Recent Advances in Wheat Head Scab Research in China
Li-Feng Chen, Gui-Hua Bai, and Anne E. Desjardins
Introduction
USDA, Agricultural
Research Service
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Introduction |
Distribution of Wheat Head Scab in China
Green = Severe
Orange = Mild
Yellow = Sporadic
Robert Stack, Fusarium Head
Blight of Small Grains,
APSnet Article
Hershman et al.,
Cooperative WHS Survey 1998-1999, 1999-2000
Wheat (Triticum aestivum) is the
second most important food crop in China. Wheat cultivation occupies
an estimated 30,000,000 hectares and is exceeded only by rice
in cultivated area and grain production (Xu and Chen, 1993).
Farmers in China grow spring wheat cultivars which are not winter
hardy, and winter wheat cultivars which are able to survive the
winter by vernalization. The first outbreak of WHS was reported
in 1936 in regions along the middle and lower reaches of the
Yangtze River in central China. In this outbreak, the percentage
of scabbed spikes was as high as 95% in cultivar Jinda 2905 in
Xuancheng County, Anhui Province (Xu and Chen, 1993). Since then,
WHS has occurred frequently in winter wheat in central China,
including the provinces of Jiangsu, Zhejiang, Anhui, Hubei, and
the municipality of Shanghai. WHS has also occurred in spring
wheat in northeast China, including Heilongjiang Province. During
the last 15 years, an increasing number of wheat-growing regions
throughout the country, including the provinces of Henan, Shaanxi,
Ningxia, Gansu, Qinghai, Hebei, Shandong, and Sichuan have been
affected by the disease (Li, 1989; Zhang, 1989; Yang and Li,
1995; Gan and Zhang et al., 1996; Liu, 1997; Yu, 1997; Lu and
Tan et al., 1998; Zhang, 1998; Guo and Li et al., 1999). In 1985,
an outbreak of WHS occurred in an area of approximately 3,300,000
hectares in Henan Province (Li, 1989). From 1987 to 1997 in Sichuan
Province, there were five serious WHS epidemics in the years
1989, 1990, 1992, 1996, and 1997, a
frequency much higher than that in other areas (Li, 1996; Liu,
1997). In 1998, wheat growers in China suffered another severe
epidemic of WHS. In the regions of the Huai River and the Yellow
River, this epidemic is the most severe WHS outbreak in the historical
record (Bai and Fan, 1998; Lu and Tan et al., 1998; Zhang, 1998;
Guo and Li et al., 1999; Shi and Wang, 1999; Zhu, 1999). All
available data indicate that WHS epidemics are becoming more
frequent, more severe, and more widespread in China.
Economic Losses Due to Wheat Head Scab
WHS epidemics can cause great, direct yield losses. In severe epidemics, the percentage of scabbed spikes (disease incidence) is usually 30 to 50%, but can exceed 60 to 70% in the most susceptible cultivars in the most severe epidemics (Zhang, 1998; Bai and Fan, 1998; Lu and Tan, 1998; Shi and Wang, 1999). The epidemic of WHS in Henan Province in 1985 caused a yield loss of 890,000 metric tons (Li, 1989). The outbreak of WHS in Sichuan Province in 1996 resulted in a yield loss of about 78,000 metric tons (Li, 1996). Data on wheat yield losses for the whole country due to head scab epidemics are not yet available.
In addition to direct yield losses, WHS causes further indirect economic losses. The quality of WHS infected wheat kernels is reduced due to changes in protein content and to contamination with mycotoxins (Xu and Chen, 1993; Wu and Peng, et al., 1996; Wu and Wu et al., 1997). When used as seeds, the infected kernels germinate poorly, and the seedlings can easily become blighted, which indirectly reduces the yield (Xu and Chen, 1993; Bai and Shaner, 1994). F. graminearum, the major pathogen of WHS, produces two major classes of mycotoxins: trichothecenes and zearalenone. Trichothecenes are toxic to human beings and animals, and cause dizziness, headaches, nausea, vomiting, abdominal pain, diarrhea, fever, and sleepiness in human beings, and cause food refusal and diarrhea in animals (Xu and Chen, 1993). Zearalenone causes infertility and abortion in pregnant female animals, especially in pigs (Xu and Chen, 1993). In China, some human diseases, such as Kashin-Beck disease and esophageal cancer, have been epidemiologically associated with consumption of trichothecenes, but the correlation has not been well established (Chu and Li, 1994; Zhang and Guo et al., 1996; Yang, 1997).
Guidelines for Human Consumption of Scabbed Wheat
Although the role of mycotoxins in human
diseases is unclear, some preventive measures have been taken
by the Chinese government. To safeguard human health, a limitation
of 5 to 6% scabbed grains in wheat was established in the early
1960s (Lu and Xue, 1995). Wheat with more than 6% scabbed grains
was not allowed for human or animal consumption. This standard
has subsequently been modified to 4% scabbed grains and accepted
as an official standard by the Chinese government (Xu and Chen,
1993). A stricter standard with 3% scabbed grains has been proposed,
but has not yet been approved by the government (Lu and Xue,
1995).
Two recent surveys of naturally contaminated wheat in China have
shown that the levels of the trichothecene toxin, deoxynivalenol
(DON), are correlated with the
percentage of scabbed kernels (Lu and Xue, 1995; Xie and Wang,
1999). Standards based on levels of DON, i.e., no more than 2,000
µgram of DON per kg of wheat, have been proposed (Luo, 1989; Xie
and Wang, 1999), and are pending official approval by the Chinese
government.
Brief History of Wheat Head Scab Research in China
In China, research on WHS began in the
1930s. First, Wu (1939) described the disease symptoms, pathogen
morphology, life cycle, and host range, and the environmental
conditions favorable for the disease. Then, Dai (1941) found
remarkable differences in wheat resistance to head scab among
41 wheat cultivars. From the 1950s to the 1960s, research focused
mainly on pathogen biology, factors affecting disease development,
and chemical control. The establishment of the China Wheat Scab
Cooperation Group (CWSCG) led to great progress in the 1970s
and 1980s. CWSCG identified 18 Fusarium species causing WHS in
China, and developed several techniques to test wheat resistance
to head scab. CWSCG also identified Sumai 3, Ning 7840, Wangshuibai,
and other good resources for WHS resistance breeding by screening
more than 30,000 collections of wheat germplasm from China and
other countries (Wang and Liu et al., 1989). In addition, CWSCG
proposed an integrated control strategy for WHS that combined
the use of resistant cultivars with application of fungicides
to reduce the yield loss due to WHS. Fungicide mixtures and application
strategies have been developed to slow the development of resistance
in F. graminearum to carbendazim, a benzimidazole fungicide
that has been widely used for WHS control in China (Shao and
Liu et al., 1998). Recently, wheat breeders have been continuing
their efforts to develop resistant cultivars by combining conventional
breeding methods and new techniques, such as tissue culture,
recurrent selection, and transfer of head scab resistant germplasm
from alien species (Liu, 1994; Lu and Jiang et al., 1995; Jiang
and Wu, 1996). Plant pathologists and wheat breeders have also
initiated research on physiological and biochemical mechanisms
of WHS resistance, which should provide basic information to
aid WHS resistance breeding.
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