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70 GULF COAST RESEARCH & EDUCATION CENTER iij
IFAS, UNIVERSITY OF FLORIDA
5007 60th Street East
Bradenton, FL 34203
Bradenton GCREC Research Report BRA1987-25 .....D-ecember-1987
FUNDAMENTAL FACTORS NECESSARY FOR A SUCCESSFUL SCREEN FOR
FUNGICIDES TO CONTROL FUSARIUM WILT OF CHRYSANTHEMUM
Arthur W. Engelhardl
A model system for chemical control of symptoms incited by Fusarium
oxysporum f. sp. chrysanthemi on chrysanthemum is described. Nitrogen
source, soil pH, chemotherapeutant, temperature, inoculum density, and
cultivar affect disease level. A model system is a mix of 5:3:3:1 parts
peat, sand, vermiculite and perlite to which 424 g dolomite, 42 g Ca(OH)2,
424 g CaCO3, 106 g superphosphate and 63.5 g [iicromaxR are added to 45
liters of mix. One Royal Trophy chrysanthemum cutting is planted per 10
cm pot. Candidate fungicides are drenched on the soil at 50 ml/pot 3 and
10 days after planting and fertilizer solution (50 ml/pot of 6.6 g/l
Ca(N03)2 + 1.5 g/l KC1) twice weekly beginning at planting. Plants are
inoculated at 11 days by drenching with 50 ml spore suspension containing
10' conidia per pot. Parallel cuts are made in the medium 2.5 cm away
from the stem to injure the roots before inoculation. In one study,
benomyl 50W at 0.6 and 0.15 g a.i./l gave a rating of 0.4 and 2.3 (0-5, 5
= dead plant) after 9 wks; thiophanate M 70W at 0.6 g a.i./l gave 1.0;
diniconazole 25W at 0.15 g a.i./l gave 1.5; triflumizole 50W at 1.2 g
a.i./l gave 2.1 and the inoculated check was 4.4.
The development of a reproducible procedure for evaluating the
chemotherapeutant ability of a chemical to control or reduce the symptoms
of Fusarium wilt of chrysanthemum (incited by Fusarium oxysporum f.sp.
chrysanthemi) usually requires many trial and error attempts because of
the many factors that significantly alter the level of symptoms.
Cultivar, inoculum density, temperature, pH, nitrogen and minor element
nutrition, and chemotherapeutant each can change the level of symptoms of
the disease. Fusarium wilt is a deadly disease.
This publication outlines the exact requirements of each of the factors
that can vary the level of the disease, explains the specific role of each
of the regulating factors, and presents an overall balanced program of all
the regulating factors that allow the evaluation of unknown chemicals for
control of symptoms of Fusarium wilt.
1Professor of Plant Pathology.
Procedures and Details of the Method
Specific day-to-day procedures are listed in outline form in the following
table. The details for each procedure are listed with the respective note
FUNGICIDE SCREEN FUSARIUM WILT CHRYSANTHEMUM
Day Specific Test Procedure See Note No.
1 Plant 1 rooted cv Royal Trophy cutting/10 cm pot 1,2
1 Fertilize drench 50 ml solution/pot 3
3 First fungicide drench 50 ml/pot 4
4 Fertilize drench with 50 ml solution/pot 3
8 Fertilize drench with 50 ml solution/pot 3
10 Second fungicide drench 50 ml/pot 4
11 Inoculate plants 5,6
12 Fertilize drench with 50 ml solution/pot 3
15 Fertilize drench with 50 ml solution/pot 3
-- Rate symptoms 7
NO. 1. A soil mix that is easy to reproduce contains 5:3:3:1 parts
peat, sand, vermiculite and perlite. Add 424 g dolomite, 42 g Ca(OH)2,
424 g CaCO3, 106 g superphosphate and 63.5 g MicromaxR (minor elements) to
45 liters of medium and mix thoroughly. The pH of this mix is maintained
at about 7.0 for at least 12 weeks. Successful disease control depends on
maintaining the pH in the above range. The level of disease increases as
the pH drops below 6.5-7.0 (2,5,8,9). The minor elements zinc, iron and
manganese, when fed in luxurious amounts, increase slightly the level of
wilt symptoms. The cultivar Royal Trophy is used because it is very
susceptible to Fusarium wilt. Other cultivars may be used (1,6,7).
