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Abstract Sugarcane smut disease ( Ustilago scitaminea Sydow) is an important fungal disease worldwide, which seriously affects yield and quality of sugarcane and threatens the stability and development of sugarcane industry. This paper reviewed disease occurrence, damage, pathogen, symptoms, genetic diversity and detection methods of sugarcane smut, response of sugarcane to sugarcane smut stress and control measures of sugarcane smut. The in-depth research of sugarcane smut was also prospected, aiming to provide theoretical reference and scientific basis for the effective control of sugarcane smut.
Key words Sugarcane; Smut; Pathogen; Stress response; Control strategies
Received: January 26, 2020Accepted: March 19, 2020
Supported by Sugar Crop Research System (CARS-170303); the Yunling Industry and Technology Leading Talent Training Program "Prevention and Control of Sugarcane Pests" (2018LJRC56);andthe Yunnan Province Agriculture Research System (YNGZTX-4-92).
Changmi WANG (1991-), male, P. R. China, research assistant, master, devoted to research about sugarcane disease.
*Corresponding author. E-mail: huangyk64@163.com.
Sugarcane is an important cash crop, mainly used for the production of sucrose, ethanol, biofuels and fiber-related products［1］. There are more than 100 sugarcane planting countries around the world, with a planting area of 260.1 million hectares and an output of about 1.83 billion tons of sugar［2］. Chinas planting area is second only to Brazil and India［3］, and the main sugarcane areas include central and southern Guangxi, southwestern Yunnan, west of Guangdong, and northern Hainan［4］. Sugarcane smut is a systemic disease. Historically, the disease was prevalent in some sugarcane areas, which caused hindered growth of sugarcane plants, reduced effective stems, increased fiber content and reduced sucrose content［5-6］, resulting in a significant economic loss. It is still causing economic losses to varying degrees. This paper described the occurrence, harm, pathogen, symptoms and genetic diversity of sugarcane smut, sugarcane response to smut stress, and methods for prevention and treatment of smut, hoping to provide a theoretical basis and scientific basis for effective prevention and control of smut.
Occurrence and Damage Conditions
Sugarcane smut is also known as sugarcane Bianheisuibing［7］. The disease was first discovered in Natal, South Africa in 1877［8］, and has since occurred throughout the world［9-12］. Before 1940, the disease only occurred in the Eastern Hemisphere［13］, spread to Hawaii, United States in 1971, and occurred in Florida in 1978, so far in all sugarcane regions except New Guinea［14］. Sugarcane smut was first reported in Guangdong, China ［7］. In recent years, it has generally occurred in major sugarcane-producing areas in Yunnan, Guangdong, Guangxi, and Hainan［5］. The main varieties such as Taitang 134, Chuantang 61-408, Guitang No.11 and Guitang 12 at different periods and the current main varieties such as Xintaitang 22 and Mintang 69-421 are facing elimination due to high infection of smut［5］. The occurrence of smut is becoming more and more serious, especially in ratoon cane. Some studies have shown that the yield of new plants is lost by 39%-56%, and the yield of ratoon plants suffers from a 52%-73% loss［15］. The loss in the yield of new plants of Indian susceptible variety Co312 was 29%, while the loss in the ratoon cane reached 70%［16］. A 48% loss was found in the new plant yield of South African susceptible variety NCo376, and a 90% loss was reached in the ratoon cane yield［17］. Australian variety Q157 showed an average yield loss of 26%, and the worst yield loss reaching 62%［18］.
Symptoms, Pathogen, Genetic diversity and Molecular detection
Symptoms
Sugarcane plants are susceptible to smut in the early stages of growth, and the tips of young shoots are also susceptible to smut in the early stages of growth［19］. After being infected, sugarcane plants showed increased tillering［20］, light green slender leaves, tall and straight apical leaves, small cane stems and sparse nodes［21］. As the smut is spreading, it gradually forms non-branched whips that point downward and inward on the tip［22］. The whips are early white and produce a large number of chlamydospore after 2-4 months, forming black whips ranging from several centimeters to tens of centimeters in length, and the black whips reach its largest length after 6-7 months［20,22］. Spores germinate at the base of immature bud scales to form appressorium［23］. The appressorium penetrates sugarcane tissue cells to form highly differentiated specific hyphae in the cells［24］, resulting in severe deformation and even vacuolation of sugarcane tissue cells, cell wall breaking, nucleus and nucleolus deformation or degradation, uneven chromatin distribution, papillae on nucleoli and complete degradation of mitochondria［25］.
