The publications in this collection do
not reflect current scientific knowledge
or recommendations. These texts
represent the historic publishing
record of the Institute for Food and
Agricultural Sciences and should be
used only to trace the historic work of
the Institute and its staff. Current IFAS
research may be found on the
Electronic Data Information Source
site maintained by the Florida
Cooperative Extension Service.
Copyright 2005, Board of Trustees, University
Fort Pierce ARC Research Report RL-1973-2
DEVELOPMENT AND CONTROL OF A SEEDLING DISEASE OF TOMATO IN
VIRGIN SANDY SOILS OF SOUTH FLORIDA
R. M. Sonodae
Two water molds, Pythium myriotylum Drechs and P. arrhenomanes Drechs.
were associated with a root and stem rot of seedling tomatoes (2)-in 'plug-
mix' (1) in virgin sandy soils of south Florida. P. myriotylum has been
isolated more frequently from diseased tissue than has P, arrhenomanesi
P. myriotylum was also isolated from diseased seedlings grown in plug-mix
in previously cropped soil at the Agricultural Research Center, Fort
Pierce (ARC-FP). P. aphanidermatum (Eds,) Fitz,. another water mold
occurred commonly on damped-off seedlings on previously cropped soil at
the ARC-FP, This latter pathogen, however, has not been encountered on
diseased seedlings in virgin soil near the ARC-FP. This paper discusses
factors that may contribute to disease development in virgin soil and
control of the disease by fungicides.
Factors Affecting Disease Development
Root and stem rot has been noticed in plug-mix plantings of tomatoes in
virgin soil in growers fields since 1971. It has occurred yearly during the
fall when warm wet weather occurred when plants were young. P. myriotylum
and P. arrhenomanes isolates obtained from diseased tomato seedlings grew
best at about 97F on corn meal agar. This high optimum temperature probably
accounts for the occurrence of the disease during late summer and early fall,
An important factor that appears to contribute to disease development
is a cultural practice employed by many growers on virgin land. Growers
frequently do a minimum of disking and in most cases plant beds are made
with soil containing living native plants. The plant material serves to
reduce erosion of the beds during the frequent heavy rains. Seeds are
usually planted before residues of these plants are completely rotted.
Pythium spp. are frequent primary colonizers of dying plant.material
Isolations from decaying native plant residues have yielded several Pythium
species including P. myriotylum. The decaying plant material in the bed
can serve as a medium for increase of the fungus and as a source of inoculum
for initial disease development.
17TAssistant Plant Pathologist UniversIty of Flori'da Insttute" of Food and
Agricultural Sciences, Agricultural Research Center, Fort Pierce, Florida.
APR 1 1974
.F.AS U .iv.-of -f Rtoda-
Diseases caused by Py-thiu! spp. are favored by high levels of moisture.
lThe materialss used in plug-uia (a 1: 1 ii:x of shredded sphagnum- peat moss and
horticultural vermiculite) maintain a high moisture level in the ii;sediate
vicinity of seeds and seedlings aiding in their growth and dev elopment. The
T:ater-holding capacity of pluv-lmix ia. s about 9,', .lhile the water-holding
capacity for most of the virgin sandy soils in i.hich plug-mi;: seeding :as
used vas about 25:,.
Twio methods of isolation were used to determine if Pythium species
were present in the commercial sources of materials used in plug-ri:x. In
one, aqueous suspensions of samples of the mix:es were plated out on four
different media selective or semi-selective for -rater-nolds. The second
r!ethod consisted of gro 'ing tomato seedlings in snaples of mi: maintained
at about 95, of .,water holding capacity at 90 day and 751 night in a
controlled environment chamber. i'o Pythiumi species were isolated by either
method from any of 24 bags of mi:: sampled at different times during the
past three years.
Since P. i-yriotylua and P. arrhenomianes were the only pathogens isolated
from diseased tissue frovi' tomato -seedling- s grown on virgin soil, fungicides
with activity against Pythiaceous fungi -'ere incorporated into plug;-mix.
After preliminary greenhouse and field tests at ARC-FP, three fungicides
were selected for use in a grov:er test. Disease incidence and severity
were reduced with 5-et'ho:z-3-trichloroI-ethyl-l ,2,,4-thiadiazole (Terraz;ole 35:)
and p-dimethyl aminobenzene diazo sodium sulfonate (Dexon 357) (Table 1).
N-((Trichloromethyl) thio)-4-cyclohexene-1 2-dicarbo.ximide (Captan 507) at
a rate equivalent to one half the concentration currently recommended for
use in plug-mAix as not effective (Table 1). Cood results with higher
levels of captain -were obtained in field tests at the ARC-FP. Of the three
compounds tested, captain is the only one currently labeled for use on
Actinomycetes antagonistic to the grmoth of P. ryriotylum and P.
arrhenomanes on culture medium have been incorporated into plug-tiirx in
attempts to control the two pathogens. So far control by this 1uethod has
1. Hayslip, I. C. 172. A 'plug-mi;" seeding method for field planting
tomatoes and other smaell-seeded hill crops. Fort Pierce ARC iiimeo
Report RL-1972-2. 5 PP.
2. Sonoda, IP. 1.73. Occurrence of a Pythium disease in virgin sandy
soils of south Florida associated with a new method of field seeding
tomatoes. Plant Dis. ,eptr. 57:260-261.
Severity of Pythium root rot on tomato seedlings in plug-mix
treated with fulgicides and deposited in virgin soil in a
growers field ./
Fungicide Concentration (gis)- J3I3 diseased
Terrazole .32 0.2 a4 12
Dexon 0.12 0.3 a 7
Captan 0.62 0.9 ab 42
None --- 1.6 b 43
/Four replicates of ten hills each per treatment, ten plants per hill.
/Grams of active ingredient per 1000 gms dry weight of Jiffy Mix.
SDSI disease severity index where 0 = no disease to 4 = plants dead.
Numbers followed by the same letter are not significantly different
at the 5% level by Duncan's multiple range test.