玉米茎及果穗的小斑病(Helmintho—Sporium maydis),在热带及亚热带的玉米栽培地区是早就有的。直到一九七○年,美国对此病害还不够重视。主要原因是由于玉米本身对这种病原菌的抗病力很强。但是自从出现了新的侵染力更强的H.maydis生理小种后,情况发生了根本的变化。在南部各州,一九七○年因小斑病减产50％以上;西北部的玉米亦受到严重的侵染。同时这种病还威胁着巴西南部及许多中美国家中的感病的玉米品种和杂交种。 Helmintho-Sporium maydis, a corn stalk and ear, is long established in tropical and subtropical maize fields. Until 1970, the United States did not pay enough attention to this disease. The main reason is due to corn itself is very resistant to this pathogen. But things have changed fundamentally since the emergence of a new, more potent H. maydis race. In the southern states, the production of small spot blight decreased by more than 50% in 1970; the northwest maize was also seriously affected. The disease also threatens susceptible maize varieties and hybrids in southern Brazil and many Central American countries.