Pathogen
Sugarcane smut is a fungal disease caused by Ustilago scitaminea Sydow［2］. The pathogen is a systemic parasite in the host, in addition to parasitizing in sugarcane, it also parasitizes cogon［5］. The pathogen has a genome size of 19.8 Mb and encodes 6 677 proteins［25］. The optimal growth temperature range is 25-30 ℃, the optimal temperature is 28 ℃, and the relative humidity is above 80%［22］. Neutral or acidic conditions are good for colony growth, and light has little effect on colony growth［26］. Spores are mainly transmitted by airflow, followed by irrigation water and rainwater［2］. Diseased seedcane is the source of long-distance transmission and the source of the first infection in the occurrence areas; and infected ratoon cane, the soil with the pathogen and field-infected weeds are also the sources of the pathogen in some areas［17-18］. Under suitable environmental conditions, winter spores germinate to form binucleated mycelia. Binucleated mycelia formed two types of haploid spores, "+" and "-". Separate "+" or "-" haploid spores could not form infectious mycelia, only "+" and "-" mating type haploid spores can produce infective mycelia. In this process, the fusion and pathogenicity of mycelia are regulated by conserved gene loci a and b, respectively［27］. Genetic diversity of U. scitaminea
There is a common phenomenon of physiological race differentiation in U. scitaminea . There are two physiological races A and B in Hawaii, at least two physiological races in Brazil, and five physiological races in Pakistan［28］. In the 1990s, it was reported that there were two physiological races in China, namely race 1 and race 2, of which race 1 was the dominant race［29］. The results of a survey of sugarcane smut disease in China in 2001 showed that there were 3 races in Taiwan and 2 elsewhere［30］. Analysis of genetic diversity showed that African and American U. scitaminea strains belong to the same lineage, which also existed in Asian populations, indicating that Asia is the origin of sugarcane smut disease［31］. The genetic diversity of 30 U. scitaminea strains in the sugarcane area of Guangdong showed a significant correlation with the geographical origins of the strains, and there was little correlation between the varieties［32］. There are 23 U. scitaminea strains in 6 sugarcane areas of Guangxi, Yunnan, Guangdong, Hainan, Fujian, and Jiangxi in China, which can be divided at the genus level and can also be distinguished among related species［33］. There is no evidence of co-evolution with sugarcane［34］.
Molecular detection
Shen et al. ［35］ found that PCR and nested PCR were used for early detection of smut. Su et al. ［36］ found that TaqMan quantitative real-time polymerase chain reaction can also be used for early detection of smut. Albert et al. ［37］ developed a specific primer pair bE4/bE8, Shen et al. ［38］ used specific primers SL1/SR2 to detect smut molecularly. Shen et al. ［39］ also developed a specific loop-mediated isothermal amplification technology that specifically detects the presence of smut based on F3/B3 and FIP/BIP primer pairs. Li［40］ found that enzyme-linked immunosorbent assay can be used for early smut detection, and Singh et al. ［41］ could use meristematic staining and light microscopy techniques to determine whether the material is infected with sugarcane smut.
