Cultural practices for root-knot control between annual crops of cigar-wrapper tobacco

Material Information

Cultural practices for root-knot control between annual crops of cigar-wrapper tobacco
Series Title:
Bulletin University of Florida. Agricultural Experiment Station
Alternate title:
Cultural practices for root knot control between annual crops of cigar wrapper tobacco
Kincaid, Randall R ( Randall Rich ), 1903-
Reeves, Jesse
Place of Publication:
Gainesville Fla
University of Florida Agricultural Experiment Station
Publication Date:
Physical Description:
16 p. : ill. ; 23 cm.


Subjects / Keywords:
Tobacco -- Diseases and pests -- Control -- Florida ( lcsh )
Root-knot -- Florida ( lcsh )
City of Gainesville ( local )
Fallow ( jstor )
Tobacco ( jstor )
Soils ( jstor )
bibliography ( marcgt )
non-fiction ( marcgt )


Bibliography: p. 16.
General Note:
Cover title.
Bulletin (University of Florida. Agricultural Experiment Station)
Statement of Responsibility:
Randall R. Kincaid and Jesse Reeves.

Record Information

Source Institution:
University of Florida
Holding Location:
University of Florida
Rights Management:
All applicable rights reserved by the source institution and holding location.
Resource Identifier:
027134731 ( ALEPH )
18235487 ( OCLC )
AEN5843 ( NOTIS )


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Bulletin 392

August, 1943






I w- V w- '-V u -
Fig. 1.-Cigar-wrapper tobacco plants stunted by root-knot: 2 plants in left
foreground are normal in development. (Photo by J. D. Warner.)

Single copies free to Florida residents upon request to


John J. Tigert, M.A., LL.D., President of the
Wilmon Newell, D.Sc., Directors
Harold Mowry, M.S.A., Asso. Director
L. O. Gratz, Ph.D., Asst. Dir., Research
W. M. Fifield, M.S., Asst. Dir., Admin.'
J. Francis Cooper, M.S.A., Editors
Clyde Beale, A.B.J., Assistant Editors
Jefferson Thomas, Assistant Editor'
Ida Keeling Cresap, Librarian
Ruby Newhall, Administrative Managers
K. H. Graham, LL.D., Business Managers
Claranelle Alderman, Accountants

