These losses may be estimated from reduc- 
tion in yields of individual plants (5, 27, 56, 
90, 127), from lack of survival during summer 
(6), from poor stands (10), from lowered 
quality of produce (22, 44), or from increased 
susceptibility to cold injury (37). The use of 
low-income, nonhost crops to control nematode 
populations also contributes to the total crop 
losses. Crop damage by nematodes has been 
attributed to decreased fertilizer efficiency 
(37, 131), decreased water uptake (37), in- 
creased susceptibility to attack by pathogenic 
fungi and bacteria (1, 19, 70, 86, 100, 104, 105, 
120), and interference with metabolic processes 
(37). 
Not all plant-parasitic nematodes are harm- 
ful; there are at least two cases of host-plant 
stimulation attributable tonematode attack. 
The development of plants possessing geneti- 
cally controlled resistance to nematodes has 
been recognized as one of the most effective 
and economical means of reducing the losses 
caused by these pests, especially the root-knot 
nematodes. However, the appearance of physi- 
ologic races has raised the question as to 
whether there can ever be permanent control 
solely by genetic means (18). This paper deals 
with the problem of physiologic races and other 
problems in breeding for nematode resistance. 
VARIABILITY IN NEMATODES 
In breeding plants for resistance to any 
plant parasite, whether it be insect, virus, 
fungus, bacterium, or nematode, the plant 
breeder, if he is to make lasting progress, 
must consider the genetic variability of the 
parasite as well as that of the host plant. 
Populations of both are plastic and dynamic. 
A paper prepared nearly 40 years ago by 
Steiner (130) presented these fundamental 
principles of population genetics: 
(1) Nematodes prefer the host species on 
which their parents lived, 
(2) This preference grows with each suc- 
ceeding generation. 
(3) Utimately the specialization may reach 
such a high degree that distinct new hosts of 
even the closest taxonomical, physiological, 
and chemical relationships are either not 
attacked or attacked very lightly. 
(4) Starved nematodes will attack a hostthat 
otherwise they would completely ignore. 
U2 
If expressed by a population geneticist, 
Steiner's principles would be: 
(1) Differences and new mutations of genes 
and chromosomes occur in all organisms. 
(2) Mutations are selectively sorted out by 
the environment and passed on to the surviving 
progenies, 
(3) They are the basis for genetic changes 
leading to noticeable variability, to genetic 
drift in a given direction, andultimately tonew 
races, 
(4) Any condition that speeds up mutation 
rate or increases selection pressure will 
automatically accelerate race development. 
Nematodes possess almost infinite variabil- 
ity. They can infest man, animals, and plants. 
They can thrive in soil and in both fresh and 
salt water. They can withstand extremes of 
temperature from -271° C. to 61.3° C. (131). 
Some species can endure dry storage for as 
long as 39 years (133). Information accumulated 
over the past century indicates that one form 
or another can parasitize most crop and 
ornamental plants grown in the world (133). 
In addition to vast and multidimensional 
patterns of existing variation, many nematode 
species reproduce sexually and thereby insure 
the optimum and virtually limitless potential 
for new genetic combinations, a distinct evo- 
lutionary advantage. Such wide diversity of 
germ plasm provides nematode populations 
with an extremely high degree of versatility to 
adapt rapidly to abruptly changing en- 
vironments. 
RACE FORMATION IN NEMATODES 
Nematodes may be destroyed in large num- 
bers, but the remaining few can rapidly build 
up large populations in the altered environ- 
ment. Survivors of soil fumigation have been 
demonstrated to restore original population 
levels completely in one growing season (92). 
This high reproductive efficiency permits 
increased numbers of new genetic re- 
combinants per generation to be tested against 
all changed conditions. The net result may be 
the evolution of new races in remarkably 
short periods. As already mentioned, any 
condition that speeds mutation rate or in- 
creases selection pressure will automatically 
accelerate race development. 