Sugarcane Response to U. scitaminea Stress
Response of different tissues to U. scitaminea stress
The host plant not only responds to U. scitaminea stress through physical structural defense, but also responds to U. scitaminea stress through changes in the content of biochemical substances in different tissues［6］. Callose deposits around the cells infected by U. scitaminea mycelia, inhibiting mycelial infection, which is a typical structural defense. The accumulation of glycosides in bud scales inhibits spore germination and mycelial growth［42］. Changes in the content of glycoproteins in fruit juice affect the cytoplasmic polarity of spore germination and inhibit germ tube production and spore germination［43］. The changes in these different tissue biochemical substances are typical physiological and biochemical response. Defense enzyme system response to U. scitaminea stress
After infecting sugarcane with U. scitaminea , arginine active enzymes with defensive function were detected in healthy and disease-resistant sugarcane juice, but not in the juice of susceptible varieties［44］. And disease-resistant varieties mayar55-14, GT2, NCo376, and ROC10 were found to have higher phenylalanine ammonia lyase activity than the susceptible varieties Barbados42231, ROC22, F134, and FN83-0607［23,45］. Catalase activity of resistant varieties Yacheng 05-179 and GT28 was always higher than that of susceptible varieties Liucheng 03-182 and ROC22［45］. Resistant varieties GT28 had higher activity of superoxide dismutase (SOD) and polyphenol oxidase (PPO) than susceptible variety ROC22［23］. Resistant varieties NCo376 and ROC10 showed that the activity of tyrosine ammonia lyase (TAL) and 4-coumaric acid coenzyme A ligase increased, which was significantly higher in amplitude and duration than highly susceptible varieties F134 and FN83-0607［45］. It was also found that β-1,3-glucanase in resistant variety Yacheng 05-179 had a longer duration than susceptible variety Liucheng 03-182［46］.
Gene response to U. scitaminea stress
After inoculation with U. scitaminea , smut-related genes responded to U. scitaminea stress with different expression patterns and differential expression. Of the 192 U. scitaminea virulence-related genes, 31 were expressed at various stages, and 32 were expressed at different stages of sugarcane infection［25］. The disease-resistant variety Yacheng 05-179 showed expression of resistance genes more significant in the treatment with the inoculation of U. scitaminea at 48 h than in the control at 24 and 120 h, also more significant than the expression in susceptible variety ROC22 at 48 h［47］, in response to U. scitaminea stress. The expression of the three candidate genes ScBAK1, ScMapkk and ScGloI in the mitogen-activated protein kinase signaling pathway［48］ and related sugarcane enzyme genes and the differential expression of lignin synthesis-related genes CCR, ubiquitin-mediated protein degradation-related gene UCH-LE5, genes related to the interaction between plants and pathogens GK, MLO and HIR1, and EIL3 related to the ethylene pathway also responded to U. scitaminea stress.
Control
Use of resistant varieties The use of smut-resistant varieties is an effective measure to prevent and control the disease［49］. Several sugarcane-growing countries in the world, such as the United States, Cuba, India, Brazil, Australia, France, and China, have made sugarcane smut resistance a major goal of variety selection［50］. According to the local practice, the selection of disease-resistant varieties Xintaitang 10, Xintaitang 16, Xintaitang 20, Yuetang 93-159, Yuetang 00-236, Yuetang 00-318, Guitang 02-901, Guitang 31, Yunzhe 01-1413, Yunzhe 03-194, Yunzhe 06-80, Yunzhe 08-2060, Funong 15 and Funong 36 is the basis of sugarcane smut control［51］.
Agricultural control
Before planting, applying sufficient base fertilizer, timely irrigation, timely fertilization and soil cultivation, and reasonable application of nitrogen, phosphorus, and potassium fertilizers are effective to promote the robust growth of sugarcane seedlings and enhance the disease resistance of sugarcane plants. It is necessary to remove diseased plants in time and centrally destroy them to reduce the source of repeated infections and control the spread of the disease. The rotation of sugarcane and rice, corn, sweet potato, peanut, soybean and alfalfa should be strengthened, to reduce the long-term infection and accumulation of U. scitaminea and reduce disease occurrence［3］.
Chemical prevention
Studies have shown that 2.5 mg/L pentachloronitrobenzene and 2.5 mg/L didecyldimethylammonium chloride, 50 mg/L propiconazole, 100 mg/L triazolone and 200 g/L cyprokonazol can significantly reduce spore germination［52］; and 250 g/L azoxystrobin suspension, 250 g/L difenoconazole, 300 g/L difenoconazole·propiconazole emulsifiable concentrate, 325 g/L difenoconazole·azoxystrobin and 250 g/L propiconazole emulsifiable concentrate have a good control effect on sugarcane smut［53］. The study of Huang et al. ［54］ showed that soaking seeds with 2%-3% lime water for 24 h, soaking seeds at 52 ℃ in hot water for 20 min, soaking seeds with 40% seed dressing double wettable powder, 40% seed dressing wettable powder, 25% triazolone wettable oxazolone powder, 80% mancozeb zinc wettable powder and 500-800 times dilution of 2.5% triticonazole suspension seed coating agent for 10 min, all have a certain control effect; and if the seeds are soaked with 50 ℃ hot water for 2 h, the disease can be thoroughly controlled.
