Maize leaf blight disease caused by Cochliobolus
heterostrophus pathogen eliminated the corn crop from
southern American regions during 1970. The complete crop
failure was due to the narrow genetic base of corn grown in
that region (Ullstrup, 1972). This epidemic was due to the
development of a new race of pathogen called Race T, which
had T-cms virulence gene. Therefore, a new type of plant
material called Texas cytoplasm was introduced, which
contained a mitochondrial sterility gene that helped to
develop hybrid seed. These hybrids were high yielding due
to hybrid vigor and were disease resistant. This resulted in
replacement of the old susceptible cultivars and 80% corn
cultivars in the USA were derived from T-cytoplasm as
parent by the end of the 1970s (Strange, 2005). In the same
way, Shah et al. (1982) reported that potato germplasm
imported from the USA, India, and the Netherlands
showed promising resistance against potato leaf roll and
blight diseases. Therefore, introduction is an important
way to enhance the genetic diversity of crop plants when
local germplasm lacks resistance. The introduction can be
of exotic cultivars through a multinational company or
from a foreign gene bank. Wolfe (1993) reported genetic
diversity as the most effective method of durable resistance
in crop plants. Efficacy of introduction for development of
disease resistant cultivars was also reported by Simmonds
(1993).