Fusarium head blight or scab of small grains is caused by the fungus Fusarium graminearum (Schwabe), although the Netherlands and other areas of central Europe report F. culmorum as the most prevalent species (Snidjers, 1989). In Poland, F. culmorum, F. graminearum and F. nivale have shown similar moderate to severe virulence levels, while F. avenaceum has proved to be mildly to be moderately virulent. However, in several studies aimed at identifying the causal organism, as many as 18 Fusarium spp. were isolated and identified (Mihuta-Grimm and Foster, 1989; Reis, 1985).
Scab is prevalent in warm, humid regions where flowering coincides with rainy periods. Incidence of this disease has been increasing over the last ten years for various reasons. Perhaps the most important reason being increased area where wheat is rotated with maize or other cereals. Other reasons are changes in the cropping system for soil protection purposes and changes in wheat cropping from traditional to more humid, non-traditional areas (Gilchrist et al., 1997).
Fusarium scab causes severe production losses worldwide and may be as high as 50 percent. Similarly, in Paraguay weather conditions in 1972 and 1975 favoured Fusarium and Septoria epidemics, which together accounted for losses of up to 70 percent (Viedma, 1989). Poland, the Netherlands, the United Kingdom, former Czechoslovakia, the Russian Federation, France and Austria are some of the European countries reporting scab incidence. Areas of Canada, the United States, Mexico, Guatemala, Brazil, Ecuador, Uruguay and Argentina in the Americas are highly affected by Fusarium (Ireta and Gilchrist, 1994).
Scab can cause significant yield and quality damage, as well as toxicoses in animals and humans (Ireta and Gilchrist, 1994; Baht et al., 1989; Luo, 1988; Snidjers, 1989; Marasas et al., 1988). Damage due to scab in the United States was estimated to be more than US$1 billion in 1993 and US$500 million in 1994. In China, the estimate is that scab may affect up to 7 million ha, and 2.5 million tonnes of grain may be lost in epidemic years. Diseases related to fusarial mycotoxin in humans have been reported in China, India and Japan, whereas in animals diseases have been reported in numerous parts of the world (Dubin et al., 1997).
Today, worldwide regulations exist for mycotoxins (Van Egmond and Dekker, 1995). However, in many countries regulations are not applied, and many people, especially from rural areas, eat the cereals without any control, either as cereal grains or in indirect form as meat coming from animals fed with contaminated grain. Table 16.1 shows the main effects on swine and poultry induced by the more important toxins produced by some Fusarium species.
Wet and warm weather conditions between heading and maturity increase scab severity. The point of entry of F. graminearum is the spike, especially the floral organs. This affects seed set and grainfilling. Infected spikelets quickly lose chlorophyll and become pale in colour. Later they turn pink or peach coloured, especially at the base and edges of the glumes (Plate 41). If the environmental conditions remain favourable, the infection advances to the adjacent spikelets and in some cases may infect the entire spike, including the rachis and its peduncle. When the infection is severe, damaged grains are covered with mycelia and take on the appearance of a pink cottony mass. If disease levels are moderate, the grain may be shrivelled, low in weight and whitish in colour (tombstone kernels).
Main toxic effects on swine and poultry induced by the more important toxins produced by some Fusarium species
Swollen red vulva; vaginal prolapse in swine; suckling piglets may show enlargement of vulvas; fertility problems
Vomitoxin (desoxynivalenol, DON)
Decreased feed intake and weight gain in pigs with DON at >2 mg/kg feed; vomiting; feed refusal at very high concentrations of DON (>20 mg/kg feed)a
Other trichothecenes T-2 toxin HT-2 toxin Diacetoxyescirpenol
More toxic than DON; reduced feed intake; emesis; skin and gastrointestinal irritation; neurotoxicity; abnormal offspring; increased sensitivity to disease; haemorrhaging
Mainly affects proximal tubules of the kidneys in swine and poultry; kidneys are grossly enlarged and pale; fatty livers in poultry
amg/kg = parts per million (ppm).
Source: Trenholm et al., 1984.
Fusarium graminearum (Plate 42) can attack wheat plants at all growth stages, causing seedling, stem and root rot diseases. Primary infections may arise from either ascospores or macroconidia deposited on glumes and extruded anthers. Temperatures of 10° to 30°C and relative humidity above 95 percent for 40 to 60 hours are usually enough for macroconidia to successfully infect the spikes (Ireta, 1989).
