II- ( A. R. Chase ',, i n
University of Florida, IFAS SEP 3 o 1994
Central Florida Research and Education Center Apopka
CFREC-Apopka Research Report, RH-91-4 University of Floridv
Pest management strategies for hibiscus diseases have been researched during the past
ten years at the CFREC-Apopka. Producers of hibiscus and other ornamentals face the
challenge of producing a variety of blemish-free crops. Due to the special conditions of
greenhouse production, however, they have a wide range of management techniques available
for both fungal and bacterial disease control (Table 1).
The most effective strategy of preventing a pathogen from infecting plants is exclusion
by quarantine. This is usually done in situations where the pathogen is very destructive and is
not present in a particular geographic area. Growers should implement their own quarantine
measures to safeguard against introduction of a pathogen. When new plants are purchased, they
can be maintained in separate structures until their health is confirmed. It is especially important
to maintain stock plants under quarantine to protect them from infections, since contaminated
stock plants produce only contaminated cuttings.
Pathogens may be introduced in plant propagules, potting media, pots, equipment, insects
and weeds, soil under benches and in walkways, and sometimes irrigation water. The specific
control method which will be most effective depends upon the source of the pathogen.
Eliminating greenhouse weeds and crop debris, removing diseased plants, and occasional
removal of diseased leaves, collectively create an environment unfavorable to pathogen spread
and favorable to early disease detection. Benches should be cleaned and sanitized between crops
to eliminate contamination of new crops. Use of clean pots, flats and tools is important. If
benches are cleaned between crops, placing new plants in contaminated pots or flats will negate
the benefit of all other control strategies. Recognition and removal of diseased plants can aid
in decreasing spread of disease between plants. Removal of infected leaves or entire plants is
also a common practice for control of some diseases which greatly reduces inoculum available
for future disease outbreaks.
'Professor, Plant Pathology, Central Florida Research and Education Center-Apopka, 2807
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Growing media are an integral part of all plant production systems for greenhouse
ornamentals. Media must be mixed or held in areas which are not contaminated with native soil.
The media should be covered with a water-proof tarp to reduce chances of contamination with
pests such as insects, weeds and plant pathogens and exposure to rainfall. Growing plants on
raised benches removes them from a very common and extremely important source of infection,
the ground. Many root and stem rot pathogens are present in native soil and move into ground
beds and pots placed on the ground.
One of the most important considerations in irrigation management is elimination of
standing water on plant foliage since this is necessary for infection to occur in most bacterial and
fungal leaf diseases. Water management encompasses the water needs of the crop plant as well
as the delivery method and humidity. Water stress through either too much or too little water
can result in more severe disease development. This is often seen in root rot diseases caused
by pythiaceous fungi which are more serious when plants are over-watered. Roots die when
kept too wet due to oxygen starvation. Once the pathogen gains entrance through dead roots it
can spread throughout the root system. The potting medium should not retain excessive water
for long periods of time since even a small amount of water can be too much. Irrigation
management of plants in a heavy potting medium is even more critical because it holds more
water and has fewer large pores for air, an ideal condition for root disease.
The method of delivering water to the plants is also an important consideration for
reducing both disease development and spread. Water systems which cause leaves to stay wet
for long periods of time or cause splashing are ideal ways to spread pathogens and increase
disease severity. Rapid leaf drying greatly reduces disease severity since many fungal spores
and bacteria cannot infect leaves without a minimum period of free water on leaves. It is even
better to use a method of irrigating which does not deliver water to the leaf surface at all.
Systems such as drip irrigation, ebb and flow systems, and capillary mats are utilized for a
variety of plants to minimize pathogen spread and disease development.
The potential for use of biological control agents in greenhouse ornamentals is relatively
high for insects and mites but not for many pathogens. Unfortunately, the zero tolerance level
for damage on the ornamental product indicates that use of biological control may be confined
to stock areas or to early stages of crop production where minimal damage may be acceptable.
