Studies On The Epidemic Dynamics Of Corn Sheath Blight As Well As The Development Of Corresponding Forecast And Precaution Model In Liaoning Province

Corn is one of the most important crops in Liaoning province and has important economical value and social value. Corn sheath blight, one of the most important diseases on corn, always damages corn production heavily in recent years and becomes the significant limiting factor to increasing production and high yield. To manage and protect the diseases better, it is necessary to study the occurrence and regularities of disease. So we make the systematical investigation and monitoring in the most corn producing areas of Liaoning province. Combined with artificial inoculation method in experimental plot, the result indicates the temporal and spatial dynamics of corn sheath blight. The influences of various high yield planting patterns on the epidemiology of corn disease were studied also. On the basis, the simulation models of epidemic dynamics and yield loss estimation on corn sheath blight was established. The results were as follows:1. The he systematical investigation and monitoring show that the incidence of corn sheath blight is usually about 40%-85% in Liaoning province, and sometimes over 90%. There were highly significant difference of the incidence and diseases index in different regions. In Dandong and Dalian, the warm and humid region of south part, corn sheath blight has become the significant limiting factor to corn production.2. One hundred and twenty-four Isolates of Rhizoctonia spp. were obtained from tissues of corn sheath blight in 11 counties of Liaoning province. According to the hyphal fusion with international standard isolates of anastomosis groups (AGs) of Rhizoctonia, the multinucleate series belong to R. solani AG1-IA and AG4-HGI. The frequency was 97.6%, 2.4%, respectively. In Liaoning province, the predominant anastomosis group is AG1-IA.The isolates of AG4-HGI were first found in corn in Liaoning.3. The resistance identification of sheath blight for 14 principal cultivated varieties of maize was carried out in the field for two years in Shenyang. The results showed that the high resistant and resistant varieties were not found, the percentage of moderate resistant varieties was 14.3%, disease susceptible varieties and highly susceptible varieties was 42.9%, respectively. The resistance of disease affection among these varieties was different, and the disease of each variety was also obviously different between 2007 and 2008. This is mainly due to the difference climate condition.4. The bio-mathematical models, which could reflect the epidemics of corn sheath blight in Shenyang region, were obtained by artificial inoculation and computer simulation. The LOGISTIC model of Y=k /(1+exp (a-b*t)) and the GOMPOERTZ model Y=aEXP(bEXP(ct)) could reflect the dynamic development of corn sheath blight with time. The LOGISTIC modelis the optimistic model, and the model format was as follows:Xt =(?),(R²=0.9953).5. The paper studied on the spatial dynamic of corn sheath blight and obtained the results as follow. The spatial transmissions haven't been observed in one growing period. The disease can't transmit by aerial hypha or contagium between infected and uninfected leaves. If the pathogen can spread by rainwater splash waits to be proved. The results are completely different from conclusion of other scholar and description in textbooks. A method of indices of aggregation and mean crowding-mean density coefficient were led into the study of spatial patterns of corn sheath blight. All indices indicate that the distribution of the plant of com sheath blight were of the random distribution pattern.6. The epidemic dynamic of disease in new cultivation patterns were studied in this paper. There are obviously differences of the increased rate of disease index among various cultivation patterns in exponential growth phase. The results indicate that the increased rate of disease index in 2-0 cultivation and double-row ridges cultivation was lower than other cultivation patterns. This is the fundamental cause for the disease severity degree was milder than other patterns. Those new cultivation patterns can create good condition for air and light penetration which will slow down the expansion speed of pathogens.7. The results of epidemic dynamic study to maize leaf spot represented by gray leaf spot of maize in various cultivation patterns showed that the season epidemic curve of corn sheath blight in the field fitted s shape. The epidemic dynamic of disease changing trends were the same at different cultivation patterns. Gray leaf spot spread from center to vicinal fields, and the models of Y=EXP (ax+b) and Y=EXP (aEXP(bx+c)) could reflect the disease development with single direction in fields. The Gauss model of Y=EXP (ax²+bx+c), which were optimal models, reflected the disease dynamic with west-east direction or north-south direction. The Circular model Y=a(x²+y²)^c and Oval model Y=EXP (a(x²+y²) +bx+cy+d) were the optimum 3-Dimensional models by comparison different mathematical models. The spatial dynamic of corn leaf spot was affected by host spatial distribution patterns and performance wind. And those new cultivation patterns could promote the transmission of corn leaf spot.8. The relationship between the disease index of corn sheath blight and the percentage of yield loss was studied. The results showed that the percentage of yield loss increased with the increase of disease index, the 100 kernel weight and kernels per row decreased with the increase of disease index. On the basis of the results achieved above, the yield-loss assessment models for disease grade or disease index and yield loss ratio were established as cubic model (Y= -0.678x³ + 5.211x²-1.388x+14.733) and exponential model (Y= 0.969^*x^0.919).