10 years ago, there is no evidence of how the 
resistance-breaking character, acquired by 
this nematode, is inherited. Hybrids between 
the original parental nematode line and the 
resistance-breaking lines must certainly have 
occurred in nature, but they have never been 
reported under controlled conditions. Such 
hybridization could markedly alter the rate of 
establishment of new races. Genetic studies of 
the nematode are asimportantas similar 
studies in the host plant. 
Hybridization of nematodes for a different 
objective was proposed by Dunnett (35). He 
suggested culturing a mixture of the Dudding- 
ton and Bogshall strains on a susceptible host 
to build up a highly pathogenic strainfor use in 
evaluation and selection of potato-breeding 
lines with combined resistance to both races. 
However, an identical program proved to be 
ineffective in the control of late blight. In the 
late blight program, potato clones were re- 
peatedly developed with genes for resistance 
matching corresponding genes for resistance 
breaking of the pathogen. Such clones based 
upon gene-for-gene interactions served only 
to increase the selection pressure, which in 
turn, usually within a year, resulted in the 
formation of a resistance-breaking race of 
the pathogen. The above program, suggested 
by Dunnett in breeding potatoes resistant to 
multiple races of the golden nematode, has the 
same basic weakness; it provides a system 
whereby the pathogen mutates faster than the 
plant breeder can evolve the matching re- 
sistance to the newly formed races. The 
development of potato clones based upon this 
kind of multiple allele system (in contrast toa 
multiple gene system) can conceivably result 
only in perpetuating the race problem, but 
never in solving it. 
ASSOCIATION OF NEMATODES 
WITH OTHER PLANT DISEASES 
Control of nematodes by genetic resistance 
in the host plant is closely related to genetic 
resistance to other soilborne parasitic orga- 
nisms. Thus, nematodes are responsible both 
directly and indirectly for diseases in plants. 
In many cases, nematodes, in addition to their 
own parasitic action, provide avenues of in- 
fection for the entities that cause bacterial, 
130 
fungal, and viral diseases. Jenkins and Coursen 
(77) found that nematodes cause a host re- 
sponse that lowers genetic resistance to other 
diseases, Likewise, the bacteria that cause 
Granville wilt of tobacco are not strong 
primary invaders and probably seldom infect 
tobacco in the absence of nematodes (83). 
Hence genetic resistance to nematodes would 
often enhance resistance to other disease- 
causing organisms, These relationships have 
been reviewed recently in detail (19, 70, 100, 
104, 105), Insome instances, nematodes caused 
no evident plant injury, but acted solely in 
providing avenues of infection (1, 86, 120). 
On the other hand, soilborne pathogens may 
in turn directly affect nematode populations. 
Increased populations of the tobacco stunt 
nematode, Tylenchorhynchus claytoni Steiner 
1937, and the meadow nematode, Pratylenchus 
penetrans (Cobb, 1917) Filipjev & Schuurmans 
Stekhoven 1941, were found surrounding roots 
of Alaska peas when Fusarium and Rhizoctonia 
were present in the soil, as compared to plots 
where the nematodes alone were present near 
the pea roots (29). 
CONTROL OF NEMATODES BY 
RESISTANT VARIETIES 
Nematode control through use of resistant 
crop varieties has occupied the time and 
efforts of many workers in the past 40 or 50 
years, and there is a rich and plentiful source 
of information on this subject. For example, a 
list of plant varieties resistant to root-knot 
nematodes required nearly 90 printed pages in 
1941 (139), Reviews of past and current work 
along these lines are available (89, 114). A 
partial summary of breeding research on some 
farm crops is shown in table 1. 
Rohde (114) indicated that plants are re- 
sistant to nematodes for one or more of the 
following reasons: 
(1) The plant is not attractive. The roots may 
lack an attractant or may produce a toxic 
substance, 
(2) The host tissues are not suitable for 
continued nematode feeding. In hypersensitive 
reactions, the plant tissues die so rapidly that 
the nematode is isolated in necrotic tissue and 
its development is hindered. 
(3) The host fails to respond to the presence 
of parasite. For example, galls or giant cells 

