Animals such as rabbits, deer, raccoons, crows, gophers, and skunks can all ruin a farmer's crop by eating or trampling them.
Bugs such as caterpillars, stink bugs, bollworms, grasshoppers, locust, etc. can ruin crops by the hundreds if there's enough of them.
Some pesticides that are used to keep pests away may do more harm to crops than they're intended to do.
Severe weather such as tornadoes, hard rain, heavy winds, sand storms, etc. can destroy farmer's crops all over the place.
Lack of rain (aka droughts) can kill crops by the thousands and cause famines and starvation.
Air and water pollution can destroy them too. This can be contributed to all the pollutants we let out in the air by car fumes, dust pollens, factory fumes, smog, etc. that is released into the air on a daily basis. Water pollutants can be contributed to all that end up in our waters from vehicle leakages, litter, etc.
Once that stuff gets in our waters, and we use it to water our crops, those pollutants get in our crops and kill them off.
Hope this helps!