STUDIES ON THE FORCASTINC AND CONTROL OF POTATO LATE BLIGHT

C.K.Lin, H. Hwang, S.H. Hwo

Appearance of sporadic diseased-plant centers has repeatedly been shown to be a good signal for forcasting the current year's outbreak of potato late blight in an individual farm. Under North China conditions, diseased-plant centers have never been found before the blooming period. Only a very few plants within a large area of potato plantation act as disease-spreaders, even if all plants are grown from infected seed-tubers. They are usually located at places with poor drainage and dense stand. Before all other plants show any trace of infection, many characteristic lesions can be seen on diseased-plant centers where the lower leaves of plants already begin to turn yellow. Then, the disease gradually spread to surrounding plants corresponding to wind direction. Effective distance of spore dispersal is apparently guite limited. In a typial case, 10 days after the discovery of a single diseased-plant center in an area of over 5,000 sq. m., actual count of the number of lesions in the entire plantatlon showed that over 90% of them are distributed in an area of not more than 800 sq. m. In the period during which epiphytotie of late blight occurs, the length of the incubation period of each secondary infection is rather constant, varying only between 3 and 4 days. Under humid conditions, abundant sporulation and penetration of the fungus may be expected almost every day. The time interval between the appearance of initial diseased-plant centers to final total blight in a potato plantation varies from 18 to 42 days, in accordance with the amount of rainfall and the age and the rate of decline of the plants themselves. If, however, a few days of continuous drought and high temperature occur soon after the disease :already .has begun to spread, the fungus in the plant lesions may die out and the disease almost entirely ceases to make further development. In 1955, in the center of a #-shaped experimental plot with an area of 540 sq. m., 4 plants were inoculated on June 23, and the disease spread over the entire plot by July 20. During July 21 to 25, rain and dew were entirely lacking and the air temperature averaged 28.9℃ with an extreme maximum as high as 39.9℃. Although it rained continually during the following 5 days and the air temperature suddenly dropped down, there were no increase in the number of lesions for a long time. Until August 8, the fungous sporulation was seen only on a few lesions. All these observations lead us to conclude that in forcasting late blight of potatoes, attention must be directed to the date of the appearance of diseased-plant centers in each plantation as well as to the knowledge of climatic and pheno- logical conditions. Here, determination of the incubation period of each infection is unnecessary and of no practical value. While studying the epiphytology of potato late blight, experimentation on its chemical control has simultaneously been undertaken. In a preliminary experiment carried out in 1953, some promising results have been obtained with dilute copper sulphate spray. Again in 1954-1955 large scale field experiments proved this spray to be highly profitable. In 1954, a serere blight year, 4-5 sprays with 0.1% CuSO_4 solution applied at 7-days interval beginning from the first of July retarded the development of late blight by 15-20 days, and resulted in an increase of yield by 30-45% over the control. It was dry in 1955 and blight occurred very late in the season. Four sprays with 0.15% CUSO_4 solution applied at 14-days interval beginning from July 12 again gave farorable results, increase of yield being almost 20% in irraga- ted land and about 10% in non-irragated land. Although dilute copper sulphate solution is less effective thsn standard bordeaux mixture under severe blight conditions, it equals or even exceeds the latter in case the late blight is less severe. It is important to note that dilute copper sulphate solution is much cheaper than bordeaux mixture and much easier to prepare, so as to permit its general application over a large area. As dilute copper sulphate spray will always give good profit under various conditions, we are of the opinion that, at least in northern North-China, it can be put into regular cultural practice. It is recommended to look carefully each year, beginning from the blooming of potato plant, for the appearance of the diseased-plant centers in every plantation in order to adjust the time for the first application of spray. When precipitations are heavy and the plants are quickly declining in vigor, it needs to repeat the spray every week. Otherwise, intervals between successive sprays may be lengthened to two weeks or longer. It is estimated that 3-5 sprays with dilute copper sulphate solutlon will be sufficient to protect the potato plants from severe blight and to insure high yield of tubers. In our spray experiments in 1955, various other micro-elements were also investigated. 0.1% zinc sulphate spray also resulted In good control of late blight and significant increase in yield, although no synergetic effect was observed in a combination of copper and zinc spray. Since zinc is more readily obtainable than copper, further studies on this problem will be justified.