Who first discovered lichens?

The potato spindle tuber disease is caused by which of the following?

$A$ scientist $X$ demonstrated that the extract of infected plants of $Y$ could cause infection in healthy plants and called the fluid as "Contagium vivum fluidum". Identify $X$ and $Y$.

What is the habitat of a virus in its free-living state?

The causative agent of mad-cow disease is a

Who recognized certain microbes as the causative organisms of the mosaic disease of tobacco?