Question: how does the microorganism infect the tomato and potato plants?

Benjamin Hall answered on 22 Jun 2013:

Thank you for this question! 🙂

The microorganism in question, Phytophthora infestans, produces spores (tiny little seeds) which land on the leaves or stems of the plants. Given the right conditions, mild and wet weather, they will begin to germinate. They send down tiny, thin structures which invade the plant tissue and form very close connections to the plant cells. Now, at this point the plant recognises it is under attack and wants to stop it. It does something called ‘programmed cell death’. The microorganism needs living tissue to feed on initially so the plant deliberately kills itself in the area where the microorganism has invaded to stop it from spreading throughout the whole plant. However, the microorganism has evolved to stop this from happening and produces molecules called ‘effector proteins’ which enter the plant cell and stop the programmed cell death, allowing the microorganism to keep growing and spreading through the plant.