THE DEPTH DISTRIBUTION OF THE ROOT-KNOT 
NEMATODE, HETERODERA RADICICOLA, 
IN FLORIDA SOILS 1 
By G. H. Godfrey 2 
Pathologist, Office of Cotton, Truck, and Forage Crop Disease Investigations , 
Bureau of Plant Industry, United States Department of Agriculture 
INTRODUCTION 
In any measure taken to decrease the 
number of root-knot nematodes in the 
soil, one of the factors that must be 
taken into consideration is the depth at 
which they occur. Whatever treat¬ 
ment may be given, whether chemical, 
heat, or cultural, must be applied with 
the idea of reaching the large majority 
of the parasites, or its effectiveness will 
be greatly diminished. The experi¬ 
ments described in this paper were 
designed to give a rough determination 
of the relative root-knot nematode con¬ 
tent of certain soils at different depths, 
and at different seasons. 
LITERATURE 
Only the most general information on 
this subject is available in the nematode 
literature. Cobb 3 indicates, in regard 
to soil-inhabiting nematodes in general, 
that they are likely to be most abund¬ 
ant near the surface and gradually 
decrease in numbers as the depth in¬ 
creases. Atkinson 4 found root knots 
on parsnips and tomatoes 15 to 18 
inches below the surface of the ground. 
Larvae released from such knots would 
naturally increase very greatly the 
numbers of nematodes at that 
depth, for a time at least. Bessey 5 
stated, in connection with the over¬ 
wintering of the organism, that the 
nematodes probably descend into the 
lower levels of the soil to avoid the 
cold. No exact information on this 
point appears to be available. Frank 6 
mentioned the occurrence of galls on 
deep rooted plants, and discussed the 
possibility of there being a positive 
geotaxis due to gravitation. 
METHODS 
The general methods followed in 
this experimental work were to collect 
soil samples at the desired depths from 
thoroughly infested soils and to grow 
plants susceptible to root-knot in them 
under uniform conditions. The num¬ 
ber of distinct knots that appeared on 
the roots within a given period was 
taken as an index of the number of 
nemas in the soil, the results being 
naturally purely relative. It was as¬ 
sumed that a sample producing twice 
as many knots as another sample had 
relatively twice as many nemas capable 
of producing infection. This assump¬ 
tion need not be exactly correct in 
order that the general results be com¬ 
parable. 
The soil samples were taken in suc¬ 
cessive layers at depths of 1 to 2 inches, 
3 to 4 inches, 5 to 6 inches, 7 to 8 inches, 
9 to 10 inches, 11 to 14 inches, 15 to 
18 inches, 19 to 22 inches, 23 to 26 
inches, 27 to 30 inches, and 31 to 34 
inches, respectively. The samples were 
taken in quadruplicate, each sample 
being sufficient to fill a 6-inch flower pot. 
In collecting the samples every pre¬ 
caution was taken to avoid contami¬ 
nating any lot with soil from another 
level. A V-shaped trench was dug to 
a depth of 34 inches. This is indicated 
diagrammatically in Figure 1. With 
a large, flat trowel the 1- to 2-inch 
sample was taken from the top of the 
soil at one end of the trench, and tfie 
excess soil to that depth lifted and dis¬ 
carded. After taking the sample, the 
trowel was sterilized by dipping in a 
boiler of hot water. Then the next 
layer was removed, the trowel sterilized, 
and so on until the 11 sets of samples 
were taken. 
1 Received for publication May 5, 1924—issued January, 1925. 
2 The writer acknowledges credit due to Dr. G. M. Armstrong, now of the Missouri Botanical 
Garden, St. Louis, Mo., for an active part in this and other nematode problems of the South during the 
summers of 1920, 1921, and 1922. 
3 Cobb, N. A.— nematodes and their relationships. U. S. Dept. Agr. Yearbook 1914: 457-490, 
llus. 1915. 
4 Atkinson, G. F.—nematode root-galls. Ala. Agr. Exp. Sta. Bui. 9, 54 p^ illus. 1889. 
5 Bessey, E. A.— root-knot and its control. U. S. Dept. Agr. Bur. Plant Indus. Bui. 217: 36 
illus. 1911. 
« Frank, B. ueber das wurzelalchen und die durch dasselbe verursachten beschadigungen 
der pflanzen. Landw. Jahrb. 14:164, illus. 1885. 
Journal of Agricultural Research, 
Washington, D. C. 
( 93 ) 
Vol. XXIX, No. 2 
July 15, 1924 
Key No. G-446 
