollen.
Spread of disease to the upper parts of the plant and to the ear is by the dissemination of spores from the lower part of the plant. In the case of F. nivale, ascospores produced in the perithecia which occur abundantly on leaf sheaths, are known to be an important means of spread. For this and the other species (which do not produce ascospores) it is assumed that spread is through rain-splashed or air- borne conidia. However, good information on the production and dissemination of spores is lacking.
Severe ear blight occurs erratically and even in 1982, when it was particularly prominent, assessments made in ADAS surveys indicated that national losses were small (less than 1 per cent).
There are varietal differences in susceptibility to ear blight and selection for resistance is now included in plant breeding programmes. There is little information available on the control of ear blight with fungicides.
The conditions which favour ear blight also favour the formation of Fusarium mycotoxins. About twenty mycotoxins occur naturally on cereals but many more related compounds have been identified in laboratory cultures. Fusarium mycotoxicosis is rare in humans but instances in livestock are reported more frequently. In the UK data on the subject has been collected by ADAS, the Flour Milling and Baking Research Association, the Edinburgh School of Agriculture and others. Examination of various species of Fusarium on wheat has shown that many isolates are toxigenic but their potential is rarely realised in the field, presumably because weather conditions are not favourable. Surveys have shown that some mycotoxins are frequently present at very low levels on home-grown grain but there is little evidence that they pose a widespread threat to man or livestock. One possible exception could be cereals used for livestock feed in parts of Scotland where the prolonged and often late harvest can favour the growth of Fusarium spp. on the grain.
Recommendations for further work
The recommendations for further work can be summarised as follows:
A study of the specific causes of Fusarium foot rots, the conditions affecting disease development and the effects of the diseases on the plants, especially in grain yield and quality. (Highest priority).
A survey for several years of the incidence and causes of Fusarium foot rots in field crops.
An assessment of the causes of ear blight in field crops and of the damage done.
Epidemiology to include sources of infection, the production and dissemination of spores.
A study of the factors affecting infection by the five Fusarium species known to cause ear blight symptoms.
Monitoring seed-borne disease.
Independent tests of new seed treatments and monitoring the effectiveness of chemicals on the market, especially in relation to fungicide resistance.
Surveillance of home-grown cereals in different parts of the UK for Fusarium mycotoxins, using the most sensitive methods available, in order to establish their incidence. The investigation to be done over several years so as to include different climatic conditions at harvest.
An assessment of the significance of Fusarium mycotoxins in grain to establish "acceptable" levels of contamination for each mycotoxin or combination of mycotoxins at which no harmful effects on man or livestock would be expected.
This Review (89 pages) was completed in October 1988.
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