Rau and Fassuliotis (106) have found evi- 
dence of genetic drift in nematode populations 
by using statistical means. Thus, shifts toward 
new race formation, and even speciation, can 
be shown. Additional studies in microevolution 
of field populations of nematodes are needed, 
Estimates of the number of organisms that 
give rise to the next year's population are an 
important parameter for rates of expected 
genetic drift. 
Among nematologists and plant breeders, the 
development of physiologic races of the golden 
nematode, Heterodera rostochiensis Wollen- 
weber, 1923, is recognized as a classic 
example of race formation in nematodes, and 
is described in the following. 
Breeding studies with resistant clones of 
Solanum tuberosum L, subspecies andigenum 
(Juz. & Buk.) Hawkes were started in Holland 
in 1946 (71) and in England in 1952 (34). This 
species was probably used because it is a 
tetraploid that readily yields fertile hybrids 
with the potato, S. tuberosum L., which is also 
a tetraploid. Nine years later a physiologic 
race of golden nematodes was reported in 
England (34, 78) and another somewhat later 
in Peru (72), In these same experiments, 
Solanum vernei Bitter & Wittm., a diploid, 
maintained its full resistance (34). In Europe 
the latter species was not generally used until 
recently, probably because it is a diploid 
species and therefore fertile hybrids with S. 
tuberosum are difficult or impossible to obtain, 
or because resistance did not “inherit ac- 
cording to simple mechanism" (73). By 1960, 
at least three distinct races of the golden 
nematode could be distinguished in the Nether- 
lands: Race A (the parental strain), race 
Bi, and race B2 (73). In England comparable 
strains were called nonagressive, Duddington, 
and Bogshall (35). It became evident that 
repeatedly growing resistant varieties on the 
same field would not eradicate the golden 
nematode. Nematode populations were sta- 
bilized at a low level after 3 years of cropping 
with resistant varieties; the crop was not 
noticeably damaged (74). However, because of 
the buildup of resistant biotypes (21), a marked 
increase in nematode populations occurred 
after the sixth year. Similar resistance- 
breaking races occurred in Germany (132). At 
the most recent count, 12 races could be 
identified. The need has been expressed for a 
128 
system of nomenclature akin to that used by 
plant pathologists, to identify the unending 
physiologic races that continually appear in the 
late blight organism and the cereal rusts. 
The race problem in the golden nematode 
and in potato late blight organism has several 
striking similarities. Because the late blight 
problem is nearing a workable solution, certain 
similarities may be clues to a solution of the 
golden nematode problem. First, in bothcases 
the factor for resistance came from a single 
species and was inherited as a single dominant 
gene (72). Second, additional races could be 
attributed to mutations at a single gene locus, 
a means by which the various races could be 
identified. Third, there seems to be aninfinite 
number of potential races in both organisms. 
Unlike late blight, which spreads through the 
air, the golden nematode spreads very slowly 
in the soil. Ithas been estimated that nematodes 
seldom move more than 1 to 2 meters per 
year (133, 140, 146), This slow spread is vital 
to effective control measures. Nematodes can, 
of course, be spread by tillage procedures, 
by being carried on potatoes, bags, shipping 
materials, and by similar means. 
The solution to the late blight problem 
appears to center on finding a type of re- 
sistance that protects a plant from all races. 
Such resistance found within the S. tuberosum 
species is inherited by an unknown number of 
multiple genes, and to date it has protected the 
plant against all known races of the pathogen. 
In the United States resistance-breaking 
biotypes of the golden nematode apparently 
have not been found (65). However, breeders 
here have for nearly 10 years been actively 
using resistance from S. vernei, as well as 
that from S, andigerum. A fertile cross withS. 
vernei resulted from an unreduced gamete of 
the diploid S. vernei, combining with the 
reduced gamete of S. tuberosum to from a 
fertile tetraploid hybrid. Subsequent genetic 
studies at the diploid level indicated multigenic 
inheritance of resistance (42, 101). 
A solution to the race problem in golden 
nematodes might very well be found in the use 
of multiple-gene resistance from S. vernei, 
coupled with known resistance from other 
Solanum species (79) to build up a multigenic 
system. The mathematical probability of the 
occurrence of corresponding multiple 
mutations in the parasite would be of lower 
