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Peronospora hyoscyami f.sp. tabacina
''Peronospora hyoscyami f.sp. tabacina'' is a plant pathogen infecting tobacco that causes blue mold. It is an oomycete (a fungus-like organism) that is highly destructive toward seed plants. The tabacina is also very alarming towards humid farming zones, like the southeastern and eastern U.S., Canada, and countries boarding the Caribbean. The disease was first identified in 1921 in Florida and Georgia 〔Ivors, Kelly, and Asimina Mila. "Burley and Flue-cured Tobacco: Blue Mold." North American Plant Disease Center Forecast. 16 July 2007. Web. 3 Mar. 2010.〕 Ten years later the same disease was found once again in the same region of the U.S. The disease began to spread into Virginia, Maryland, and North Carolina. A few years later the disease reached Kentucky and Tennessee. In 1960 a blue mold epidemic spread on tobacco in approximately eleven countries. There was approximately twenty five million dollars in losses which is nearly thirty percent of tobacco plants.〔 Each year Peronospora Hyoscyami is introduced as blue mold, known as windblown spores from outside the region by infected transplants.
== Symptoms ==
Tobacco plants with an upright position of small patches of the seed bed are typically early signs of blue mold. These leaves are usually two to four centimeters in diameter, and normally spotted with yellow, gray, or a bluish mold in the lower surface. Younger plants that are up to four weeks old and are affected by the disease are easily killed by the fungus. Older plants become deformed and dark, indicating that parts of the plant are dead. In more severe cases the stem and root become infected and turn to a brownish color. The observation of downy mold in the lower surface of the leaf is the most reliable symptom of blue mold. Downy mold carries thousands of small reproductive units called conidia 〔Zhang, Shouan. "Plant and Soil." SpringerLink. Springer Netherlands, 2 Nov. 2004. Web. 4 Mar. 2010.〕 If conidia forms at different spots in the seedbed an outbreak is more than likely to occur. The symptoms usually take place before it’s obvious. The symptoms evolve rapidly and are typically noticeable over night.
The fungus frequently takes place when the vein of the leaves reaches vascular tissue of the stem, causing weakness and lodging. This infection is called "systemic infection". The infection causes a stunt in growth and the leaves become narrow and short. The vascular system of the plants usually turns brown in color and the weakened stalk usually causing the stalk to fall over.
The fungus is highly weather sensitive. During the time when the weather is cool, wet, or when there’s an overcast the disease can evolve in a greenhouse or field. The disease spreads rapidly because of the fungal pathogen. The rate of continental spread is based on the potential for high levels of inoculum and effective wind spores. When the weather is clear, dry, and hot the disease usually stops spreading and more than likely stops all together.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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