wat is necrosis ...do plants get cancer.. 

Necrosis means death of  living cells or tissues. Plants develop necrosis due to deficiency of  certain nutrients such as calcium, copper, potassium and magnesium. Necrosis of leaves is most common. 

Yes, plants do get cancer. The crown gall (gall tumor) of tobacco plant is example of this. This is caused by a soil bacterium which is Agrobacterium tumifaciens. But cancers of plants are different considerably with the cancers of humans/animals.