W. E. Stokes, M.S., Agronomist1
Fred H. Hull, Ph.D.. Agronomist
G. B. Killinger, Ph.D., Agronomist
G. E. Ritchey, M.S., Associates
W. A. Carver, Ph.D., Associate
Roy E. Blaser, M.S., Associate
H. C. Harris, Ph.D., Asso. Agronomist
R. W. Bledsoe, Ph.D., Asso. Agronomist
Fred A. Clark, B.S., Assistant
A. L. Shealy, D.V.M., An. Industrialist1 '
R. B. Becker, Ph.D., Dairy Husbandman'
E. L. Fouts, Ph.D., Dairy Technologists
D. A. Sanders, D.V.M., Veterinarian
M. W. Emmel, D.V.M., Veterinarians
L. E. Swanson, D.V.M., Parasitologist4
N. R. Mehrhof, M.Agr., Poultry Hush.3
G. K. Davis, Ph.D., Animal Nutritionist
T. R. Freeman, Ph.D., Asso. in Dairy Mfg.
R. S. Glasscock. Ph.D., Asso. An. Hush.
C. L. Comar, Ph.D., Asso. Biochemist
D. J. Smith, B.S.A., Asst. An. Husb.'
P. T. Dix Arnold, M.S.A., Asst. Dairy Husb.s
L. E. Mull, M.S., Asst. in Dairy Tech.'
0. K. Moore, M.S., Asst. Poultry Hush.3
J. E. Pace, B.S., Asst. An. Husb.3
S. P. Marshall, M.S., Asst. in An. Nutr.
C. B. Reeves, B.S., Asst. Dairy Tech.
C. V. Noble, Ph.D., Agr. Economist1
Zach Savage, M.S.A., Associate
A. H. Spurlock, M.S.A., Associate
Max E. Brunk, M.S., Assistant
Ouida D. Abbott, Ph.D., Home Econ.1
Ruth O. Townsend. R.N., Assistant
R. B. French, Ph.D., Biochemist
J. R. Watson, A.M., Entomologist1
A. N. Tissot, Ph.D., Associates
H. E. Bratley, M.S.A., Assistant
G. H. Blackmon, M.S.A., Horticulturist1
A. L. Stahl, Ph.D., Associate
F. S. Jamison, Ph.D., Truck Hort.
Byron E. Janes. Ph.D.. Asst. Hort.
R. J. Wilmot, M,S.A., Asst. Hort.
R. D. Dickey, M.S.A., Asst. Hort.'
J. Carlton Cain, B.S.A., Asst. Hort.'
Victor F. Nettles, M.S.A., Asso. Hort.4
A. L. Kenworthy, M.S., Asst. Hort.2
F. S. Lagasse, Ph.D., Asso. Hort.2
II. M. Sell, Ph.D., Asso. Hort.2
W. B. Tisdale, Ph.D., Plant Pathologist'
Phares Decker, Ph.D., Asso. Plant Pathologist
Erdman West, M.S., Mycologist
Lillian E. Arnold, M.S., Asst. Botanist
R. V. Allison, Ph.D., Chemist'1
Gaylord M. Volk, M.S., Chemist
F. B. Smith, Ph.D., Microbiologist3
C. E. Bell, Ph.D., Associate Chemist
L. E. Ensminger, Ph.D., Soils Chem.
J. R. Henderson, M.S.A., Soil Technologist
L. H. Rogers, Ph.D., Asso. Biochemist'
R. A. Carrigan, B.S., Asso. Biochemist'
J. N. Howard, B.S., Asst. Chemist
T. C. Erwin, Assistant Chemist
H. W. Winsor, B.S.A., Assistant Chemist
Gee. D. Thornton, M.S., Asst. Microbiologists
R. E. Caldwell, M.S.A., Asst. Soil Surveyor4
Olaf C. Olson, B.S., Asst. Soil Surveyor4


H. P. Adair, Chairman, Jacksonville
R. H. Gore, Fort Lauderdale
N. B. Jordan, Quincy
T. T. Scott, Live Oak
Thos. W. Bryant, Lakeland
J. T. Diamond, Secretary, Tallasassee
J. D. Warner, M.S., Agronomist in Charge
R. R. Kincaid, Ph.D., Plant Pathologist
V. E. Whitehurst, Jr., B.S.A., Asst. An. Hush.4
W. C. McCormick, B.S.A., Asst. An. Hush.
Jesse Reeves, Asst. Agron., Tobacco
W. H. Chapman, M.S., Asst. Agron.'
Mobile Unit, Monticello
R. W. Wallace, B.S., Asso. Agronomist
Mobile Unit, Milton
Ralph L. Smith, M.S., Asso. Agronomist
A. F. Camp, Ph.D., Horticulturist in Charge
V. C. Jamison, Ph.D., Soils Chemist
B. R. Fudge, Ph.D., Associate Chemist
W. L. Thompson, B.S., Entomologist
C. R. Stearns, Jr., B.S.A., Chemist
W. W. Lawless, B.S., Asst. Horticulturist'
R. K. Voorhees, Ph.D., Asso. Plant Path.
H. O. Sterling, B.S., Asst. Hort.
T. W. Young, Ph.D., Asso. Horticulturist
J. W. Sites, M.S.A., Asso. Horticulturist
J. R. Neller, Ph.D.. BineBhemit in Charge
J. W. Wilson, Sc.D., Entomologist4
F. D. Stevens, .., buii L,.e Agron.
Thomas Bregger, Ph.D., Sugarcane
G. R. Townsend, Ph.D., Plant Pathologist
R. W. Kidder, M.S.. Asst. An. Husb.
W. T. Forsee, Jr., Ph.D., Asso. Chemist
B. S. Clayton, B.S.C.E., Drainage Eng.'
F. S. Andrews, Ph.D., Asso. Truck Hort.4
Roy A. Bair, Ph.D., Asst. Agron.
E. C. Minnum, M.S., Asst. Truck Hort.
N. C. Hayslip, B.S.A., Asst. Entomologist
Geo. D. Ruehle, Ph.D., Plant Path. In Charge
S. J. Lynch, B.S.A., Asso. Horticulturist
Clement D. Gordon, Ph.D., Asso. Poultry
Geneticist in Charge2
W. G. Kirk, Ph.D., An. Husb. in Charge
E. M. Hodges. Ph.D., Asso. Agron., Wauchula
Gilbert A. Tucker, B.S.A., Asst. An. Husb.'
M. N. Walker, Ph.D., Plant Path. in Charges
E. M. Andersen, Ph.D., Associate Horticul-
turist Acting in Charge
Plant City
A. N. Brooks, Ph.D., Plant Pathologist
A. H. Eddins, Ph.D.. Plant Pathologist
E. N. McCubbin, Ph.D., Truck Horticulturist
S. 0. Hill, B.S., Asst. Entomologists 4
A. M. Phillips, B.S., Asst. Entomologist'
J. R. Beckenbach, Ph.D., Horticulturist in
E. G. Kelsheimer, Ph.D., Entomologist
F. T. McLean, Ph.D., Horticulturist
A. L. Harrison, Ph.D., Plant Pathologist
David G. Kelbert, Asst. Plant Pathologist
R. W. Ruprecht, Ph.D., Chemist in Charge
J. C. Russell, M.S., Asst. Entomologist
E. S. Ellison, Meteorologist2 5
Harry Armstrong, Meteorologist2

