关于柑桔黄龙病几个问题的讨论

柑桔黄龙病在我国北部产区及南部高山发生得很轻微或完全不发生。在南部的重病区中,由于地势、防风林、间作果树等所造成的遮蔭条件也不利于病害的发生。大气候和小气候与黄龙病的相关有多种可能的原因。但首先应当研究它们对病毒传播媒介和侵染后发病的影响。用病芽作为接穗所长出的新梢在相当长时间内没有病状。最容易发病的是着果很多的幼树。初期发病的柑桔树通过重修剪和靠接辅助砧木的方法治疗有成功的实例。这些现象都可以从树冠树根比例与发病的关系来解释。柑桔类的种和品种间显然存在着耐病力的差异。这种差异也可能存在于同一品种的不同株系和个体间,因为用病区老树的接穗比幼树的接穗育成的苗木发病率较低。在黄龙病传播和流行上的许多不一致的现象还可能决定于病毒浓度和株系间毒力的差异及其相互关系。上述问题的研究可能导致栽培免疫、品种免疫和人工接种免疫方法的应用。

NOTES ON CITRUS YELLOW SHOOT DISEASE

This is a discussion of the natural phenomena and the reported experimental data of citrus yellow shoot disease in China which come to the writer''s notice.The disease,allied with tristeza virus disease,has long been known.However,in the northern citrus growing regions and at high elavations in the southern part,it is entirely ab-sent or only rarely occurs.Shading provided by topography,such as hills of a valley,and other trees,such as wind break and interplanted deciduous fruit trees,is also unfavorable for its outbreak in individual orchards,despite of plenty of severely diseased ones in the sur-roundings.There are several possible explanations for the correlation of macro-and micro-climates with the incidence of the disease.But it is considered important to find out first whether the influences are exerted on the vector of the virus or on the post-infectional host-virus relationship.The scions taken from diseased shoots develop into apparently normal shoots which remain healthy for a considerable length of time.Yellow shoots most frequently make their first appearance on young trees following a year of over-loading of fruits.There is one case of successful permanent cure of the young tree at its early stage of disease by simultaneous heavy pruning and side-grafting.These phenomena may be explained on the basis of the relation of top/root ratio to pathogenesis of the virus disease.Apparently,there are specific and varietal differences in tolerance of citrus trees to the virus infection.Similar differences may also exist among clones and individuals within the same variety,because scions taken from old healthy-looking trees in diseased area give lower percentage of diseased plants than those taken from young trees.Variations in transmission and occurrence of the disease may also be due to the difference in virulence of the virus strains which interfere with each other and the fluctuations or un-eveness of concentration of the virus within its host.The writer hereby expresses the beleif that further researches might lead to the applica-tion of cultural,varietal and vaccinal immunization against the hazard of citrus yellow shoot disease.