Biological control
It has been reported that fungi Fusarium moniliforme ［Gibberella fujikuroi］var. subglutinans , Aspergillus niger , A. flavus and Penicillium sp. have inhibitory effects on the spore germination of sugarcane smut［55］. Liao et al. ［56］ isolated a few bacterial strains having a strong antagonistic ability against U. scitaminea . The bacterial strain ST4 isolated from the rhizosphere can block the sexual mating of U. scitaminea ［57］. The Bacillus bacterium BGMRC03005 has an inhibitory effect on the morphology of three species of U. scitaminea ［57］. At present, no effective studies of these biological agents in field control of sugarcane smut have been reported［58］. Changmi WANG et al. Research Progress and Prospect of Sugarcane Smut
Prospects
At home and abroad, researches on the occurrence and damage of sugarcane smut, the interaction between pathogens and hosts, and prevention and control have achieved a lot of results, but the following aspects need further exploration.
Distribution of physiological races: The pathogenicity of pathogens and the differentiation of physiological races are common; and defining the distribution of physiological races is very important for the prevention of smut and the selection of resistant varieties. Analysis of different rotation methods: Although the rotation of sugarcane with rice, corn, sweet potato, peanut, soybean and alfalfa reduce the occurrence of diseases, the mechanism explanation and the exploration of the occurrence law need to be further deepened. Use of smut resistance genes:Introducing smut resistance genes through Agrobacterium -mediated transformation or gene gun transformation can be performed to increase the expression of smut resistance genes; and the expression of smut-causing genes can be silenced by techniques such as gene methylation or RNA interference, thereby improving the resistance of sugarcane plants to smut. Research and development of new biological agents: The use of biological agents has a small impact on people and the environment, has little pollution to agricultural products, and is also conducive to the protection of beneficial insects. Therefore, excavation of biochemical substances with antimicrobial function and screening of antagonistic fungi and bacteria are of great significance for the prevention of smut.
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Editor: Yingzhi GUANGProofreader: Xinxiu ZHU
Key words Sugarcane; Smut; Pathogen; Stress response; Control strategies
Received: January 26, 2020Accepted: March 19, 2020
Supported by Sugar Crop Research System (CARS-170303); the Yunling Industry and Technology Leading Talent Training Program "Prevention and Control of Sugarcane Pests" (2018LJRC56);andthe Yunnan Province Agriculture Research System (YNGZTX-4-92).
Changmi WANG (1991-), male, P. R. China, research assistant, master, devoted to research about sugarcane disease.
*Corresponding author. E-mail: huangyk64@163.com.
Sugarcane is an important cash crop, mainly used for the production of sucrose, ethanol, biofuels and fiber-related products［1］. There are more than 100 sugarcane planting countries around the world, with a planting area of 260.1 million hectares and an output of about 1.83 billion tons of sugar［2］. Chinas planting area is second only to Brazil and India［3］, and the main sugarcane areas include central and southern Guangxi, southwestern Yunnan, west of Guangdong, and northern Hainan［4］. Sugarcane smut is a systemic disease. Historically, the disease was prevalent in some sugarcane areas, which caused hindered growth of sugarcane plants, reduced effective stems, increased fiber content and reduced sucrose content［5-6］, resulting in a significant economic loss. It is still causing economic losses to varying degrees. This paper described the occurrence, harm, pathogen, symptoms and genetic diversity of sugarcane smut, sugarcane response to smut stress, and methods for prevention and treatment of smut, hoping to provide a theoretical basis and scientific basis for effective prevention and control of smut.
