马铃薯晚疫病的田间动态观察及防治试验

Occurrence of sporadic diseased plant centers during or after the bloomingperiod is considered as reliable indication of the current year's forthcomingoutbreak of late blight in a potato farm. Incubation period of each secondary infection being about 4 days,is ratherconstant during the blight epiphytotic season under Chahar conditions.The timeinterval between the appearance of initial diseased plant centers to final totalblight in a potato field varies greatly,however,not only from locality to localitywith different climatic conditions but also from field to field under similarclimatic conditions.Our data record a range of 18-42 days.Circumstantial eviden-ces indicate that this variation is due,on the one hand,to the rainfall whichinfluences the amounts of sporulation and the frequency of penetration of thepathogene,and on the other hand,to cultural and other factors that determinethe vigor and the rate of aging of the plant itself. Actual counts of lesions formed from secondary infection in the early periodof the spread of the disease in a potato field,reveal that their distribution bearsdirect relation with the direction of the prevailing wind.It is estimated that over90% of spores fall within a rectangle of 800 sq.m.around a diseased plantcenter along the loci of wind directions. On the basis of possible function of absorbed copper ions in the enhancementof plant resistance to late blight,field experiments have been conducted by using0.1 - 0.2% copper sulfate solutions as spray.Results show that spraying 2-3 timesat 7-10 days' interval,started soon after the appearance of diseased plant center,gives remarkable control of late blight and corresponding reduction of loss inyield.The copper sulfate spray,though less effective and stable in comparisonwith the bordeaux spray,has the advantage of being more economical and easierto apply.Further experiments with the pu:pose of modifying and improving themethod are in progress.