SHead of Department.
2 In cooperation with U. S.
3 Cooperative, other divisions, U. of F.
In Military Service.
5 On leave.

Sift of Ianiiii offit


Page Page
Introduction -........... .. .............. .......... 3 Results ................................ ............. 9
Review of Li....terature ......... ............. 6 Discussion ................................................. 11
Experimental ............................ ............. 7 Summary ........... ....................................... 15
Materials and Methods ........................ 7 Literature Cited ..................................... 16

Root-knot is at present the most serious disease of cigar-
wrapper tobacco in Florida. Losses in crop value range from
a trace in nearly every shade to about 50 percent in a few shades,
averaging perhaps 5 percent. Additional losses are incurred
by the necessity of resting or moving the shades (Fig. 2). This
prevents the grower from deriving maximum returns from the

1 The assistance of Mary W. Love, especially with the calculations and
photography, is hereby gratefully acknowledged.
Fig. 2.-Exterior view of cigar-wrapper tobacco shade.


Page Page
Introduction -........... .. .............. .......... 3 Results ................................ ............. 9
Review of Li....terature ......... ............. 6 Discussion ................................................. 11
Experimental ............................ ............. 7 Summary ........... ....................................... 15
Materials and Methods ........................ 7 Literature Cited ..................................... 16

Root-knot is at present the most serious disease of cigar-
wrapper tobacco in Florida. Losses in crop value range from
a trace in nearly every shade to about 50 percent in a few shades,
averaging perhaps 5 percent. Additional losses are incurred
by the necessity of resting or moving the shades (Fig. 2). This
prevents the grower from deriving maximum returns from the

1 The assistance of Mary W. Love, especially with the calculations and
photography, is hereby gratefully acknowledged.
Fig. 2.-Exterior view of cigar-wrapper tobacco shade.

Florida Agricultural Experiment Station

large investment which he must make in shade structure and
heavy fertilization, and from the superior quality of tobacco
which he could usually produce on old shades in continuous
The root-knot disease is caused by small eel worms or nema-
todes (Heterodera marioni Goodey) which enter the root tips
of tobacco or other host plants and live there. The activities
of the worms cause the roots to swell into the form of knots or
galls (Fig. 3), and the entire plant becomes more or less yel-
lowed and stunted and more subject to wilting during periods
of dry weather or hot sunshine (Fig. 1).


Fig. 3.-Roots of a cigar-wrapper tobacco plant heavily infected with
root-knot nematodes.
A female nematode matures at the age of 3 weeks or more,
depending on the temperature, and lays hundreds of eggs, which
are sometimes held together in masses by a gelatinous material.

Fig. 4.-Stages in the life history of the root-knot nematode: Eggs and larvae (left) and mature, egg-laying female (right).
Magnified about 125 times.