Occurrence and Damage Conditions
Sugarcane smut is also known as sugarcane Bianheisuibing［7］. The disease was first discovered in Natal, South Africa in 1877［8］, and has since occurred throughout the world［9-12］. Before 1940, the disease only occurred in the Eastern Hemisphere［13］, spread to Hawaii, United States in 1971, and occurred in Florida in 1978, so far in all sugarcane regions except New Guinea［14］. Sugarcane smut was first reported in Guangdong, China ［7］. In recent years, it has generally occurred in major sugarcane-producing areas in Yunnan, Guangdong, Guangxi, and Hainan［5］. The main varieties such as Taitang 134, Chuantang 61-408, Guitang No.11 and Guitang 12 at different periods and the current main varieties such as Xintaitang 22 and Mintang 69-421 are facing elimination due to high infection of smut［5］. The occurrence of smut is becoming more and more serious, especially in ratoon cane. Some studies have shown that the yield of new plants is lost by 39%-56%, and the yield of ratoon plants suffers from a 52%-73% loss［15］. The loss in the yield of new plants of Indian susceptible variety Co312 was 29%, while the loss in the ratoon cane reached 70%［16］. A 48% loss was found in the new plant yield of South African susceptible variety NCo376, and a 90% loss was reached in the ratoon cane yield［17］. Australian variety Q157 showed an average yield loss of 26%, and the worst yield loss reaching 62%［18］.
Symptoms, Pathogen, Genetic diversity and Molecular detection
Symptoms
Sugarcane plants are susceptible to smut in the early stages of growth, and the tips of young shoots are also susceptible to smut in the early stages of growth［19］. After being infected, sugarcane plants showed increased tillering［20］, light green slender leaves, tall and straight apical leaves, small cane stems and sparse nodes［21］. As the smut is spreading, it gradually forms non-branched whips that point downward and inward on the tip［22］. The whips are early white and produce a large number of chlamydospore after 2-4 months, forming black whips ranging from several centimeters to tens of centimeters in length, and the black whips reach its largest length after 6-7 months［20,22］. Spores germinate at the base of immature bud scales to form appressorium［23］. The appressorium penetrates sugarcane tissue cells to form highly differentiated specific hyphae in the cells［24］, resulting in severe deformation and even vacuolation of sugarcane tissue cells, cell wall breaking, nucleus and nucleolus deformation or degradation, uneven chromatin distribution, papillae on nucleoli and complete degradation of mitochondria［25］.
Pathogen
Sugarcane smut is a fungal disease caused by Ustilago scitaminea Sydow［2］. The pathogen is a systemic parasite in the host, in addition to parasitizing in sugarcane, it also parasitizes cogon［5］. The pathogen has a genome size of 19.8 Mb and encodes 6 677 proteins［25］. The optimal growth temperature range is 25-30 ℃, the optimal temperature is 28 ℃, and the relative humidity is above 80%［22］. Neutral or acidic conditions are good for colony growth, and light has little effect on colony growth［26］. Spores are mainly transmitted by airflow, followed by irrigation water and rainwater［2］. Diseased seedcane is the source of long-distance transmission and the source of the first infection in the occurrence areas; and infected ratoon cane, the soil with the pathogen and field-infected weeds are also the sources of the pathogen in some areas［17-18］. Under suitable environmental conditions, winter spores germinate to form binucleated mycelia. Binucleated mycelia formed two types of haploid spores, "+" and "-". Separate "+" or "-" haploid spores could not form infectious mycelia, only "+" and "-" mating type haploid spores can produce infective mycelia. In this process, the fusion and pathogenicity of mycelia are regulated by conserved gene loci a and b, respectively［27］. Genetic diversity of U. scitaminea
There is a common phenomenon of physiological race differentiation in U. scitaminea . There are two physiological races A and B in Hawaii, at least two physiological races in Brazil, and five physiological races in Pakistan［28］. In the 1990s, it was reported that there were two physiological races in China, namely race 1 and race 2, of which race 1 was the dominant race［29］. The results of a survey of sugarcane smut disease in China in 2001 showed that there were 3 races in Taiwan and 2 elsewhere［30］. Analysis of genetic diversity showed that African and American U. scitaminea strains belong to the same lineage, which also existed in Asian populations, indicating that Asia is the origin of sugarcane smut disease［31］. The genetic diversity of 30 U. scitaminea strains in the sugarcane area of Guangdong showed a significant correlation with the geographical origins of the strains, and there was little correlation between the varieties［32］. There are 23 U. scitaminea strains in 6 sugarcane areas of Guangxi, Yunnan, Guangdong, Hainan, Fujian, and Jiangxi in China, which can be divided at the genus level and can also be distinguished among related species［33］. There is no evidence of co-evolution with sugarcane［34］.
