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Introduction

Pathogen Biology

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Resistance

Mechanisms

Evaluation

Disease Control

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NAL

Manitoba Agriculture and Food,
Fusarium Head Blight,
Fact Sheet

Donald Hershman
Head Scab of Small Grains in Kentucky,
PPA-38

McMullen and Stack, Fusarium Head Blight of Small Grains, NDSU State Extension Service

Administrative Regions of China

Agricultural Regions of China

Since no single measure can provide adequate protection of wheat from WHS, integrated management strategies have been employed (Bai and Chen et al., 1999). In addition to development and utilization of resistant cultivars, as mentioned above, some cultural practices have been applied in China to reduce disease severity. These practices include draining wheat fields to lower the water table and create an environment unfavorable to WHS; plowing fields to bury crop residues or removing crop residues before sowing to reduce the primary inoculum; and changing the sowing date to avoid climate conditions that are favorable for WHS at the flowering stage (Liu, 1990; Xu and Chen, 1993). However, utilization of resistant cultivars and application of fungicides have been the most common and effective disease control methods. Because no highly resistant cultivars with high yield potential are available to date, moderately resistant cultivars with better agronomic traits have been widely used for commercial wheat production. Under favorable conditions at the flowering stage, these cultivars may become infected and, thus, application of fungicides is necessary to protect these moderately resistant cultivars and susceptible cultivars from the disease. Biological control by using antagonistic organisms is a promising approach to reduce the loss due to WHS, but much work is needed before its application in the field.

Biological Control

Biological control with antagonistic microorganisms has been shown to successfully suppress some fungal diseases of wheat. Biological control strategies also have the potential to suppress WHS. Tests of WHS control by antagonistic organisms have been conducted in the laboratory and in experimental plots, but, to date, successful applications on a large scale have not been reported in China.

Strains of Bacillus spp. have shown potential as useful antagonistic microorganisms for biological control of WHS (Shi and Wang, 1991; Huang and Li et al., 1993). Strain A014 of B. subtilis showed a significant inhibitory effect on F. graminearum when the two organisms were co-cultured (Wang and Liu et al., 1992). Application of B. subtilis strain A014, and Bacillus spp. Isolate-2 and Isolate 8-2 by spraying bacterial suspensions over wheat spikes decreased visible symptoms of WHS by approximately 50% in the greenhouse and in field plots (Shi and Wang, 1991; Wang and Liu et al., 1992). Several possible mechanisms for the antagonism of Bacillus spp. to F. graminearum have been identified. Twenty-two of 184 strains of Bacillus spp. isolated from wheat leaves and spikes were inhibitory to F. graminearum in culture and induced production of large numbers of chlamydospores near the inhibition zone (Shi and Wang, 1991). Bacillus spp. Isolate-2 induced sectored growth, Isolate 91-2 induced hyphal lysis of the pathogen, and Isolates 5-1 and 5-2 induced granular condensation of the hyphal cytoplasm in F. graminearum. Some microbial metabolites may also play a role in controlling WHS. An antibiotic isolated from a strain of Streptomyces roseoalutaceus var. pallidus gave 80 to 97% control of WHS in experimental plots, but further characterization of this antibiotic has not been reported (Jin, 1989). Other biological control studies have used nutritionally competitive microbes that are epiphytes of flowers of healthy plants. When applied on wheat spikes, nutritionally competitive microbes reproduce and occupy potential infection sites for F. graminearum. Nutritionally competitive microbes (13 bacterial strains and three yeast strains) have been isolated from flowers of white gourd, maize, cucumber, towel gourd, morning glory, hot pepper, Chinese rose, and chrysanthemum in China (Dai and Zhou, 1995). A wide range of microbes (118 bacterial strains and five actinomyces strains) that are antagonistic to F. graminearum in culture have also been isolated from samples of wheat seeds, spikes, and leaves, from rice stubble, and from a variety of field soils (Dai and Zhou, 1995). Among the 139 isolates tested, only three bacterial strains (YN1 and GJ6 from soils, and Y-12 from flower of Chinese rose) gave a high level of WHS control both in greenhouse tests and in field tests, reducing visible symptoms by 60 to 70%. These strains, however, have not been identified to the species level or characterized further. Much work is needed to develop these potentially useful strains to control WHS under field conditions.

