Rust Occurrence,loss Evaluation And Control On Apocynum Venetum In Altay Prefecture

Apocynum venetum（Apocynaceae）is a perennial herbaceous plant that is widely distributed throughout Europe,Asia,and North America.It has a variety of uses,including as a plant food for cattle and sheep,textile production,and importantly,for producing medicinal products including a tea.China is one of the largest producers of A.venetum,using leaves obtained mainly from the harvesting of the wild plants.In recent years,significant effort has been carried out to domesticate and breed the species for cultivation.Disease has become a main limiting factor on the production of A.venetum and this has been the focus of this study.Naturally growing and cultivated A.venetum in the Altay Prefecture were used for a series of experiments relate to a serious rust problem that severely affects this plant.The study involved pathogen identification,disease epidemiology and control measures of the rust in laboratory,greenhouse and field conditions.The objectives of this study were to provide support for sustainable utilization of A.venetum.The main results obtained from the research are as follows.1.In the Altay Prefecture,we found three kinds of disease on A.venetum,and these were a rust,a spot blight and black spot disease.The causal fungal pathogens were Melampsora apocyni,Septoria apocyni and Alternaria catharanthicola,respectively.To our knowledge,the presence of S.apocyni was the first report of it causing a disease on the genera Apocynum and Poacynum in China.The presence of the black spot disease caused by A.catharanthicola was the first report on A.venetum in China,as well as worldwide.The rust species,M.apocyni,also infected Poacynum pictum and P.hendersonii,plant species closely related to A.venetum.The maximum disease incidence of rust on A.venetum,P.pictum and P.hendersonii,respectively,were 73%,83%and 99%.From 2013 to 2015,thirty genera of airborne fungi,including Alternaria,Septoria,Fusarium,Aspergillus,and Penicillium were captured and identified in the field of cultivated A.venetum used in our study,through use of the microbiological air sampler.We found the presence of airborne fungi had significant（P<0.05）positive correlations with the daily mean relative humidity and the daily precipitation in the seven days before being detected.2.The rust on cultivated plants first developed on the lower leaves in late May to early June,but developed 24 to 48 days later in wild plants.Rust progressively spread to the leaves of the middle and then upper branches.The maximum disease index(DI_max)had large variations across times and sites and these variations could be linked to a series of factors leading to the rust epidemics.The correlation analysis indicated that DI_max on cultivated and wild plants had significant（P<0.05）positive correlations with the rainfall,the total vegetation coverage,the density of A.venetum and the number of urediniospores.The correlation coefficient（r²）were,respectively,0.461 and 0.805,0.558 and 0.445,0.564 and 0.353,0.716 and 0.749.The partial least squares path modeling analysis showed that precipitation（Path coefficient,PC=0.313 and 0.544）was the key driver leading to variation in DI_max at these two fields.The topsoil water content and total vegetation coverage at the cultivated and wild field,respectively,play main roles in influencing rust epidemics by their effects on the microclimate（PC=0.681 and 0.989,P<0.05）.In these two fields,the effects of host factors on the rust intensity via the microclimate（PC=0.841 and 0.862,P<0.05）were higher than that via the inoculum（PC=0.705,P<0.05 and=0.130,P>0.05）,and the indirect effects led to DI_max variation（Mean effect size=0.699 and 0.200,95%Confidence interval>0）.3.The dry weight of disease leaves was 11.1²5.9%less than that of healthy leaves resulting from the net photosynthetic rate of disease leaf decreasing with increasing severity exponentially(Y=14.41e^-0.02x,R²=0.477).The transpiration rates were 15.3¹11.9%when leaf rust degree was between 1 and 4 on the scale,higher than that of healthy leaves,leading to the leaf wilt and defoliation.The photosynthetic CO₂ assimilation and the defoliation were important components of plant yield loss.The leaf dry weight of individual plants decreased exponentially with increasing disease index（Y=-0.060x+6.959,R²=0.531）.Compared to healthy leaves,total flavanone of disease leaves when the rust degree was 5 on the scale was reduced by 20.3%（P<0.05）,chlorogenic acid of disease leaves when the rust degree was 4 and 5 on the scale was reduced 12.1%and 33.5%（P<0.05）,and the total amino acids of disease leaves when the rust degree was between 1 to 5 on the scale was reduced 13.9%to 49%（P<0.05）.4.In the greenhouse,we studied the relationship between the soil water content and the rust occurrence on A.venetum.We found that A.venetum plants exposed to drought stress（25%and 50%relative soil water content）have an enhanced resistance to the rust disease caused by M.apocyni,and the disease index was 15.9%to 64.8%（P<0.05）less than that of 75%relative soil water content,probably the consequence of the closure of stomata of leaves,high content of H₂O₂,as well as the enhanced activity of POD,PPO and PAL in leaves.Compared to the 75%relative soil water treatment,the content of H₂O₂ increased 12.6%to 26.7%（P<0.05）,and the activity of POD,PPO and PAL,respectively,increased 13.7%to 26.7%（P<0.05）,44.1%to 95.3%（P<0.05）and 10.2%to 12.3%（P<0.05）.The net photosynthetic rate of A.venetum plants exposed to combined stress of drought and disease was 39.8 to 55.5%（P<0.05）less than that of the 75%relative soil water treatment.The dry weight of disease leaves had no significant difference with the healthy leaves,but the dry weight of leaves under drought stress was 35.2%to 56.8%less than that of the 75%relative soil water treatment.5.In the cultivated A.venetum field,we conducted a experiment that had five irrigation frequencies as follow;interval of three,five,seven,nine and eleven days.The soil water content under the interval of three and five days treatments was higher than 75%.The soil water content was higher than 50%but lower than 75%when the interval of irrigation was seven days and it was lower than 50%when the interval of irrigation was more than seven days.With irrigation interval of seven,nine and eleven days,the DI_max were 35.4%to 61.9%,and the AUDPC were 36.4%to 65.6%,lower than that of interval of three days treatment.However,compared to the treatment of a three days interval,the net profit of A.venetum leaves under the interval of nine and eleven days treatments was reduced 26.4%to 70.5%（P<0.05）.For the interval of seven days treatment,the economic value of applied irrigation water was the highest and the amount of water applied was 54.8%lower than that of the interval of three days treatment,and thus it was determined as the optimum irrigation regime.6.The concentration of seed dressing using fungicide triadimefon was 0.1 to0.5 g a.i./kg that had little negative effects on germination of A.venetum seed,but substantially reduced the rust incidence,being 49.7%to 66.7%（P<0.05）less than control treatment.In addition,we found that the mixed seed dressing using triadimefon（0.3 g a.i./kg）and fludioxonil（0.3 g a.i./kg）not only reduced rust but also increased the rate of emergence and the root length.For A.venetum in the adult stage,the relative control effect on the rust after the fungicide triadimefon or fludioxonil was sprayed onto leaves was higher than 95%,and fungicide effect last more than three weeks.