Molecular detection
Shen et al. ［35］ found that PCR and nested PCR were used for early detection of smut. Su et al. ［36］ found that TaqMan quantitative real-time polymerase chain reaction can also be used for early detection of smut. Albert et al. ［37］ developed a specific primer pair bE4/bE8, Shen et al. ［38］ used specific primers SL1/SR2 to detect smut molecularly. Shen et al. ［39］ also developed a specific loop-mediated isothermal amplification technology that specifically detects the presence of smut based on F3/B3 and FIP/BIP primer pairs. Li［40］ found that enzyme-linked immunosorbent assay can be used for early smut detection, and Singh et al. ［41］ could use meristematic staining and light microscopy techniques to determine whether the material is infected with sugarcane smut.
Sugarcane Response to U. scitaminea Stress
Response of different tissues to U. scitaminea stress
The host plant not only responds to U. scitaminea stress through physical structural defense, but also responds to U. scitaminea stress through changes in the content of biochemical substances in different tissues［6］. Callose deposits around the cells infected by U. scitaminea mycelia, inhibiting mycelial infection, which is a typical structural defense. The accumulation of glycosides in bud scales inhibits spore germination and mycelial growth［42］. Changes in the content of glycoproteins in fruit juice affect the cytoplasmic polarity of spore germination and inhibit germ tube production and spore germination［43］. The changes in these different tissue biochemical substances are typical physiological and biochemical response. Defense enzyme system response to U. scitaminea stress
After infecting sugarcane with U. scitaminea , arginine active enzymes with defensive function were detected in healthy and disease-resistant sugarcane juice, but not in the juice of susceptible varieties［44］. And disease-resistant varieties mayar55-14, GT2, NCo376, and ROC10 were found to have higher phenylalanine ammonia lyase activity than the susceptible varieties Barbados42231, ROC22, F134, and FN83-0607［23,45］. Catalase activity of resistant varieties Yacheng 05-179 and GT28 was always higher than that of susceptible varieties Liucheng 03-182 and ROC22［45］. Resistant varieties GT28 had higher activity of superoxide dismutase (SOD) and polyphenol oxidase (PPO) than susceptible variety ROC22［23］. Resistant varieties NCo376 and ROC10 showed that the activity of tyrosine ammonia lyase (TAL) and 4-coumaric acid coenzyme A ligase increased, which was significantly higher in amplitude and duration than highly susceptible varieties F134 and FN83-0607［45］. It was also found that β-1,3-glucanase in resistant variety Yacheng 05-179 had a longer duration than susceptible variety Liucheng 03-182［46］.
Gene response to U. scitaminea stress
After inoculation with U. scitaminea , smut-related genes responded to U. scitaminea stress with different expression patterns and differential expression. Of the 192 U. scitaminea virulence-related genes, 31 were expressed at various stages, and 32 were expressed at different stages of sugarcane infection［25］. The disease-resistant variety Yacheng 05-179 showed expression of resistance genes more significant in the treatment with the inoculation of U. scitaminea at 48 h than in the control at 24 and 120 h, also more significant than the expression in susceptible variety ROC22 at 48 h［47］, in response to U. scitaminea stress. The expression of the three candidate genes ScBAK1, ScMapkk and ScGloI in the mitogen-activated protein kinase signaling pathway［48］ and related sugarcane enzyme genes and the differential expression of lignin synthesis-related genes CCR, ubiquitin-mediated protein degradation-related gene UCH-LE5, genes related to the interaction between plants and pathogens GK, MLO and HIR1, and EIL3 related to the ethylene pathway also responded to U. scitaminea stress.