NO. 2. The success of this screening procedure depends on
maintaining the temperature in the 800 + 50 F (26.6 + 2.50 C) range.
Fusarium wilt is a warm season disease and few symptoms develop if
temperature drops below 740 F (23.30 C). Grow chrysanthemums in a long-
day photoperiod regime (illuminate from 10 PM 2 AM nightly) to maintain
the plants in a vegetative state. The entire test may be conducted with
plants maintained in the vegetative state. If flowering (reproduction) is
desired, maintain the plants in the vegetative state for about 3 weeks,
then expose plants to a 14-hr uninterrupted dark period daily for the
duration of the experiment. Chrysanthemum plants become reproductive when
exposed to long nights (short days). Flowers are mature about 9 weeks
after exposure to long nights.
NO. 3. The fertilizer program is an extremely important part of this
procedure because the level of disease is raised or lowered depending on
the type of nitrogen used. The level of disease is reduced when only NO3-
N (Ca(N03)2 or KNO3) is used and increased when NH4-N (NH4N03 or
(NH4)2SO4) is used (2,5,10). For this procedure drench plants twice
weekly, starting with the day of planting, with 50 ml of fertilizer
solution (6.6 g/1 Ca(N03)2 + 1.5 g/1 KC1). Distribute the fertilizer
uniformly over the entire soil surface because the fertilizer elements
affect the growth and virulence of the pathogen in the soil (10,11).
NO. 4. Add water to the chemical and slurry to insure getting a
uniform suspension and/or solution. Distribute the liquid uniformly to
the entire root system because effective chemicals usually are systemic
and therefore are taken up by the roots. Apply the chemicals on the 3rd
and 10th days after planting. New roots develop in 3 days and extend to
the side of the pot in 10 days. Benomyl 501I, a substituted benzimidazole
compound that has systemic properties, is the standard of comparison that
has given good control of symptoms when a non-resistant strain of the
pathogen is used. At 0.6 and 0.15 g a.i./l a disease rating of 0.4 (in
some tests this was 0) and 2.3 respectively (0 = no symptoms, 5 = dead
plant) was obtained while the control was 4.4 after 9 weeks (Table 1).
Thiophanate M, a related benzimidazole chemical also may be included as a
standard of comparison. It gave a rating of 1.0 at 0.6 g a.i./l. See
Table 1 for other results.
NO. 5. The plants are inoculated 11 days after planting. The roots
grow to the edge of the pot in 10 days. Make parallel cuts into the soil
medium (to the bottom of the pot) 2.5 cm away from the stem to injure the
roots. Infection by Fusarium oxysporum is facilitated by the presence of
wounds so this wounding procedure makes the test more strenuous. Pour 50
ml of the inoculum suspension (containing 10 million spores) uniformly
into the 2 cuts in the soil.
NO. 6. Fusarium oxysporum f. sp. chrysanthemi is plated onto Petri
plates containing Difco potato dextrose agar and incubated at 26.70 C (800
F) in constant fluorescent light. Spores are harvested by flooding the
plates with distilled or deionized water, gently stroking the agar surface
with a rubber policeman and a small brush, and filtering the suspension
through a double layer of cheese cloth. The spore concentration of the
suspension is adjusted to 200,000 spore/ml using a Levy Blood cell
counting chamber. Apply fifty ml containing 200,000 spores/ml or a total
of 10 million spores/10 cm pot onto each pot. It appears that the age of
the spores up to 10 days has little affect on the results. One Petri
plate, on which a single inoculum chip is placed in the center and growth
has extended to the edge, can yield a liter of suspension containing
NO. 7. Disease is rated 0-5 according to the symptoms on the plant
(1). Some variation may occur among the plants but the rating system
generally is adequate.
