A detailed review of the inheritance of plant 
resistance has been made by Hare (63). 
Another aspect of breeding for nematode 
control that should not be overlooked is to 
search for tolerant hosts that will yield well in 
the presence of nematodes. Although these are 
less satisfactory than resistant hosts, because 
they help to maintain nematode populations, 
they are nevertheless useful for making acrop 
when other types of nematode control are not 
feasible. 
NATURE OF NEMATODE 
RESISTANCE 
The nature of some forms of nematode re- 
sistance has been studied in detail (32, 43, 
54, On, ‘675 Or; 112, 113, 114, 118, 129, 139, 
146), This kind of research is the keystone to 
any sustained program of developing genetic 
resistance to nematodes. Some interesting 
developments in this field should be noted. 
Rohde (112) found the enzyme cholinesterase 
in all species of nematodes tested, and he 
attributed the nematocidal effect of an extract 
from asparagus to a cholinesterase inhibitor. 
Many of the recent organic phosphate chemicals 
so effective in controlling serious plant pests 
are likewise esterase inhibitors (17). A sys- 
temic organic phosphate has been found to 
eliminate nematodes from the tubers and roots 
of potato plants under greenhouse conditions 
(67). 
Potato clones can be developed that have 
significant differences in cholinesterase inhi- 
bition of foliar extracts (99). Genetic resistance 
to potato leafhoppers and flea beetles has been 
positively correlated with a higher content of 
cholinesterase-inhibiting substance (69). It 
would be interesting to learn whether these 
same insect-resistant lines would also be 
resistant to nematodes for the same reason. 
The mechanisms involved in these observations 
need further study, especially since Steele and 
Price (129) reported no toxic effect of dif- 
fusates and extracts of asparagus roots on the 
sugarbeet nematode, Heterodera schachtii, in 
contrast to Rohde's results. 
Mountain and Patrick (93) found that 
amygdalin from peach roots was hydrolyzed 
by nematode enzymes, releasing hydrocyanic 
acid, to the detriment of both host and para- 
site. The nematode enzyme, a beta-blucosi- 
132 
dase, may enter into other host-parasite re- 
actions. In plants there are many other 
naturally occurring glycosides that could be 
involved in similar breakdown and result in 
resistance to nematodes. 
METHODS OF BREEDING 
NEMATODE-RESISTANT PLANTS 
When nematode resistance is found within a 
particular crop species, no serious problems 
of breeding are encountered, unless the re- 
sistance character is closely linked to some 
other highly undesirable character, as in the 
case of tobacco (20). Transfer of resistance to 
acceptable crop varieties is largely through 
hybridization followed by selection in the 
segregating progenies. Mass selection is some- 
times preferable to the backcross method in 
such programs (124), Thus, by intraspecific 
hybridization, the resistance in Nemagreen, 
a root knot-resistant lima bean, was developed 
from a lima bean grown by the Hopi Indians. 
Likewise, the root knot resistance of the sweet 
potato variety Nemagold came from types 
grown in New Jersey, the origin of which is 
obscure, Similarly, Catskill strawberry, Conch 
cowpea, Copper Gem onion, Auburn 56 cotton, 
Oakview Wonder Pepper, Menaguard peach, 
Delmar and Bragg soybeans, Nemastan and 
Lahontan alfalfa, and Bluebonnet 50 rice all 
came from resistant segregates within the 
original crop species. No doubt additional 
sources of intraspecific resistance can be 
found in almost any crop, with continued 
search and improved methods of testing. For 
example, plants both resistant and susceptible 
to the stem nematode are thought to occur in 
all commercial alfalfa varieties and can un- 
doutedly be found by proper evaluation experi- 
ments (124). 
However, the most valuable germ plasm for 
nematode resistance is found frequently in 
species quite distantly related. In such in- 
stances, special techniques must be used, 
Thomason and Smith (134) and McFarlane 
et al. (84) were able to cross the root knot- 
resistant Lycopersicon peruvianum (L.) Mill. 
with the tomato by culturing the hybrid embryos 
on aseptic agar media. Attempts tocross these 
two species without using embryo culture 
techniques had previously failed, though it is 
