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-NORTH FLORIDA EXPERIMENT STATION
Quincyj Florida ."''.
SOILS DEPARTMENT, MAIN STATION
Gainesville, Florida Il FEL 6 Jb57
August 14, 1956
NFES MIMEO RPT. 57-4 -
SOILS DEPT. MIMEO RPT. 57-2
EFFECT OF SOIL pH ON THE INCIDENCE OF THREE SOIL-BORNE
DISEASES OF TOBACCO
by Randall R. Kincaid, Plant Pathologist, North Florida
Experiment Station and Nathan Gammon, Jr., Soils
Chemist, Main Station
Test crops of tobacco have been grown under shade for three seasons on
quadruplicate field plots, adjusted to five different pH levels with elemental
sulphur and hydrated lime. Each year the incidence of blackshank in a suscep-
tible variety varied directly with soil pH, as previously reported on the basis
of the first year's results (1954); differences were highly significant. There
was some indication that blackshank in resistant varieties was less severe at
soil pH about 5.8 than at lower or higher pH values. The incidence of root knot
and of coarse root (nematode root rot) varied inversely with soil pH; differences
were significant, especially for 1956.
INTRODUCTION AND METHODS
The incidence of blackshank varied directly with soil pH for 1954, as
reported in Plant Disease Reporter 38(12): 852-853, Dec. 15, 1954. The experiment
has been continued through the 1955 and 1956 seasons. Further applications of
sulphur and lime have been made between crop seasons to individual plots, in an
attempt to maintain differences between the five pH levels, and to reduce
differences among the four plots at each level. Soil pH readings were generally
increasing during the crop season. The pH values reported are simple averages
of readings made for two rows (fumigated and unfumigated) in each plot at each of
three dates during the crop season.
Varieties of tobacco used in the experiment were as follows:
No. 870 Highly susceptible to blackshank.
Connecticut 49 Moderately susceptible to blackshank, susceptible to
root knot and coarse root.
Rg Resistant to blackshank, susceptible to root knot and coarse root.
Dixie Highly resistant to blackshank, moderately resistant to root
knot, and highly susceptible to coarse root.
The three rows in each plot were managed as follows: One row (unfumigate
was interplanted with 870 and Dixie in 1954, and with 870, Connecticut, and Dixie
in 1955 and 1956. A second row (fumigated each year with ethylene dibromide) was
transplanted to Dixie in 1954, and to Rg in 1955 and 1956. The third row (unfumi-
gated) was transplanted to Dixie each season.
NFES Mimeo Rpt. 57-4 and Soils Dept4 Mimeo Rpt. 57-2 cont'd.
The number of plants dead or severely stunted by blackshank was counted
at intervals of about a week. The percentage of affected plants of the susceptible
variety about 1 month after transplanting, and of all varieties about 3 months
after transplanting are reported. Shortly after the end of leaf harvest, all of
the plants were pulled and rated for severity of root diseases on scales of 0 to
100; the average ratings are reported as root disease indexes.
Results, given in Table 1, include least significant differences between
means; the occurrence of differences significant at the 5% and 1% levels is indicate
in the table by asterisks. Some interesting and significant results may be pointed
Blackshank in the highly susceptible variety (No. 870) varied directly
with soil pH for each of the three seasons; differences were highly significant.
Blackshank in moderately to highly resistant varieties (Connecticut 49,
Rg, and Dixie) rose with increasing soil pH to 5.5 5.7 (significant in six
instances), and then fell at about pH 5.8 (significant in one instance). Above
pH 5.8, incidence was again generally rising (not significant) to pH 6.2, the limit
of this experiment to date.
Root knot varied inversely with soil pH for 1954 and 1955, but results
were not significant. All four sets of results for 1956 were significant, three
of them being highly significant.
Coarse root varied inversely with soil pH for 1954 and 1956. DifferenceL
were significant in one set of results out of two for 1954, and in three out of
four for 1956.
Blackshank and the nematode diseases, root knot and coarse root, responded
in opposite directions to soil pH as adjusted by additions of sulphur and lime.
Yield response, as affected by the incidence of these diseases, would probably
depend on their relative severity.
Yield data for 1954 and 1955 showed significant differences in only one
instance; the yield of Rg on fumigated soil for 1955 was inversely related to soil
pH. Blackshank, which showed relatively large differences, was probably the most
important disease factor in determining the yield, because the incidence of root
knot was so low that it was probably not important, and the incidence of coarse
root was quite uniformly high.
The experiment is being continued. A paper on crop and soil results for
the three years of the experiment is in preparation.
NORTH FLORIDA EXPERIMENT STATION
Table 1. Incidence of blackshank, root knot, and coarse root at five soil pH levels, 1954-1956.
Blackshank (dead or stunted)
Dixie Fum. fum.
V % %
Dizie Fum. fun.
Root knot index
ConnI Di-Yel Fun, fumn
Coarse root inoe-"
ConnI Dixie1 P:m,
L.S.D.,5% 26* 24*
3% 36* 34*
1955 5.04 43 77
5.28 89 97
5.48 100 100
5.72 98 100
6.00 100 100
L.S.D.,5% 14* 10*
1% 20* 13*
1956 5.26 12 100
5.57 16 100
5.76 36 100
5.84 49 100
6.24 52 100
L.S.D.,5% 17* -
1% 24* -
3Grown in the interplanted row
- 22* 14
- 31 19
of each plot.
2Substituted for Dixie in 1955 and 1956.
91 89 92
82 80 82
83 77 79
81 73 80
78 66 77
9* 8* 8*
13* 12* 12*