在农业生产中,推广应用一个丰产而又抗病的品种,往往能够获得较大的经济效益,但是,如果在一个地区范围内,过份地扩大该品种的种植面积,以致造成品种单一化,对这个抗病品种能致病的生理小种,则会迅速繁殖、蔓延起来,导致这个品种的抗病性丧失,引起病害流行。如“窄叶青”品种的育成、推广后不久,丧失抗性,就是一例。 In agricultural production, popularization and application of a high yield and disease-resistant variety can often achieve greater economic benefits. However, if the cultivated area of ​​the variety is excessively expanded within a certain area so as to result in the simplification of varieties, Physiological races that can cause disease in this resistant variety will multiply and spread rapidly, resulting in loss of disease resistance and epidemics of this variety. Such as “narrow leaf green” breed breeding, promotion shortly after the loss of resistance, is an example.