Experimental setup

Randomly (?) allocated the 140 genotypes to 6 pots each. With 5 plants per pot. Response is 0,0.5,1,1.5,2,2.5,3 (why not multiply by 2?) measuring the how diseased that plant is.

The client is currently fitting

  1. normality of residuals
  2. constant variance

Both of these seem easily violated. The residuals are not going to be normally distributed due to the response only taking on 7 values. For genotypes that have low (high) disease, the values will be near zero (3) and there will not be much variability. In contrast, middling disease will result in a values (possibly) spanning the range from 0-3 and therefore have higher variance.

Do a permutation test. Ordinal regression model?

But we aren’t really sure this is at all what you want.