"It is agronomically, ecologically, nutritionally, and economically risky and unsustainable to rely almost exclusively on a handful of major crops to provide food for the world's (future population)" (Dempewolf).
The 'agronomic' problem is the need for crop diversification to achieve adaptation and resilience of our food production systems in the face of climate change.
The 'ecological' problem is that monocultures are an unbalanced hole in the ecosystem which can generate disease and spread toxins.
The 'nutritional' problem is that a diverse and varied diet is vital for our nutrition and health.
The 'economic' problem stems from all of the above.