Selection of the Predicting Factors for Rice Stripe Virus and the Application of Forecasting Model
RSV(Rice stripe virus)is a kind of viral disease transmitted by Laodelphax striatellus(LS).Once paddy rice infected with the virus,it is difficult to control the disease which would cause great loss of rice production.In recent years,as the cropping system changed and climatic conditions suited,the trend of Rice stripe virus RSV disease tend to go up acutely in north part of Zhejiang,especially in Jiaxing and Huzhou.The two cities became the most severe area Rice stripe virus RSV occurred.The RSV forecasting and prediction was the foundation to control and prevent the disease.From 2005 to 2007,we selected the key predict factors based on the study of RSV occurrence patterns established forecasting model and predicted the disease successfully in 2006 and 2007.The integrated results reorganized as following:
1 Materials and Methods
1.1 Systemic investigation of disease and pest conditions
In the observation area,investigate the quantity of LS in wheat field,the seedling field time and rice growth stage,analysis the RSV disease rate under the conditions of different pest amount.
Determine the disease rate of the winter generation of LS with quick-detection method and biological assay,compare the conversion relation between the two methods,and investigate the disease condition in the steady-going periods,analysis the relation between the carrier rate and the occurrence of the disease.
1.2 Experiment of seeding in stages
Sow the seeds in four stages equally in 2006 and 2007,the sowing time respectively be May 15,on May 22,on May 29 and on June 5.There is 7-days interval between every two stages,duplicates 3 times in each stage,and the field area is 333m2in each plot.Sow the seeds by stages,transplant the seedling in turn when they are 30-days-growing periods.In the steady disease occurring time,investigate cluster disease incidence and individual disease incidence,analysis the influence of sowing time did on RSV.
In the steady disease occurring time,do a large-area survey of RSV disease on the main paddy rice variety in the whole city,observes the anti-(bears)degree of each variety to RSV.
1.3 Predictors screening and establishment of forecasting models
Collect historical materials including cultivation,planting,variety and meteorology,analysis the relations between them and the plant disease.Use has gradually regression and simulating model methods to set up forecasting model,and predict the disease trend.
2 Results and Analysis
2.1 Analysis of disease occurring factors and selection of predict factors
2.1.1 The carrier rate of LS In 2005,2006 and 2007 we determined the carrier rate of LS by biological assay in Xiuzhou area,the city of Jiaxing,Zhejiang province.The results were 0.0%,2.01%and 2.69%respectively.We had a survey in the unprevented field in the steady disease occurring time in the middle ten days of July,and the result showed that disease conditions among the fields were remarkably different.The carrier rate could not be tested in 2005,so the disease was not serious and the average diseased cluster rates of early planting field,middle time planting field,late planting field were 3.1%,1.2%and 0.0%respectively.The disease aggravated in 2006,the carrier rate increased to 2.01%and the average diseased cluster rates of early planting field,middle time planting field,late planting field were 18.5%,10.1%and 1.2%respectively.When the carrier rate was 2.69%,the disease continually aggravated and the average diseased cluster rates of early planting field,middle time planting field,late planting field were 21.5%,16.5%and 1.5%respectively.It showed that the carrier rate affected the prevalence of disease directly,and the relationship between the two was very close.The higher the carrier rate was,the greater the disease incidence would be,instead the disease incidence lighter.According to the Japanese experience,when the carrier rate(qick-detection method)is more than 3%,the prevalence of RSV is more likely to happen.If the carrier rate ＞12%,there would be a seriously epidemic trend.According to the comparison of carrier rate and biological assay in recent years,the results tested by the two methods were different.The result of biological assay was significantly lower than that qick-detection method,and the ratio was about 1:1.75.Therefore,we chosed biological assay for prediction.When the carrier rate ＞1.71%,the disease had moderate trend,when the carrier rate ＞6.86%,there was a pandemic disease trend.
Table 1 The relatonship between the rate of viruliferous LS and the ccurrence of Rice stripe virus RSV (Xiuzhou Jiaxing,2005～2007)
2.1.2 Amount of LS Under certain conditions of carrier rate,amount of LS is an important factor to determine whether the RSV would be prevail or not.The higher the volume of insects was,the greater the incidence was,instead the incidence lighter.In recent years,amount of LS continued increase.In years 2005,2006,2007,the amount of first generation LS per 667m2 in wheat field reached 2.98 million,2.877 million and 4.093 million.LS transferred to the nearby weeds or other crops after the wheat harvest.They moved to the seedling when rice is planting and caused RSV occurring.It has been observed that the first peak incidence(7/early) caused by the first genenration LS generation was more serious than the second peak incidence(8/early)caused by the second generation insect,because the first generation LS had a large amount a high rate of virus spreading.But as the second generation LS lived in high-temperature season,the insect amount of was small and rate of virus spreading was low,so the disease was lighter.
