This is a very important question.

Cassava like other plants has many diseases but the most important are Cassava Mosaic Disease and Cassava Brown Streak Virus.

The best way to prevent them establishing is to use clean material. This means that when you get seeds from a neighbour or seed supplier try to check the field from where the seeds came. You can use Nuru to check. This is what a farmer in Busia did https://plantvillage.psu.edu/blogpost...

In your own field only choose plants with healthy leaves from which to collect seeds.

Also, check the young plants and remove them when you see infection

The viruses are also spread by whitefly. So, try to spray the underside of leaves with soapy water to kill the young whitefly

here is our cassava page https://plantvillage.psu.edu/topics/c...
Thanks