Control
Use of resistant varieties The use of smut-resistant varieties is an effective measure to prevent and control the disease［49］. Several sugarcane-growing countries in the world, such as the United States, Cuba, India, Brazil, Australia, France, and China, have made sugarcane smut resistance a major goal of variety selection［50］. According to the local practice, the selection of disease-resistant varieties Xintaitang 10, Xintaitang 16, Xintaitang 20, Yuetang 93-159, Yuetang 00-236, Yuetang 00-318, Guitang 02-901, Guitang 31, Yunzhe 01-1413, Yunzhe 03-194, Yunzhe 06-80, Yunzhe 08-2060, Funong 15 and Funong 36 is the basis of sugarcane smut control［51］.
Agricultural control
Before planting, applying sufficient base fertilizer, timely irrigation, timely fertilization and soil cultivation, and reasonable application of nitrogen, phosphorus, and potassium fertilizers are effective to promote the robust growth of sugarcane seedlings and enhance the disease resistance of sugarcane plants. It is necessary to remove diseased plants in time and centrally destroy them to reduce the source of repeated infections and control the spread of the disease. The rotation of sugarcane and rice, corn, sweet potato, peanut, soybean and alfalfa should be strengthened, to reduce the long-term infection and accumulation of U. scitaminea and reduce disease occurrence［3］.
Chemical prevention
Studies have shown that 2.5 mg/L pentachloronitrobenzene and 2.5 mg/L didecyldimethylammonium chloride, 50 mg/L propiconazole, 100 mg/L triazolone and 200 g/L cyprokonazol can significantly reduce spore germination［52］; and 250 g/L azoxystrobin suspension, 250 g/L difenoconazole, 300 g/L difenoconazole·propiconazole emulsifiable concentrate, 325 g/L difenoconazole·azoxystrobin and 250 g/L propiconazole emulsifiable concentrate have a good control effect on sugarcane smut［53］. The study of Huang et al. ［54］ showed that soaking seeds with 2%-3% lime water for 24 h, soaking seeds at 52 ℃ in hot water for 20 min, soaking seeds with 40% seed dressing double wettable powder, 40% seed dressing wettable powder, 25% triazolone wettable oxazolone powder, 80% mancozeb zinc wettable powder and 500-800 times dilution of 2.5% triticonazole suspension seed coating agent for 10 min, all have a certain control effect; and if the seeds are soaked with 50 ℃ hot water for 2 h, the disease can be thoroughly controlled.
Biological control
It has been reported that fungi Fusarium moniliforme ［Gibberella fujikuroi］var. subglutinans , Aspergillus niger , A. flavus and Penicillium sp. have inhibitory effects on the spore germination of sugarcane smut［55］. Liao et al. ［56］ isolated a few bacterial strains having a strong antagonistic ability against U. scitaminea . The bacterial strain ST4 isolated from the rhizosphere can block the sexual mating of U. scitaminea ［57］. The Bacillus bacterium BGMRC03005 has an inhibitory effect on the morphology of three species of U. scitaminea ［57］. At present, no effective studies of these biological agents in field control of sugarcane smut have been reported［58］. Changmi WANG et al. Research Progress and Prospect of Sugarcane Smut
Prospects
At home and abroad, researches on the occurrence and damage of sugarcane smut, the interaction between pathogens and hosts, and prevention and control have achieved a lot of results, but the following aspects need further exploration.
Distribution of physiological races: The pathogenicity of pathogens and the differentiation of physiological races are common; and defining the distribution of physiological races is very important for the prevention of smut and the selection of resistant varieties. Analysis of different rotation methods: Although the rotation of sugarcane with rice, corn, sweet potato, peanut, soybean and alfalfa reduce the occurrence of diseases, the mechanism explanation and the exploration of the occurrence law need to be further deepened. Use of smut resistance genes:Introducing smut resistance genes through Agrobacterium -mediated transformation or gene gun transformation can be performed to increase the expression of smut resistance genes; and the expression of smut-causing genes can be silenced by techniques such as gene methylation or RNA interference, thereby improving the resistance of sugarcane plants to smut. Research and development of new biological agents: The use of biological agents has a small impact on people and the environment, has little pollution to agricultural products, and is also conducive to the protection of beneficial insects. Therefore, excavation of biochemical substances with antimicrobial function and screening of antagonistic fungi and bacteria are of great significance for the prevention of smut.
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