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Copyright 2005, Board of Trustees, University
of Florida

Mimeographed Report 60-1 November, 1959

University of Florida
Sub-Tropical Experiment Station
Homestead, Florida

CONTROL OF LEAF AND FLOWER DISEASES OF CHRYSANTHEMUM

Lorne A. McFadden
Assistant Horticulturist

Florists' chrysanthemums are subject to several leaf and flower diseases in

Florida. Those occurring on the flowers include Botrytis petal spot, Ascochyta ray

blight and ray speck while bacterial leaf spot, Ascochyta leaf blight, Septoria

leaf spot, foliar nematode and yellow strapleaf disease occur on leaves of

chrysanthemum.

The purpose of this report is to discuss the control of three of these diseases;

namely, Botrytis petal spot, Ascochyta ray blight and bacterial leaf spot. Foliar

nematode of chrysanthemums has been observed only on rare occasions in commercial

plantings. Growers using insecticides such as parathion have not experienced

losses due to foliar nematode. Septoria leaf spot, although an important disease

of chrysanthemums in Florida, has not been difficult to control when plants are

sprayed regularly with a good fungicide. Ray speck, caused by a Stemphyllium fungus

has been observed on a few occasions. but its distribution appears limited. The

yellow strapleaf disease is of recent occurrence and its cause and control are

under investigation at the Gulf Coast Experiment Station.

Disease Symptoms

The Ascochyta fungus attacks chrysanthemum flowers usually on one side causing

a tan to dark brown rot. The fungus grows into the peduncle causing it to urn -,

dark brown to black. Unless controlled, the fungus can cause almost tp ; loss of :
-' .r ..\

f~ c j
-.1, '^ ( -) ^ *

-2-

flowers either in the field or during shipment to wholesale markets. The ray

blight fungus also causes irregular brown to dark brown lesions or blotches on

leaves and stems of chrysanthemums in Florida. Under optimum conditions, such as

frequent rains, fungus spores are spread to adjacent plants.

Botrytis petal spot is evident as small spots varying in color (pinkish,

purplish, brownish), depending on the variety of chrysanthemum attacked. Under

optimum conditions the spots become numerous resulting in reduction of flower

quality and appearance.

A "new" bacterial leaf spot was observed during the fall of 1957 causing almost

total loss of stock plantings of Bluechip chrysanthemums. The disease appears

during the rainy seasons of the spring, summer and fall months. The spots first

appear on the lower,.mature leaves but under continued wet weather conditions may

reach the top of plants at time of flowering. The spots are dark brown to black

with lighter tan centers. At times, numerous spots coalesce to form large, necrotic

areas on the leaf. A careful examination of spots when dry show characteristic

concentric rings or target patterns. There is no yellowing around the spots as in

the case of Septoria leaf spot.

Control

Experiments carried out at the Sub-Tropical Experiment Station over the past

2 years indicate that all three diseases can be controlled by using frequent

applications of protective fungicides in conjunction with a strict program of

sanitation around the nursery.

The fungicide tests were carried out in outdoor replicated plots provided with

overhead irrigation and lights. Sprays were applied twice weekly regardless of

weather conditions, using a 12 gallon Myers Baronet sprayer at 200 pounds pressure.

- 3 -

To assure optimum disease development the flowers of Whitetop chrysanthemums were

artificially inoculated with the Ascochyta fungus while flowers of Bluechip

chrysanthemums were inoculated with the Botrytis fungus. Overhead irrigation at

frequent intervals provided optimum conditions for infection. No attempts were made

to inoculate the Bluechip and Iceberg plants with the bacterial leaf spot pathogen

although frequent use of overhead irrigation made conditions suitable for disease

spread.

The results of the tests against Ascochyta ray blight and Botrytis petal spot

are summarized in Table 1. Over a 2 year period most of the commercially available

fungicides have been tested against these diseases. A number of materials proved

unsatisfactory either due to lack of disease control or phytotoxicity. Ascochyta

was best controlled with spray applications of maneb, maneb + captain, captain, zineb

and zineb + captain. Botrytis was best controlled using sprays of maneb, maneb +

captain, zineb and zineb + captain. No injury resulted using these materials on open

flowers of Bluechip and Whitetop chrysanthemums.

