a farmer has two variety of wheat crop in his field one is good yield but is prone to diseases the other is resistant to diseases but does not give good yield.how can you help the farmer

We can suggest the farmer to produce a hybrid crop from both the wheat varieties he has. 

Hybridisation is the process by which two plants( like two varieties of wheat) with desired characteristics are mated or crossed. The resultant plant will have the characteristics of both the parent plants.

So, as the result of hybridisation, the farmer will get a wheat variety with both good yield and disease resistance.