Florida Agricultural Experiment Station

Under favorable conditions these eggs hatch into larvae, which
remain within the same root or migrate through the soil and
infect other roots. Newly hatched larvae are slender and about
1/50 inch long, too small to be seen with the naked eye. Mature
egg-laying females are pear-shaped or nearly round, and their
bodies are about 1/30 inch in diameter or about the size of the
period at the end of this sentence. They can be seen as pearly
white bodies when an old gall is broken or crushed (Fig. 4).

Conditions such as temperature, moisture and aeration, as
they affect the development of root-knot nematodes, have been
studied by many investigators. A few findings of most interest
to the present study will be mentioned here.
Nematodes develop at any temperature between 500 and 900
F.; at the most favorable temperature, about 810, the time re-
quired for a complete generation is 23 to 36 days. Above 810
activity is reduced. Freezing at 320 does not kill larvae, a tem-
perature of 0 for 2 hours being required (6)2.
All larvae are killed at 1040 in about 2 hours and at 1090 in
8 minutes; eggs in masses survive for 41/2 days at 104 (3).
Nematodes are active throughout a wide range of moisture
conditions, from wet to fairly dry. Whenever soil in the field
becomes extremely dry nematodes unprotected by living roots
are rather easily killed; normally, however, only the uppermost
layer of the soil becomes dry enough to kill the nematode (7).
The soil atmosphere below the dry surface layer is saturated with
water vapor, and this condition favors the long-continued sur-
vival of the larvae, because they are not so active as to deplete
their store of reserve energy (6).
Aeration of the soil is favorable to hatching of the eggs;
a crust on the soil reduces aeration and delays hatching (7).
After the eggs hatch if no susceptible plants are available the
larvae gradually exhaust themselves and die. Larvae are more
susceptible than eggs to killing by various adverse conditions.
Several instances have been reported where cultural practices
have been found effective for root-knot control on field crops.
The Georgia Coastal Plain Experiment Station (2) recom-
mended fall cultural and sanitary practices for root-knot control
in flue-cured tobacco. These practices included "the plowing
2 Italic figures in parentheses refer to "Literature Cited" in the back
of this bulletin.

Cultural Practices for Root-Knot Control

out of tobacco stalks immediately after harvest and exposing
the roots to the sun, and of turning the land several times from
August to November."
Watson and Goff (7) found that summer fallow, "which con-
sists in keeping the soil entirely free of all plants during the
summer," was effective in reducing the nematode population of
Florida truck lands. Their objection to this method of root-
knot control was that it depleted soil fertility, and they recom-
mended rather the growing of an "immune" cover crop such
as velvet beans or Crotalaria spectabilis.
Le Roux and Stofberg (4), working in the Union of South
Africa, reported that a 6-month period of clean cultivation in
either winter or summer gave commercial control of root-knot
on at least 1 succeeding susceptible crop. The 2 6-month periods
which they tried corresponded to October to April and April
to October, respectively, in the northern hemisphere.

Experiments on fall cultural practices for root-knot control
were begun at the North Florida Experiment Station in the fall
of 1937. The field selected was a shade in which 15 consecutive
crops of cigar-wrapper tobacco had been grown. The soil was
heavily infested with nematodes, as shown by knotting of roots
and stunting of plants in the 1937 crop. The soil was a fine
sandy loam in texture. About 70 percent of the field was of
the Magnolia series and most of the remainder was of the closely
related Faceville series.
Twenty plots of about 1/25 acre each were used for 5 treat-
ments in quadruplicate. The 5 treatments were arranged in ran-
dom order in each 1/ of the area. Each plot consisted of 6
70-foot rows, 41/2 feet apart. Only the 4 inside rows of each
plot were harvested for experimental purposes. Border effects,
such as the transfer of nematodes, were thereby minimized.
The general layout of the plots is shown in Fig. 5.
The 5 treatments which were begun in July after the 1937
crop was harvested were as follows:
1. Fallow (roots removed). Tobacco roots pulled and re-
moved from the field; soil kept clean fallow by listing (bedding
and rebedding) at intervals to be mentioned later.
2. Fallow. Roots not removed; clean fallow.

Fig. 5.-General view of plots in root-knot control experiment, October 3,
1940. Numbers refer to treatments. (Photo by Ray Robie.)

