interesting that Porte and Walker (103) sub- 
sequently made this same cross without re- 
sorting to embryo culture techniques. The 
resistance from L, peruvianum is somewhat 
unique because it involves similtaneous re- 
sistance to four species in the genus 
Meloidogyne, though a somewhat similar con- 
dition was found in Korean lespedeza (59). It 
is also interesting that the plants homozygous 
for the dominant allele have a resistance index 
approximately twice that of the heterozygous 
plants (7, 147). 
A more complex technique was required to 
transfer the high level of resistance from 
Nicotiana repanda Willd to tobacco, N. tabacum 
L. N. repanda was found tobe highly resistant 
to six species of root-knot nematodes (16). 
However, hybridization of N. repanda with both 
diploid and tetraploid forms of N, tabacum 
resulted in failures for Burk (unpublished), 
though sterile and abnormal hybrids and to- 
baccolike plants with no resistance were re- 
ported elsewhere (80, 121). Burk and Dropkin 
(16) suggested a bridging technique, whereby 
the resistance of N. repanda could first be 
transferred to N. sylvestris Speg. and Comes 
and from N. sylvestris to N. tabacum. This 
has resulted in tabacum-like plants, which are 
highly resistant or immune to the same six 
species of nematodes for which the original 
parental species showed resistance, 
SUMMARY 
In spite of the problem of physiologic races 
of nematodes, plant breeders have developed 
numerous varieties of plants with resistance 
to several species of nematodes. The net result 
has been the reduction in crop losses, caused 
by nematodes and other organisms, that may 
be associated with the disease complex. Germ 
plasm for resistance may usually be found 
within a crop species or transferred by several 
well-known plant breeding techniques from a 
related species to that crop species. We would 
suggest that the use of multigenic resistance, 
coupled with rotations of susceptible and re- 
sistant host and nonhost plants, can reduce or 
even prevent the tendency toward race forma- 
tion. Despite the fact that breeding research to 
control nematodes started nearly 50 years 
ago, there are virtually limitless unexploited 
opportunities in the field of effective biological 
control of nematodes through the use of re- 
sistant crop plants. 
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