问题 问答题 简答题

玉米大斑病的主要症状是什么?

答案

参考答案:

主要为害叶片、叶鞘和苞叶,以叶片受害最重。一般先从植株下部叶片开始发病,逐渐向上扩展。发病初期,叶片上出现水渍状青灰色斑点,然后沿叶脉逐渐向两端扩展,形成纺锤形或梭形大斑,病斑中央黄褐色或青灰色,边缘暗褐色。在感病品种上病斑一般长5~10cm,宽1cm,严重的长可达15~20cm,宽2m左右。发病后期,多个病斑连接成片,叶片变黄枯死。潮湿时,病斑上产生大量黑色霉层,即病菌的分生孢子梗和分生孢子。在抗病品种上病斑小,表现为褐色坏死条纹,周围有褪绿晕圈,病部不产生或极少产生黑色霉层。

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   The first newspapers were written by hand and put up on walls in public place. 1_____. In the 700's

the world's first printed newspaper was published. Europe didn't have a regularly published newspaper

until 1609, when one was started in Germany.

   The first regularly published newspaper in the English language was printed in Amsterdam in 1620. In

1621, an English newspaper was started in London and was published once a week. 2___________.

   3__________. But not long after it was first published, the government stopped the paper. In 1704,

John Campbell started The Boston Newsletter (波斯顿新闻通讯), the first newspaper published in the

American colonies. By 1760, the colonies had more than thirty daily newspapers. 4___________.

   Today, as a group, English language newspapers have the largest circulation (发行量) in the world.

5________. It sells more than eleven million copies every day.

A. The earliest daily newspaper was started in Rome in 59 BC

B. The first daily English newspaper was the Daily Courant (每日新闻). It came out in March 1702.

C. In 1690, Benjamin Harris printed the first American newspaper in Boston

D. There are now about 1,800 daily papers in the United States.

E. But the largest circulation for a newspaper is that of the Japanese newspaper Asahi Dhimbun (朝日新闻)

F. reading newspaper is a must for some people.

G. newspaper has a long history but not all of us know it.