3. Fallow-oats. Roots not removed; clean fallow, followed
by a cover crop of oats sowed during the second week in October.
4. Sorghum-fallow. Roots not removed; cover crop of sor-
ghum, followed in November by fallow.
5. Grass and weeds. Roots not removed; cover crop of native
vegetation, mostly grasses.
Stalks on all plots were covered and left to decay in the soil.
In January all plots were fertilized with stable manure, about
8 tons per acre, then listed, and thereafter treated exactly alike
until the end of the tobacco crop season in July.

Fig. 6.-Method of listing the soil, leaving a balk in the center of the furrow.

Cultural Practices for Root-Knot Control

The dates of listing before October 1 for each year were as
follows: July 30, August 18, September 9 and 22, 1937; July
11, August 4 and 18, September 5 and 19, 1938; July 6 and 24,
August 3, 14 and 22, September 1, 12 and 25, 1939; July 26,
August 8 and 23, September 9 and 23, 1940. During the months
of October to January the interval between listings was gradually
lengthened to about 2 months.
The method of listing the soil during periods of clean fallow
is shown in Fig. 6. The balk left in the furrow served to in-
crease the effects of aeration and drying. The balks were plowed
each time before the soil was rebedded.
Tobacco crops of the Rg variety were grown each year and
harvested, cured, sweated and graded according to usual com-
mercial practices.
After the crop was harvested a representative sample of the
plants from each plot was pulled and examined for root-knot,
and the plants were placed in 5 groups to which were assigned
the following numerical values: None, 0; very slight, 10; slight,
20; medium, 60; and heavy, 100. The average value for the
plot was the root-knot index, a measure of the amount of root-
knot infection.
The grade index was the average value per pound, calculated
from the yield of leaves in each grade and the comparative
values of the grades. This served as a measure of leaf quality.
The crop index was obtained by multiplying the yield in pounds
per acre by the grade index. The crop index was a measure of
the total value of the crop.
Daily minimum and maximum air temperatures and rainfall
were observed about 100 yards from the plots. During the first
2 years of the experiment daily minimum and maximum soil
temperatures were measured by means of registering ther-
mometers; readings were taken at depths of 1 and 4 inches
about 100 feet from the plots in the same shade.

Table 1 gives the root-knot index, yield, grade index, and crop
index for each treatment in each of the 4 crop years. The results
are averages for the 4 plots of a given treatment. For con-
venience in comparing the results of the various years the yield,
grade index and crop index are also calculated on a comparative
basis, using a value of 100 percent for the results of Treatment
3 each year.


Treat- Root. Yield Grade Results Compared
Crop Year ment Treatment Knot Per A. Index Crop with Treatment 3
No. Index* Index Grade Crop
__Yield Index Index
lbs. % % %
1937-1938 1 Fallow (roots removed) 47 1395 0.872 1216 101 98 99
2 Fallow 41 1446 0.893 1291 105 100 105
3 Fallow-oats 45 1380 0.891 1230 100 100 100
4 Sorghum-fallow 62+ 1136- 0.898 1020 82- 101 83
5 Grass and weeds 52 1246 0.913 1138 90 102 93
Least significant difference** 11 234 0.049 253

1938-1939 1 Fallow (roots removed) 14 795 1.056 840 94 103 97
2 Fallow 8 844 1.042 879 99 102 91
3 Fallow-oats 10 849 1.023 869 100 100 100
4 Sorghum-fallow 19+ 724- 1.037 751 85- 101 86
5 Grass and weeds 17+ 567- 1.019 578- 67- 100 67-
Least significant difference** 6 115 0.046 131

1939-1940 la Soybeans-fallow 27+ 1262 0.793 1001- 96 94 90-
2 Fallow 17 1264 0.844 1067 96 100 96
3 Fallow-oats 11 1321 0.841 1111 100 100 100
4a Crotalaria-fallow 14 1291 0.800 1033 98 95 93
5 Grass and weeds 8 1281 0.905+ 1159 97 108+ 104
Least significant difference** 12 71 0.054 88