2.1.3 Sowing date of paddy rice Paddy rice sowing date is an important factor to influence RSV.The incidence would be heavier when the sowing time was sooner,otherwise the incidence was lighter or even not onset.We had a investigation in a stable condition period(mid-July)among different rice sowing time in years 2005~2007,and found that sowing time were closely related to the occurrence of diseases.For example,in accordance with the four sowing times in 2006,the cluster disease rates were 18.5%,10.0%,5.67%and 1.5%in turn,the individual disease rates were 4.07%,2.69%,1.89%and 0.42%in turn.With the sowing time delayed,the disease lightened.
Among the four sowing times in 2007,disease in early planting field was the most serious,the cluster disease rates were 16.2%and 16.67%respectively,the individual disease rates were 5.18%and 7.31%respectively.Disease in late planting field was light,the cluster disease rates were 2.7%and 1.5%respectively,the individual disease rates were 1.9%and 0.7%respectively.
Table 2 RSV occurrence in different sowing date (Nanhu Jiaxing,2006～2007)
2.1.4 Cultivation and planting system In recent years,the wheat area has expanded,which wasconducive to the survival and reproduction of the overwintering generation and first generation LS.A wide range of weeds in the field create good conditions for the insects to transit to the rice.According to the observation in 2006,that disease rates of all rice seedling fields near the wheat field was evidently higher than fields away from the wheat field.In the directseeding fields,disease rates in the verge was evidently higher than that in the medial part of the field.As a result of the sowing time was early in the transplanting field,the seedlings were easily infected by massive LS and the disease arised heavily.
2.1.5 Paddy rice varieties There are certain differences among different varieties of RSV,generally,disease of sticky rice was heavier than late rice,disease of hybrid Japonica ricewas heavier than ordinary late rice,disease of Late japonica rice was heavier than mid japonica rice,desease of indica rice was lighter.Speaking of the present Zhejiang Jiaxing paddy rice,the disease resistance of the main varieties are all poor. A general disease survey in 2006 showed that the disease of Show You No.5 was the heaviest,incidences of Xiushui 09,Xiushui 110,Jiahua No.1 and Jia 991 were also heavier,Jialeyou 2 was lighter,see Table 3.
Table 3 The resistance of rice variety to RSV (Jiaxing, 2006)
2.1.6 Climatic conditions Winter temperatures directly influence overwinter LS According to information from meteorological departments in Jiaxing City:the average temperature in February-March in 2007 was 10.2°C,1.8℃higher than the same period in 2006,3.6°C higher than 2005.Because the winter temperature continued high,the survival rates of winter generation LS is high and quantity of insect was large.The quantity of overwintering insects was significantly higher than that of 2006,but in 2006 the quantity was higher than that in 2005.
2.2 Establishment of forecasting model
The three-body interaction among virus source,media and susceptible.Varieties was the important factor in the prevalence of RSV.Based on the carrier rate and RSV disease rate in the virus-transmitting period,or the relation between the early and late disease rate,key predictors was screened and a forecasting model was established as following:Y=1.6893X-1.6935
X-carrier rate of LS,Y-RSV disease rate.
2.3 Application of the forecasting model
There were many key factors which were conducive to the transition and migration of LS,for example,a higher carrier rate,a large amount of LS,the poor resistance of the main variety,the coincidence of rice-sowing time with the first adult generation insect virus-transmitting peak,the expanding wheat area a wide range of weeds in the fields and so on.In 2006,we had a comprehensive analysis of these key factors,predicted RSV trend quantitatively with forecasting model on May,18.It predicted that RSV disease would be moderate biased towards popular while measured result was moderate pandemic,the incidence area of the whole city was up to 15500 hectares.
Compared to the previous year,the carrier rate in 2007 declined slightly,but the quantity of LS increased largely,and the other factors did not change significantly.We had a comprehensive analysis over such factors and predicted RSV disease trend with the forecasting model in April,26.We predicted the RSV disease would be moderately popular in our city and measured moderate pandemic,the incidence area was 16400 hectares.
Two years application of forecast indicated that the forecasting time could be earlier if we used this forecasting model and predictors to analysis and forecasting the trend of RSV disease.We can make out accurate prediction in late April and before late May.The predictions above were generally in line with the actual results and provided a scientific basis for"pest control and disease prevention".
3 Summary and Discussion
Using methods of the system investigation and stepwise regression analysis,we screened primary predictors including carrier rate,the LS insect quantity,paddy rice varieties and sowing time,cropping and cultivation system and meteorological forecasting factor.Among these factors,the carrier rate(X)related to the individual rice disease rate(Y)most closely.We established the following forecasting model:Y=1.6893X-1.6935.Using this model and predict factor-integrated analysis,the mid-long period trend of RSV in 2006 and 2007 in Jiaxing City,Zhejiang was predicted and was generally in line with the actual results.The prediction provided a scientific basis for decision making of disease control and the production application has obtained obvious socio-economic and ecology benefits.
