Interesting. Ghosts with firmly limited powers. I would think that if they can bear weapons, and be struck by regular weapons without harm, that they could simply attempt to seize the weapons of the intruders and struggle for possession should their own be broken. Go to Comment
Now, it will depend on what part of the world you are looking at, and just how many of the cereals are lost. If Rice or Maize/Corn are spared, then civilizations as we know them are still possible, perhaps more restricted in their ranges.
There are other possible plants to domesticate apart from grasses, but the timelines to do so would prevent them from stopping civilization collapse.
(Been reading Guns, Germs and Steel (Excellent book!) and the author asserts the availability of grains as one of the key instigators of civilization..) Go to Comment