1940-1941 lb Cockleburs-fallow 64+ 1023 0.861 881 92 100 92
2 Fallow 34 1057 0.843 891 95 98 93
3 Fallow-oats 41 1110 0.861 956 100 100 100
4a Crotalaria-fallow 59 1058 0.853 902 95 99 94
S5 Grass and weeds 24 1045 0.934+ 976 94 108+ 102
SLeast significant difference** 22 90 0.043 104
All 4 years 2 Fallow 25 1153 0.905 1032 99 100 99
3 Fallow-oats 27 1165 0.904 1041 100 100 100
5 Grass and weeds 25 | 1035- 0.943+ 962- 89- 104+ 92-
_Least significant difference** 6 56 0.030 74 _
Average values at beginning of experiment, 71.
** Odds for significance 19:1. (+indicates values significantly higher than those of treatment 3; --lower).

Cultural Practices for Root-Knot Control

CROP I Tempera- I Dat of I Root-Knot Grade
YEAR Rainfall ture Setting Rainfall Index* Yield Index
1937-1938 above normal low IApril 1 normal medium high fairly low
1938-1939 low high Mar. 31 normal** low low high
1939-1940 low moderate April 10 normal low high low
1940-1941 low low April 18 lowt medium medium low
High at beginning of experiment.
** Excessive during harvesting period.
t Normal during harvesting period.

Table 2 gives general observations on the results by crop years,
with particular reference to Treatment 3, fallow followed by
oats. The root-knot index was high at the beginning of the
experiment, fell to medium for the first year, dropped to a low
reading for the second, remained low for the third and rose to
medium for the fourth. The 1938 fall season, which gave a
striking reduction in root-knot, had low rainfall and high tem-
perature, both favorable to root-knot control, as will be men-
tioned again in the discussion of the results. The 1940 fall
season, which gave an increase in root-knot, had low rainfall
but the temperature was also low.


Treatment 3, fallow followed by oats, gave slightly the best
4-year average yield of the 3 treatments which were continued
throughout the experiment. Results of this treatment are used
as a basis for evaluating results of other treatments.
Treatment 2, fallow only, gave a little less root-knot 3 years
out of 4 than Treatment 3; the quality was about the same.
Yield and total value were highest of all treatments the first
year and then steadily declined in relation to those of Treat-
ment 3. This decline was probably due to a gradual depletion
of the organic matter reserve in the soil.
Treatment 1, which differed from Treatment 2 in having the
tobacco roots removed from the field, gave slightly poorer re-
sults than Treatment 2 but the difference was within the limit
of experimental error. After 2 years the original Treatment 1
was discontinued and a cover crop of Otootan soybeans was sub-
stituted; these were moderately infected with nematodes and
root-knot in the succeeding tobacco crop showed a considerable
increase. The last year of Treatment 1 a few cockleburs
(Xanthium sp.) were grown to maturity and all other vegeta-

Florida Agricultural Experiment Station

tion was controlled by shallow plowing and hoeing. The suc-
seeding tobacco crop had high root-knot and low yield. Cockle-
burs have not shown root-knot at this Station, even in highly
infested soil, but shallow cultivation was apparently not effec-
tive in killing the nematodes already present.
Treatment 4, a cover crop of sorghum, Texas Seeded Ribbon
Cane, planted in rows and followed in November by fallow was
unsatisfactory because of the poor stand and growth of the
sorghum, leaving the rows occupied mostly by native vegeta-
tion. Root-knot was high and yield was low in both years. After
2 years, Crotalaria spectabilis Roth was substituted for sor-
ghum. This made fair growth before frost and the results were
better than those with sorghum but not as good as fallow fol-
lowed by oats. The explanation of the poor results with fall
cover crops probably lies in the fact that only about 1/2 of the
soil surface can be cultivated and then not so thoroughly as
with clean fallow. This would favor the survival of most of
the nematodes present in the soil.
Treatment 5, a cover crop of native vegetation mostly
grasses gave various results. Root-knot varied from next to
highest for the first 2 years to lowest for the last 2 years.
Yields were lower every year than for fallow followed by oats;
they were extremely low in 1939 when the plants were badly
stunted. The quality was about the same as that for fallow
followed by oats for the first 2 years and considerably higher
for the last 2 years.
The native vegetation on plots of Treatment 5 consisted mainly
of 3 grasses: Goose grass (Eleusine indica Gaertn.), crab grass
(Digitaria serotina Michx.) and jungle rice (Echinochloa colo-
num Link).3 Several other species of grasses and weeds were
observed in small numbers. The native cover crop made a dense
growth before it was killed by frost in November. This was
turned under in January and appeared to be well decayed by
transplanting time.
Tobacco plants from plots of Treatment 5 showed a high per-
centage of abnormal root systems (Fig. 7). The lower portion
was coarse and deficient in small fibrous roots. The upper por-
tion of the root systems was usually normal in appearance.
The explanation for the abnormal root systems may be that
decomposition of the grass residues caused an unfavorable soil
3Identifications by Erdman West, Department of Plant Pathology.