Fusarium graminearum is one of the few Fusarium species that produces perithecia under field conditions (Plate 43, Plate, 44). Perithecia represent the sexual stage of the fungus, Gibberella zeae, and are produced on wheat glumes. Perithecia play an important role in the pathogens survival from year to year (Khonga and Sutton, 1988) and coexist with mycelia in residues of the previous crop to constitute the initial inoculum source for scab. Research carried out in China demonstrated that the lowest temperature for producing perithecia was 7° to 10°C, and the most suitable was 15° to 20°C. The lowest soil moisture content for perithecia production was 50 to 60 percent, and the most suitable was 70 to 80 percent (Wang, 1997).
The factors determining disease development include climate, inoculum levels and wheat growth stage. Wheat scab epidemics depend mainly on the amount of primary inoculum, rather than secondary inoculum. Invasion occurs mostly during the flowering period. The secondary infection with conidia (Plate 45) occurs after the diseased spikes appear in the field.
Crop residues and cultural practices play an important role in the preservation of F. graminearum and, consequently, epidemics. Infections on wheat sown in a field of maize residue may be two or three times more severe (Teich and Nelson, 1984). In general, the pathogen was only saprophytic on rice stubble in the rice-wheat rotation areas and on maize stems in the dry planting areas of China (Wang, 1997). If the residue is ploughed under, perithecia survival decreases and reduces the primary inoculum source (Reis, 1989).
Fusarium graminearum is a facultative parasite and is pathogenic on many other grasses, including common weeds and cereal crops (rye, rice, barley and triticale). If practices, such as rotation with non-host crops or the management of crop residues, are not effective alone, they may in combination reduce the source of primary inoculum. Disease control is effectively based on integrated management, including proper agronomic practices, utilization of resistant or tolerant cultivars and chemical applications.
The slow progress in obtaining scab-resistant, high-yielding cultivars is due to several reasons: the complexity of resistance inheritance, poor agronomic characters linked to resistance and low industrial quality of the well-known resistance sources. Furthermore, the environment strongly interacts with the screening (evaluation) process (Galich, 1997).
Three mechanisms of scab resistance in wheat have been reported (Schroeder and Christensen, 1963; Miller et al., 1985; Miller and Armison, 1986; Wang and Miller, 1988). Type I is referred to as penetration resistance, which prevents initial fungal penetration; Type II is spread resistance, where spread of the fungus is reduced within the spike; and Type III is related to biochemical resistance, where toxins produced by the fungus are broken down by the host plant.
Scab resistance is thought to be controlled by two to three major genes with several minor genes as modifiers (Chen et al., 1997). Bai (1995) found that additive effects accounted for most of the genetic variance in scab resistance, although dominance and epistasis were also important in some spring crosses. Snijders (1990) reported similar results for winter wheat.
Large-scale application of fungicides is generally not practical in years of abundant rainfall and results in low efficiency, high cost and environmental pollution. Thus, development and utilization of disease-resistant germplasm is the most economical and efficient approach for controlling scab (Chen et al., 1997).
International Maize and Wheat Improvement Center (CIMMYT) germplasm derived from crosses with Chinese germplasm shows superior levels of scab resistance in high-yielding plant types (Diaz de Ackermann and Kohli, 1997). Although scab resistance is limited, resistance from old genetic sources from Brazil, Japan and China has been incorporated into high-yielding genotypes. Despite the slow process, significant progress has been made, and the first resistant lines (Ningmai 7, Longmai 19 and Chuanmai 25) are being released in areas of China (Gilchrist et al., 1997).
Effective seed treatment chemicals for control of seedling blight are available and include mancozeb, thiabendazole and defenoconazole (Dill-Macky, 1997). Foliar spray experimental data in Uruguay show that two applications of benomyl, prochloraz or tebuconazole at stage 61 and 65 (Zadoks scale) achieve bet-ter results than a single application at either stage 61 or 65. This provided between 49 and 76 percent of the efficiency of control con-sidered adequate under field conditions (Diaz de Ackermann and Kohli, 1997). In China, the most effective fungicide was carbendazim and thiophanate methyl. A mixed fungicide composed of carbendazim and triadimefon had a significant synergistic action. The level of control achieved was 80 to 90 percent, and yield increased by 20 percent (Wang, 1997). Highly susceptible genotypes cannot be fully protected under severe epidemic conditions.
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