how do plaants fight off disese because yoou can never tell a sick plant from a healthy one ???

Question: how do plaants fight off disese because yoou can never tell a sick plant from a healthy one ???

Asked by acybaby to Michaela, Steve on 16 Jun 2010 in Categories: Your Research. This question was also asked by beithade.
- Michaela Livingstone answered on 16 Jun 2010:
  
  I don’t know anything really about plants, but you can tell some sick plants apart I think – when their leaves are discoloured for example. Yvette and Steve are the people you want for this I think.
- Steven Kiddle answered on 16 Jun 2010:
  
  Like human diseases, plant diseases have symptoms. Some extreme systems like plant death are very easy to see. The disease i work with Botrytis cinerea, causes plants to die, and during infection the plant gradually becomes brown and furry so pretty easy to see.
  
  You are right though, some plant diseases are hard to spot, for example some diseases can only live if the plant is otherwise healthy. But of course we are less interested in this type of disease as it is not a problem for humans.

Question: how do plaants fight off disese because yoou can never tell a sick plant from a healthy one ???