Cultural Practices for Root-Knot Control

Fig. 7.-Abnormal tobacco root system, grown after a cover crop of native
vegetation (right); normal root system (left).

condition which disappeared before the end of the tobacco grow-
ing season. The decomposition of the grass may also have
caused the reduction in root-knot index for certain years, as
was demonstrated for a coarse grass (Panicum barbinode Trin.,
Para grass) by Linford, Yap and Oliveira (5). Results of this
experiment might have been quite different if the cover crop
had been turned under a month or 2 earlier.
Nematode control obtained by clean fallow under the condi-
tions of this experiment was probably due to a combination of
4 separate factors, which are listed and discussed.
1. Aeration.-Cultivation supplies oxygen to the soil, pro-
viding conditions more favorable to the hatching of nematode
eggs than in soil allowed to become crusted and deficient in
oxygen (7). Larvae are more susceptible than eggs to killing
by other factors to be mentioned.
2. Starvation.-To obtain most effective control of nematodes
by starvation, according to Tyler (6), weeds must be destroyed
every 10 days in warm weather, to keep the worms from matur-
ing and producing eggs; the interval between plowings may
gradually be lengthened to a month or 2 in cold weather. Since
rain often interferes with the plowing schedule it is probably

Florida Agricultural Experiment Station

desirable during July, August and September to list the soil at
weekly intervals, or as nearly so as the weather permits.
3. Heat.-Direct readings of soil temperature at a depth of
1 inch showed that the soil frequently became hot enough to kill
all larvae in about 2 hours (1040 F.) and occasionally in only 8
minutes (1090 F.). Some larvae are killed by shorter exposures
than those mentioned (3).
During August and September, 1938, soil temperatures at a
depth of 1 inch reached 1040 or above on 12 days, including 2
consecutive days with readings of 1100. During the same period
temperatures at a depth of 4 inches reached 990 on several days
and 100 once, but these temperatures would not likely kill nema-
4. Drying.-A certain layer of surface soil may become dry
enough to kill nematodes (6, 7). This surface layer is changed
at each cultivation. Listing exposes more soil surface for dry-
ing (Fig. 6) than flat cultivation with a turning plow or harrow,
and incidentally reduces the amount of erosion. Listing of a
field may be done in the direction which best conforms to the
These factors of heat and drying operate most effectively dur-
ing the hot weather from July to the first or middle of October.
Plots sowed in oats in October showed almost as good root-knot
control and maintained their yield and quality better than those
where clean fallow was continued until January. Therefore,
following clean fallow a cover crop of oats sowed about the mid-
dle of October is recommended for shades which are to be
planted in tobacco the following year. The oat crop should be
plowed under and completely destroyed by February 1 to avoid
excessive trouble with cutworms (1).
An incidental benefit from fall plowing was mentioned by
Chamberlin and Madden (1). They reported that 1 fall plowing
of the soil to a depth of about 7 inches killed about 1/2 of the
hornworm pupae which would otherwise have passed the winter
successfully, and stated that a still greater mortality would be
expected to result from several cultivations of soil infested with
Clean fallow has been recognized by the Agricultural Adjust-
ment Administration as an alternate qualifying practice for
tobacco shades in 1942.4

"Maintenance of clean fallow by complete listing at least 6 times
during the period from the completion of harvest to October 1, 1942. The

