Σ(B  n) : Sum of the number of plants (n) and corresponding index of
infection (B). N: total number of the infected plants.
All varieties tested were classified in five different categories based on the
average index of infection.
0: immune, 0.1-1.0: highly resistant, 1.1-3.0: moderately resistant, 3.1- 4.0:
moderately susceptible, 4.1-5.0: highly susceptible.