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GC4 Central Science
GULF COAST RESEARCH & EDUCATION CENTER OCT 23 1987
IFAS, UNIVERSITY OF FLORIA
5007 60TH STREET EAST
BRADENTON, FL 34203 University of Florida
Bradenton GCREC Research Report BRA1987-23 October 1987
CHOANEPHORA WET ROT OF POINSETTIA
Arthur W. Engelhard1
Choanephora cucurbitarum incites a soft, wet rot of poinsettia in Florida.
Plants at all ages from those under mist propagation to production pots to
large stock plants are affected. The pathogen can kill plants in all
stages of production. Disease occurs during warm, wet, humid periods in
August to October and occurs on plants grown outdoors and in greenhouses.
The fungus also causes a soft, wet rot of petunia flowers, green peppers,
yellow squash, okra, southern peas and cucumbers. It is not active after
temperatures drop below 14.40C (580F). Fungicidal controls have not been
developed for this disease.
Choanephora wet rot is a disease found in Florida during warm, wet, humid
weather. It occurs primarily during August, September, and October.
Choanephora wet rot is caused by the fungus Choanephora cucurbitarum
(Berk. & Rav.) Thaxter (6). The symptoms are similar to those reported
for Rhizopus blight (3,5). The fungus also has been reported to cause
disease in swamp hibiscus (Hibiscus coccineus), yellow squash (2,7),
pepper (2,4,7), okra, southern peas, cucumbers, hibiscus, cotton and rose
of sharon in Florida (7). Choanephora infundibulifera (Currey) Sacc. was
found on cultivated hibiscus (Hibiscus rosa-sinesis L.) causing a soft,
wet rot of flowers and young stems. This apparently is the first report
of Choanephora cucurbitarum on poinsettia plants in the United States.
The disease occurs on poinsettia cuttings during propagation, on small-to-
large stock plants and plants in production pots. It occurs in plants
grown outdoors as well as in greenhouses when the weather is optimum for
disease development. The objectives of this paper are to describe the
disease, where and when it occurs in Florida and under what environmental
conditions, and how the disease can be distinguished from Rhizopus blight.
The symptoms of wet rot on poinsettia plants are a soft, wet, mushy decay
of leaves, petioles and stems. If the stems are green and actively
growing they become soft, wet, flaccid and wilt or droop before
1Professor of Plant Pathology.
collapsing. If the stems are somewhat woody, only necrosis occurs. Small
plants may be destroyed within days if weather conditions are conducive
for disease development whereas large, multistemmed, stock plants in 3-
gallon containers may be affected in only a part of the plant.
The pathogen usually sporulates profusely on diseased parts of the plant,
especially on stems. The pathogen produces long, non-septate
sporangiophores, each bearing a sporangium on the end. The
sporangiophores are long enough and abundant enough so that they give the
appearance of a coarse, hairy or "whiskery" growth. Large numbers of
sporangiospores are produced in each sporangium. Implicated in the spread
of spores are water, wind, movement of plants, hands, and insects (7).
Mycological Distinction Between Choanephora and Rhizopus
The "whiskery" growth of Choanephora is composed of sporangiophores which
arise from diseased tissue or fungus mycelium. The sporangiophores are
erect, unbranched, non-septate, up to 10 mm long, and bear on the tips
round structures called sporangia. The sporangia are initially light
colored but darken to nearly black with age. At maturity the sporangia
split open to expose what appear as individual "heads" of spores. Six or
more "heads" or clusters may be distinguished. Individual
sporangiospores, also called sporangiola, comprise each of the exposed
clusters. The sporangiospores (or sporangiola) are ellipsoid, brown to
reddish brown to pale brown in color and usually have distinct
longitudinal striations which are visible only with the aid of a compound
microscope. The striated sporangiospores usually have several fine,
hyaline appendages at each pole and a small cylindrical pedicel where it
was attached at one pole. These features also require a compound
microscope to see. Variations to the above development occur, such as
when small sporangia develop, according to Kirk (6) who published a
monograph on this family of fungi in 1984. Dr. Kirk identified the
species of Choanephora and Rhizopus in this study. However, the
description above generally describes the fungus on diseased plant
specimens observed by the writer in Florida.