Cultural Practices for Root-Knot Control

It is commonly observed that root-knot on cigar-wrapper to-
bacco is more severe on light sandy soil than on heavier soil.
Wherever there is a choice of soil suitable for shades the heavier
soil is usually planted, partly for this reason. The difference
in severity of root-knot is explained by the fact that the lighter
the soil the more readily the larvae can travel to reach the host
plant (6).
Although the experimental work was conducted on fine sandy
loams, it would be logical to suppose that the effects of heat
and drying in killing nematodes would be greater in a lighter
soil. On the other hand, depletion of organic matter might be
more serious, unless this can be offset by cover crops of oats
and heavy application of stable manure.
The effects of clean fallow will be studied further in connec-
tion with an experiment now in progress on 2-year rotations
for root-knot control. In this experiment tobacco is rotated
with various other crops, including native weeds and grasses
as well as cultivated crops. As a check on the 2-year rotations,
1 treatment is continuous tobacco, followed each year by clean
fallow and then oats, corresponding to Treatment 3 of the pre-
vious experiment.
Experiments were conducted on the effect of certain fall cul-
tural practices for root-knot control in a cigar-wrapper tobacco
shade, beginning with soil heavily infested after the 15th con-
secutive crop of shade tobacco had been harvested from the
field. The various treatments were applied from July to January
of each crop year, after which all plots were treated exactly
alike until the end of the harvesting season in July.
Clean fallow, maintained by frequent listing of the soil,
followed in October by a cover crop of oats, gave the most con-
sistently good results of the treatments tried. The results of
this treatment were used each year as a basis for evaluating
the results of other treatments.
Clean fallow for the entire 6-month period gave slightly less
root-knot than clean fallow followed by a cover crop of oats
but about the same quality; the yield was higher for the first
year only and then steadily declined, due probably to a gradual
first listing is to be made not later than 10 days after completion of harvest
and subsequent listing at intervals of not less than 7 days and not more
than 15 days apart."-AAA Handbook for Florida for 1942. U. S. D. A.
Agricultural Adjustment Administration, 1941, p. 14.

Florida Agricultural Experiment Station

depletion of the organic matter in the soil. The further pre-
caution of removing the tobacco roots from the field gave slightly
poorer results, but the difference was within the limit of experi-
mental error.
A cover crop of Crotalaria spectabilis, planted in 27-inch rows
and cultivated several times, followed in November by fallow,
gave somewhat poorer results than clean fallow followed by
a cover crop of oats.
A cover crop of native vegetation, mostly grasses, plowed
under in January, gave variable results, with slightly to greatly
reduced yields, but average quality or better. The tobacco root
systems were abnormally coarse and deficient in fibrous roots.
The nematode control obtained with clean fallow treatments
is explainable by the operation of 4 factors: (1) Aeration to
promote hatching of nematode eggs; (2) starvation of larvae
in the absence of host plants; (3) heat and (4) drying in a
surface layer of soil, which was changed at each listing.

1. CHAMBERLIN, F. S., and A. H. MADDEN. Insect pests of cigar-type
tobaccos in the southern districts. U. S. D. A. Circ. 639. 54 pp. 1942.
2. GEORGIA COASTAL PLAIN EXP. STA. Root-knot control by cultural prac-
tices. In its 19th Ann. Rpt. 1938-1939; 126-127. 1939.
3. HOSHINO, HELENE MORITA, and G. H. GODFREY. Thermal death point of
Heterodera radicicola in relation to time. Phytopath. 23: 260-270.
4. LE Roux, J. C., and F. J. STOFBERG. Control of the root-knot nematode
by cultural practices. Union So. Africa Dept. Agr. and Forestry,
Science Bul. 188. 29 pp. 1939.
5. LINFORD, M. B., FRANCIS YAP and J. M. OLIVEIRA. Reduction of soil
populations of the root-knot nematode during decomposition of organic
matter. Soil Science 45: 127-142. 1938.
6. TYLER, JOCELYN. The root-knot nematode. Calif. Agr. Exp. Sta. Cir.
330. 35 pp. 1933. Reprinted, 1937.
7. WATSON, J. R., and C. C. GOFF. Control of root-knot in Florida. Fla.
Agr. Exp. Sta. Bul. 311. 22 pp. 1937.