Application of Fungicides

Application of fungicides has been a common practice to control WHS in China. Effective systemic fungicides include carbendazim, thiophanate-methyl, and diniconazole. Carbendazim has been the most effective and has been widely used in China to control WHS since 1972 (Xu and Chen, 1993). The timing of fungicide application is critical for effective control. Fungicides are applied once or twice from the heading stage to the flowering stage, depending on the environmental conditions and the susceptibility of wheat cultivars. Spraying carbendazim at the heading or flowering stages of wheat generally gives 80 to 90% control of visible symptoms (Xu and Chen, 1993). Spraying carbendazim before the heading stage or after the flowering stage may not protect wheat from fungal infection (Xu and Chen, 1993). Diniconazole was recently reported to be as effective as carbendazim, but verification of its efficacy is needed (Jing and Shang et al., 1996).

To reduce application costs in practice, carbendazim is usually applied in mixtures with other fungicides, such as triadimefon, and/or insecticides. Pesticide mixtures are used widely to control a range of fungal diseases of wheat, such as WHS, powdery mildew, and rusts, and to control insects, such as aphids (Xu and Li et al., 1990; Gao and Tan et al., 1991; Zhai and Liao et al., 1991 Chen and Chen et al., 1991; Ye and Zhu et al., 1996; Zhu and Ye et al., 1997; Yu and Wang, 1998).

^a Data from Zhou, Ming-Guo et al. (1999, personal communication)
^b Percentage of resistant isolate (number of resistant isolates / number of isolates tested)
^c Not surveyed

Resistance to Fungicides

Resistance to benzimidazole fungicide can develop rapidly, due in part to their single major target site, ßtubulin. In most cases, after two to three years' exposure to benzimidazole fungicides, pathogens become resistant to the fungicides. However, despite widespread use of carbendazim to control WHS for more than 20 years both in China and in other countries, no carbendazim-resistant isolates of F. graminearum were identified among field isolates until 1992 (Xu and Chen, 1993; Lu and Zhou et al., 1998). In 1991, a carbendazim-resistant isolate of F. graminearum was obtained in the laboratory by UV induction (Zhou and Ye et al., 1995). One year later, the first naturally-occurring, carbendazim-resistant isolate was found in the field in Haining County, Zhejiang Province (Zhou and Ye et al., 1994). Thereafter, additional resistant isolates were isolated in Jiangsu Province, in Wuxian county in 1994, and in Tongzhou county in 1997 (Yang et al., 1998). Monitoring carbendazim resistance in recent years has shown that the proportion of resistant isolates in the pathogen population has been increasing rapidly. In a survey in 1999, the proportion of carbendazim-resistant isolates in Zhejiang Province was over 25% (Table 2) (Lu and Zhou et al., 1998; Zhou, M-G, 1999, personal communication). If this trend continues in the near future, a large proportion of the pathogen population is expected to become resistant to carbendazim, and application of benzimidazole fungicides may no longer be effective in controlling the disease. To date, there have no reports of large-scale failure of carbendazim application to control WHS in the field. Chinese scientists are searching for additional fungicides that are as effective as carbendazim in controlling WHS.

Application of mixtures of fungicides with different target sites or with different mechanisms of action appears to block or slow the development of resistance. Kangjunling, a new mixture of carbendazim, triadimefon, and thiram, has been more effective than carbendazim alone in field trials at various sites in China (Shao and Liu et al., 1998; Xu and Pan, 1998; Wu, 1998). This mixture is especially suitable for regions where carbendazim-resistant isolates have been found (Shao and Liu et al., 1998). Diniconazole may also be used instead of carbendazim wherever necessary (Jing and Shang et al., 1996).

The carbendazim resistance mechanism in F. graminearum may be different from that found in other fungi. In benzimidazole-resistant isolates of Aspergillus nidulans and Neurospora crassa, amino acid at sites 6, 50, 134, 165, 198, 200, and 257 of ß-tubulin have been changed (Lu and Zhou et al., 1998). The resistance in F. graminearum was also considered to be related to changes in amino acids of ß-tubulin, but preliminary studies showed that carbendazim resistance in F. graminearum is not associated with changes in amino acids of ß-tubulin at sites from 135 to 407 (Lu and Zhou et al., 1998). Other proteins may also be involved in carbendazim resistance in F. graminearum (Lu and Zhou et al., 1998). Much research is needed to clarify the unique mechanism of resistance to carbendazim in F. graminearum.

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