Tricyclazole belongs to melanin biosynthesis inhibitors and is a unique fungicide with a very low mechanism of action. It is usually less toxic to pathogens. Most melanin synthesis inhibitors such as tricyclazole, rice amide, cyclopropionamide, praziquantel, etc., are special agents for controlling rice blast pathogens.

Melanin is present in the attached spores of pathogenic fungi and cannot form normal adherent spores without melanin. The attached spore is an infective structure in the pathogenesis of rice blast. Its function is to absorb the energy and mechanical turgor which is required for the infection of the bacteria to reach the host epidermis and form secondary hyphae. Inhibition of melanin production, the attachment of spores will lose normal infectivity, and pathogens will lose pathogenicity. Melanin biosynthesis inhibitors are a class of indirect bactericides that are non-toxic and active, acting on pathogenic bacteria, also known as anti-penetration compounds. Such an agent that does not directly kill the pathogenic bacteria but inhibits the invasion and development of the disease has a small selection pressure on the pathogenic bacteria, a complicated mechanism of action, and a low frequency of pathogenic bacteria resistance variation.

1. R&D registration

Tricyclazole was first developed by Eli Lilly Company of the United States. It was registered in 1979 and promoted to all over the world. It was developed under the code name EL-291 and registered in Japan in 1981 under the trade name Beam. In the 1980s, tricyclazole was introduced and used in China. At present, there are 307 registered products in China, including 11 original drugs , 257 wettable powders, 32 suspension agents, 5 water-dispersible granules, and 2 suspoemulsions.

2. Development of resistance

Tricyclazole is a systemic fungicide for controlling rice blast. It has the characteristics of high efficiency, long-lasting effect and low frequency of use. It has been used in large areas for nearly 30 years in China. As the rice blast fungus in China became resistant to fungicides such as rice blast, rice glutinous rice, and rice glutinous rice, the control effect of various chemicals decreased, and tricyclazole gradually became the main agent for controlling rice blast in the field. In recent years, reports of the decline in the efficacy of tricyclazole have also appeared in some parts of China.

Huang Chunyan et al used the indoor bacteriostasis test method to test the resistance of 98 pure strains from 160 rice blast samples from different regions of Heilongjiang Province. The results showed that the average inhibitory concentration of tricyclazole against Magnaporthe oryzae in different areas (EC50) ) There is a big difference. The average inhibitory concentration of sensitive strains was 10-20 mg/Kg for Harbin strains, 20-40 mg/Kg for Mudanjiang strains, 50 mg/Kg for Jiamusi and Suihua strains, and 40 mg/Kg for state farms. The average inhibitory concentration of the strains was greater than 50 mg/Kg. The results showed that the rice blast fungus in most rice areas of Heilongjiang Province had certain resistance to tricyclazole, and the resistant strain inhibited the medium concentration to be sensitive strains. 1.25 to 5 times.

Hou Dongyan et al. measured the sensitivity of 53 rice blast strains to tricyclazole in 7 cities of Liaoning Province. The results showed that the susceptibility of rice blast strains to tricyclazole in different regions was different. According to statistical analysis, the most sensitive strain was Dandong strain. The average EC50 values ​​of the strains in Dalian and Dalian were 0.3412 and 0.3461 mg/L, respectively. The most insensitive strains were Benxi strain and Shenyang strain. The average EC50 values ​​were 0.5171 and 0.4938 mg/L, respectively. The difference reached a significant level of 0.05, and the mean EC50 values ​​of the remaining strains were not significant.

Tricyclazole inhibited the mycelial growth of 19 rice blast fungi in Heilongjiang Province. The concentration of mycelium in rice cultivars in Fujin was the highest, which was 234.5 mg/Kg. This may be because the same variety of tricyclazoles have been used for a long time in the Fujin area, resulting in a certain degree of resistance to the rice blast fungus strains in the area. Tricyclazole has different inhibition rates on strains in different regions of Heilongjiang Province, and the inhibition rate of strains on concentrated river farms is higher than 30%; for Shangzhi, Fujin, Jiamusi, Luobei, Lianzhushan, Beilin, Ning'an The inhibition rate of strains in the Jiangnan region was less than 30%. It is indicated that the rice blast fungus of Shangzhi, Fujin, Jiamusi, Luobei, Lianzhushan, Beilin and Ning'an Jiangnan has developed a certain degree of resistance to tricyclazole. The reason may be that the drug has a long service life, and as the screening algebra increases, the sensitivity of the pathogen to tricyclazole gradually decreases.

The resistance levels of Magnaporthe oryzae to tricyclazole in rice growing areas in Guizhou Province are still below the low resistance, and there are no strains with intermediate resistance. Among them, the strain with the lowest level of resistance and the region with the highest frequency of resistance appeared in Zunyi. The resistance level of Magnaporthe oryzae in this area was the largest, reaching 2.69 times, and the frequency of resistance was the highest, reaching 7.29%. In Zunyi, where the level of agricultural cultivation is relatively high, whether it is the long-term application of pesticides has led to a significant increase in the frequency of resistant genetic variation and drug-resistant strains of Magnaporthe oryzae in this area, and further research is needed.

There are some differences in the sensitivity of the rice blast fungus strains from different regions of Yunnan to tricyclazole. The most sensitive is the strain in Dali region, the EC50 value is 49.0043 mg / L, the most unclear is the Yiliang area strain, its EC50 value is 65.0004 mg / L, the difference is 1.33 times, the difference is not large. The reason may be caused by individual differences in strains or due to drug resistance problems, and further research is needed.

At present, tricyclazole is still an ideal agent for controlling rice blast in a large area. In view of the existing test results, it is necessary to strengthen the drug resistance test, and do a good job in the screening of negative cross-resistance agents, so as to provide a basis for the effective and rational use of tricyclazole.