0 No symptoms.
1 Leaf chlorosis plus vascular discoloration in leaf veins
2 NO. 1 plus stem and/or leaf curvature.
3 NO. 2 plus extensive wilting.
4 NO. 3 plus plant with retarded growth.
5 Plant dead.
In this system, 6 disease-regulating components, cultivar, pH, nitrogen
source, temperature, inoculum density and a chemotherapeutant standard are
balanced in such a manner that few or no symptoms of disease occur at the
high concentration rate of the standard treatment. A higher level of
disease develops at the low rate of the standard treatment. Most of the
plants are killed in the inoculated treatment receiving water rather than
a chemotherapeutant. This system reduces the probability that chemicals
with therapeutic value for the control of Fusarium wilt will be missed
because if plants are fertilized with NH4NO3 rather than Ca(N03)2, the
plants are killed, even when treated with the effective standard treatment
benomyl 50W (2).
This system gives reproducible results with chrysanthemum and also is
feasible for Fusarium wilt of carnation and aster (3,4). It is predicted
this system also would be applicable to any vascular wilt disease incited
by a formae specials of Fusarium oxysporum.
CHEMICAL CONTROL OF FUSARIUM WILT ON POTTED ROYAL TROPHY
Rate a.1/100 gal
Control not inoculated
Thiophanate M 70W
Benomyl + captain 50W
Prochloraz NN 50WP
SN 596 25DF
SN 84364 50W
8 + 24 oz.
1Nean of 5 replications.
text for details).
0 = no symptoms, 5 = dead plant (see no. 7 in
1. Engelhard, Arthur W. and S. S. Woltz.
chrysanthemum:Symptomatology and cultivar
State Hort. Soc. 84:351-354.
2. Engelhard, A. W.
1971. Fusarium wilt of
reactions. Proc. Fla.
Effect of pH and nitrogen source on the
fungicides in controlling Fusarium wilt of
Am. Phytopath. Soc. 1:121.
3. Engelhard, A. W. 1975. Aster Fusarium wilt:Complete symptom control
with an integrated fungicide NO3- pH control system. Proc. Am.
Phytopath. Soc. 2:62.
4. Engelhard, A. W. 1979. Control of Fusarium wilt of carnation with
an integrated nitrate-nitrogen and systemic fungicide control
program. Phytopathology 69(9):1027,
5. Engelhard, Arthur W. and S. S. Woltz. 1973, Fusarium wilt of
chrysanthemum:Complete control of symptoms with an integrated
fungicide lime nitrate regime. Phytopathology 63:1256-1259.
6. Strider, D. L. 1985. Fusarium wilt of chrysanthemum:Cultivar
susceptibility and chemical control. Plant Disease 69:564-568.
7. Strider, D. L. 1986. Resistance of chrysanthemum cultivars to
Fusarium wilt:An updated compilation. N. C. Flower Growers 30(6):8-
8. Woltz, S. S. and Arthur W. Engelhard. 1973. Fusarium Wilt of
Chrysanthemum:Effect of nitrogen source and lime on disease
development. Phytopathology 63:155-157.
9. Woltz, S. S. and A. W. Engelhard. 1986. Effect of liming materials,
lime rates and media on Fusarium wilt in Royal Trophy chrysanthemum.
Univ. Fla., IFAS, Bradenton GCREC Research Report BRA1986-21. 4 pp.
10. Woltz, S. S. and A. W. Engelhard. 1986. Effect of nitrogen source
and liming procedure on severity of Fusarium wilt of Yellow Delaware
chrysanthemum. Univ. Fla., IFAS, Bradenton GCREC Research Report
BRA1986-23. 3 pp.
11. Woltz, S. S. and J. P. Jones. 1981. Nutritional requirements of
Fusarium oxysporum:Basis for a disease control system. In: Nelson,
P. E., T. A. Tousson and R. J. Cook. 1981. Fusarium:Disease,
Biology and Taxonomy. Pennsylvania State University Press. pp 340-
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