This may not be fact… but this is what the “Old Timers” Say,

Cheap way is mix about two cups of Clorox to a gallon of water. This should cover a 10ft x 20ft plot. Nematodes will kill out your plants quick by eating the roots. If the roots is cracked or scarred when you pull a plant up that’s dying (not from cold snaps) then you have nematodes.