3. Tricyclazole resistance mechanism

The bactericide is mainly resistant to the action of the target pathogen, and the bactericide having multiple sites is generally not susceptible to drug resistance. At the same time, several sites of action produce mutations that result in lower frequency of drug resistance, and thus many protective fungicides have rarely produced drug resistance since their development.

Tricyclazole has many mechanisms of action against rice blast. One is trihydroxynaphthol reductase and tetrahydroxynaphthol reductase, which are mainly used in the melanin biosynthesis pathway, and can induce enzyme activities related to resistance reactions in rice. Improve, the second is to inhibit the infection of the pathogens on the plants. This may be the reason why tricyclazole is not easily resistant, and it is one of the pesticides with low resistance risk.

There is a different opinion on the resistance of tricyclazole. One thought that tricyclazole has no effect on the mycelial growth and spore germination of Magnaporthe grisea under in vitro conditions, so the sensitivity of field pathogens is much lower than that of drug use. Another view is that tricyclazole has a significant effect on mycelial growth and spore germination of rice blast fungus, and the concentration is related to the sensitivity of pathogenic bacteria.

The mycelial growth inhibition rate and spore germination inhibition rate of rice blast fungus were determined by 20 μg/mL solution of tricyclazole. The results showed that the inhibitory rate of tricyclazole against rice blast fungus growth was only 0.39%, and the inhibition was not obvious. The average germination rate of spores was 95%, and there was no significant inhibition.

Inoculated with rice blast fungus after treatment with tricyclazole for 24 h, the control effect was 81.3%; rice seedlings were inoculated with rice blast fungus for 24 h and then treated with tricyclazole spray, the control effect was 35.6%; tricyclazole and rice blast The spores were simultaneously sprayed to treat rice seedlings with a control effect of 46.3%. The statistical results show that the use of the same concentration of agents to control rice blast, the time of inoculation and drug treatment is different, and the difference in control effect is significant. After inoculation of rice blast fungus for 24 h, the conidia had been melaninized, and the process of invading the host was completed. At this time, the use of tricyclazole could not achieve the purpose of controlling the disease well.

The attached spores of the rice blast spores were all melaninized within 24 h. If the mechanism of action of tricyclazole is only to inhibit the biosynthesis of melanin, then there should be no obvious disease prevention effect after the melanin has been completely formed, but the test results from Chen Yan show that the drug is administered at 14, 16 and 24 h after inoculation. The effects were 91.2%, 90.3%, and 87.2%, respectively. Tricyclazole prevention did not only inhibit melanin biosynthesis.

The inhibitory effect of 10 g/mL tricyclazole on reinfection at 48 h after inoculation was 63.28%. It indicated that after the completion of the penetration of the rice leaf by the host leaf (>10 h at 25 °C), the treatment with tricyclazole could not control the development of the disease of the treated plant, but it could effectively inhibit the disease. The reinfection source causes the onset of other healthy plants that are not treated with the drug.

Induced resistance is an effective method to control plant diseases, but it is still less practical to put into production. Tricyclazole may induce resistance through JA pathway, prevent reinfection of pathogens in plants, and thus prolong The duration of the drug.

4. Resistance treatment of tricyclazole

The results of Huang Xing et al. showed that the reduction of field control effect of tricyclazole in rice blast disease is not the resistance of rice blast pathogens. It is the key to scientifically and rationally using tricyclazole.

The seedlings are immersed in the tricyclazole solution before transplanting, which can effectively prevent the occurrence of rice blast, and the method is simple and easy, and the effect is remarkable. After the first treatment with the tricyclazole spray for 3 days, the treatment can be improved, the effect of the drug can be prolonged, and the rice can be safely passed through the most susceptible period. In actual production, attention should also be paid to the use time of the drug. If the pathogen-attached spore has completed melaninization and the conidia invade the rice seedling and then use tricyclazole, the control effect is reduced, and the purpose of controlling the disease is not achieved. Field wind, rain and farming operations may cause wounds in rice leaves. When the leaves have wounds, the pathogens do not need to complete the penetrating process, and can directly enter the rice body from the wound to colonize. This situation will cause the drug to lose its effectiveness. Cause damage. Therefore, it is necessary to avoid causing wounds on rice leaves during farming operations. Rainfall within a short period of time after administration of tricyclazole will result in a decrease in control effectiveness. The shorter the interval between rainfall and application, the greater the effect on control. Tricyclazole was used in the field, and it was necessary to ensure no rain during 3 hours.

Mixing with other agents is one of the important ways to improve the efficacy of the drug. The correlation coefficients of the pathogenic bacteria against the EC50 of tricyclazole and prochloraz and azoxystrobin were 0.6373 (R2=0.4061) and 0.0877 (R2=0.0077), respectively, and the difference was not significant, indicating that tricyclazole and prochloraz and pyrimidine There is no cross-resistance between the bacteriocins.

Mixing prochloraz with tricyclazole can combine the different mechanisms of action against Magnaporthe oryzae to achieve synergistic effects. It can inhibit the growth of hyphae and the sporulation of Magnaporthe oryzae, improve the therapeutic effect on rice blast, and inhibit the spread of lesions. The control period of the medicament is prolonged, and the systemic and rain-resistant scouring ability is enhanced. The mixture preferably concentrates the advantages of tricyclazole and prochloraz in a single agent, complements the advantages, and improves the efficacy of the medicament.

Although there is still no report on the resistance of large-scale rice blast to tricyclazole, tricyclazole is currently the main agent for controlling rice blast in China, and its development is worthy of close attention, while strengthening resistance monitoring. It is also very important to use the agent scientifically and reasonably.

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