文档介绍：Melons/Cucumber Diseases
1. Downy Mildew 瓜类霜霉病
Occurring and damage
The disease is most important in the word on Cucumber muskmelon, pumpkin, and winter squash, though all cucurbits are susceptible. If appropriate conditions are present, plantings can be infected so rapidly that they appear to have been frosted .
世界性病害。我国各地都有发生,露地和保护地栽培的黄瓜,常因此病为害而遭受很大损失。流行速度快,菜农称之为“跑马干”,一两周内即可使除顶端嫩叶外的其他所有叶片枯死,减产高达30%~50%,有的地块因此病为害只采1~2次瓜后就提早拉秧。
Symptoms
May harm in entire period of life to the melon, mature period heavy. Mainly damages the leaf blade.
Foliar symptoms vary depending on how quickly infection occurs. Upper surfaces of leaves may show angular pale-green areas bounded by leaf veins that give the impression of mosaic(花叶). These areas change to yellow angular spots and e necrotic as the disease progresses.
When leaves are infected by a heavy spore shower, small individual necrotic(坏死) and chloroticflecks (褪绿斑) appear on the upper surface In a moist environment, sporulation(产孢) occurs on the lower leaf surface. The chlorotic flecks appear as light brown areas when held up to the light. Areas with active sporulation give the undersurface a purplish or dark brown sooty appearance.
Pathogens (病原物)
scientific name and taxon
Pseudoperonospora cubensis
古巴假霜霉菌
属鞭毛菌亚门假霜霉菌属
Sporangiophore (孢囊梗)
Sporangium(孢子囊)
Oospore(卵孢子)
Shape(形态)
The mycelium(菌丝体)no septa(隔膜), colorless, spread in the host intercellular space, or enters the host cell to absorpt nutrient by the oval or branching finger haustorium(吸器).
无性繁殖产生孢囊梗(sporangiophore)和孢子囊(sporangium )。孢囊梗由寄主叶片的气孔伸出,单生或2~5根丛生,无色,主干上有3~5次锐角分枝,分枝顶端产生孢子囊。孢子囊淡褐色,椭圆形或卵圆形,顶端具乳突。孢子囊在水中萌发产生6~8个游动孢子。
The fungal growth consists of large lemon-shaped spores called sporangia that are borne on branched structures called conidiophores.

游动孢子无色,圆形或卵形,有2根鞭毛,在水中游动30~60min后形成休止孢,再萌发产生芽管,从寄主气孔侵入。孢子囊在较高温度和湿度不充足的条件下,也可直接萌发产生芽管侵入寄主。
This fungus may produce the oospore, but is extremely rare.
Pathogen(病原)biology