Rhizopus stolonifera incites Rhizopus rot on poinsettia (3,5). It also
causes a soft, wet decay and produces long sporangiophores with dark
sporangia on the apices. The sporangium at the apex of the
sporangiophore of Rhizopus consists of a mass of spores all formed and
attached around a central, enlarged or bulbous tip called the columnella.
The spores are round (not ellipsoid) and not striate as in Choanephora nor
do they have the fine, hyaline appendages at each pole. Rhizopus and
Choanephora thus cause similar symptoms on poinsettia plants and both
produce long sporangiophores with dark, round sporangia on the tips.
Generally, the two can be distinguished with a hand lens by carefully
examining the differences in the sporangia.
Discussion and Control
Choanephora wet rot is a disease of poinsettia that has been observed
during the wet, humid periods in August, September and October of 1985-87.
It occurred in plants grown outdoors and in greenhouses. In certain
instances it has caused losses to 30% of the pots in some plantings. In
other cases, the losses have been minimal in the 1-5% range.
The disease also has been observed by the author on hibiscus, petunia,
green peppers and yellow squash. A soft, wet rot develops regardless of
the host. Blazquez (1) observed the disease in south Florida on green
peppers generally from August-October periodically between 1966-86. He
observed the disease appeared endemic on swamp hibiscus (Hibiscus
coccineus walt.) in the area. Blazquez (2) indicated sporulation on both
pepper and swamp hibiscus ceased soon after the ambient temperature
dropped below 14.40C (580F).
Currently, no effective fungicide treatments are known for this disease on
any of the ornamental and vegetable crops on which it occurs. The fast
development of symptoms of this potentially destructive disease, the omni-
presence of favorable epidemiological conditions (warm, wet, humid) in
Florida during August, September, and October and the occasional serious
economic losses sustained on poinsettia plants, indicate producers should
have some disease management practices in effect during the August-
September-October period to reduce the probability of a destructive
outbreak of wet rot. It is suggested plants be spaced to allow maximum
air movement and drying, culturally controlling wetness of foliage and air
humidity by watering plants individually rather than watering overhead
which would keep the soil moist, foliage wet and humidity elevated,
removing diseased plants (on which the pathogen sporulates), by placing in
plastic bags, scouting plants frequently (such as daily during mist
propagation and several times per week during problem periods), not have
wild or cultivated susceptible plants such as petunia, squash, and wild
hibiscus in the area, and maintaining mancozeb in the spray program as is
suggested for control of Choanephora blossom blight of yellow squash (8).
Dicloran (Botran) has been used to control Rhizopus rot on some stone
1. Blazquez, C. H. 1976. Weather and disease surveillance in southwest
Florida. Proc. Fla. State Hort. Soc. 88:243-248.
2. Blazquez, C. H. 1986. Choanephora wet rot of pepper. Proc. Fla.
State Hort. Soc. 99:321-325.
3. Chase, A. R. and A. W. Engelhard. 1984. Rhizopus blight of
ornamental crops in Florida. Univ. of Fla. AREC-A Research Report
4. Dougherty, D. E. 1979. Bud rot of pepper. Proc. Fla. State Hort.
5. Engelhard, A. W. 1984. Diseases and their control. Pages 37-47.
In: Commercial Poinsettia Production in Florida. B. 0. Tjia, ed.
Dept. Ornam. Hort. Univ. of Fla., Gainesville. 60 pp.
6. Kirk, P. M. 1984. A monograph of the Choanephoraceae. Commonwealth
Mycological Institute, Mycological Paper No. 152. 61 pp.
7. Kucharek, T. and G. Simone. Wet rot of vegetable crops. Plant Path.
Fact Sheet, PP-11. Fla. Coop. Ext. Service, IFAS, Uhiv. of Fla. 2
8. Kucharek, T., G. W. Simone and R. S. Mullin. 1983. Florida Plant
Disease Control Guide. Univ. of Fla., Plant Pathology Dept.
Extension Plant Pathology, Gainesville.