The data on bacterial leaf spot control (Table 2) shows tribasic copper sulfate

and Agri-mycin 500 superior to Agri-mycin 100 and Agri-Strep. Slight yellowing of

foliage occurred in plants receiving Agri-mycin 100 and Agri-Strep and to a less

extent with Agri-mycin 500, but phytotoxicity was not severe even after repeated

applications. Flowers of Bluechip and Iceberg chrysanthemums were not injured by

the above mentioned materials.

-4-

Table 1. A summary of tests carried out over a 2 year period to control Botrytis
petal spot and Ascochyta ray blight of chrysanthemums.

Concentration Disease Control
Materials per 100 gallons or ppm Ascochyta Botrytis Plant Safety

Actidione

Captan

CM-19

CM-19 + captain

Dyrene

Elcide 70

G-28810

Glyodin

Maneb (70%)

Maneb + captain

Nabac

Natriphene

Omadine (Zn)

Zineb (65%)

Zineb + captain

Permachem

Phaltan

Phygon

Phytoactin

Phytoactin + captain

Spor-rid

Thiram (65%)

Tribasic Copper sulfate (53a)

Concentration reduced to

1 1/2 ppm

1 1/2 Ibs.

1 pt.

1 pt. + 3/4 lb.

1 lb.

300 ppm*

1 lb.

1 qt.

1 lb.

1/2 lb. + 3/4 lb.

1/2 lb.

1/2 lb.

1 lb.

1 lb.

1/2 lb. + 3/4 lb.

1 qt.

1 1/2 Ibs.

1/2 lb.

1 qt.

1/2 qt. + 3/4 lb.

1 qt.

1 1/2 lbs.

3 lbs.

7.0%

90.5

20.0

50.0

95.0

20.0

28.0

87.0

94.0

20.0

7.0

50.0

82.5

94.0

30.0

44.0

75.0

78.0

89.0

22.0

52.5

30.0

150 ppm but due to severe injury

9.0% Safe

73.0 "

41.5

34.0

65.0

-- Injur:

10.0 "

7.0 Safe

89.5 Injur:

84.0 Safe

0 "

17.0 "

45.0

76.0

78.0

20.0

76.0

60.0 Injur:

83.0

83.0

47.0

50.0 Safe

25.0 "

all plants died.

ious

ious

ious

- 5 -

Table 2. Results of tests to control bacterial blight of chrysanthemums.

Treatment Concentration Disease Control Plant Safety

Tribasic Copper sulfate (53%) 4 lbs. 100 gals. 70% Safe

Agri-mycin 500* 3 lbs. 100 gals. 60 Fairly safe

Agri-mycin 100** 60 ppm 50 Fairly safe

Agri-Strep*** 60 ppm 47 Fairly safe

Control (water sprayed) 0

Streptomycin 15.01, Terramycin 0.5% Chas. Pfizer & Co.
** Streptomycin sulfate 37% Merck & Co.
*** Streptomycin 1.75ao, Terramycin 0.18o%, tri-basic copper sulfate 42.4o -
Chas. Pfizer & Co.

CAUTION: Materials containing streptomycin can cause foliar yellowing, stunting and
hardening of chrysanthemums if used at concentrations in excess of those
suggested.

Recommendations.--The data presented show maneb, zineb and captain to be

effective in controlling both Ascochyta and Botrytis. Although maneb at 1 lb. -

100 gallons is highly effective against both diseases, preference is given to maneb

1/2 Ib + captain 3/4 lb. per 100 gallons since it is less likely to cause injury to

open flowers and it leaves less objectionable residues. Zineb at 1 lb. or zineb

1/2 lb. + captain 3/4 lb. per 100 gallons also gives good disease control. Sprays

should be applied a minimum of once weekly on vegetative growth and twice weekly

(more frequently during rainy periods) on open blooms. To avoid possible spray

injury plants should dry quickly following fungicide application. Ascochyta must

also be controlled during vegetative growth and the use of the same fungicides is

suggested.

Bacterial leaf spot may be controlled using either tribasic copper sulfate

at 4 lbs. 100 gallons or Agri-mycin 500 at 3 Ibs. per 100 gallons. Sprays should

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be applied weekly to susceptible varieties (Bluechip, Iceberg, etc.) particularly

during periods of frequent and heavy rains. Stock plantings should also be sprayed

weekly for best results.

In addition to employing the most effective materials, thorough spray coverage

of plants is necessary for maximum control.

150 copies
November, 1959