Another problem with implementing biological control in the greenhouse is the common use of
a multitude of pesticides which can eliminate non-target biological control agents as effectively
as the target pest. Biological control is not currently a method which can be used to control
most diseases of greenhouse ornamentals. Although some examples of successful biological
control of a plant disease are available they have not been readily employed commercially since
they are specific to a particular disease on a relatively small group of crops.
Most potted flowering plants are produced from cuttings of tips or stems which can be
contaminated with a wide variety of fungi, bacteria and viruses. Utilization of pre-plant dips
to control diseases is usually ineffective for a variety of reasons. First, the chemicals which are
available are not eradicants and the degree of control is less than 100% even under ideal
conditions. Second, the act of immersing cuttings in a water solution creates ideal conditions
for pathogen spread. Even a very low incidence of contamination or infection in dipped cuttings
can result in contamination of all cuttings. Finally, immersing cuttings with latent infections into
water can trigger the development of many diseases.
Establishment of pathogen-free blocks of mother or stock plants is an important step
toward eliminating introduction of contaminated cuttings. Pathogen-free plant propagules are
available for some plants from tissue-culture. While some growers utilize pathogen-free
tissue-cultured plantlets as a propagative material others use these plants to establish blocks of
stock plants to produce cuttings. These blocks must be maintained under quarantine type
conditions to keep them as free of plant pathogens as possible. Stock plantings should have a
limited life to maintain both high productivity and pathogen-free status.
SPECIFIC METHODS TESTED FOR HIBISCUS DISEASES
Knowing the optimal temperatures for development of a given disease is better used for
timing control measures rather than altering the growing environment. Sometimes this
information is helpful to disease diagnosticians since they can make special effort to look for
target pathogens at certain times of the year. Hibiscus produced in Florida are subject to three
bacterial leaf spot diseases which occur during different times of the year (Table 2). During the
winter months, Pseudomonas syringae pv. hibisci occurs with Pseudomonas cichorii common
during the fall and spring. In general, only Xanthomonas campestris pv. malvacearum is active
during the summer months. Scouting for diseases only at times when temperatures are favorable
allows for better management of personnel resources. In addition, preventive pesticide
applications should be recommended only when disease development is possible and not on a
year-round basis which is costly and potentially hazardous for the plants, the workers, and
development of resistant populations.
Host nutrition has been shown to affect severity of many diseases of ornamental plants.
Both the rate of fertilizer and its source can affect disease development. Trials were conducted
with hibiscus grown with different levels of slow-release fertilizer (Osmocote 19-6-12). Both
plant quality and severity of Xanthomonas leaf spot were affected by fertilizer level (Fig. 1).
Plant quality was very high when they received between 5 and 20 g of fertilizer per 5-inch pot.
i fertilizer ral
per pot resulted in excellent quality plants with very low levels of Xanthomonas leaf spot.
Resistant varieties should be used since they can be produced in the presence of thc
pathogen with a minimum loss to disease. The available information falls short of answering
most questions concerning disease resistance of hibiscus but some information on four disease.
common to Florida hibiscus is available (Table 3). Although American Beauty is relatively)
resistant to Phytophthora and both Pseudomonas leaf spots, it is highly susceptible t(
Xanthomonas leaf spot (Table 3). Most other cultivars tested were also resistant to some
pathogens while susceptible to others. Choice of cultivars could be made on the basis of disease<
resistance as long as all diseases were not common in the nursery.
Pesticides and growth regulators
Pesticide usage remains the backbone of control for many severe diseases, due to the
need for very high quality ornamental products. Some excellent pesticides are labeled foi
control of many diseases common to hibiscus with the exception of those caused by bacteria
pathogens. Research has centered on controlling these diseases with fosetyl aluminum (Aliett<
80WP), bromine treated mist (Agribrom) and the growth regulator chlormequat chloride
(Cycocel) (Fig. 2).
Aliette 80WP has been shown to greatly reduce a variety of Xanthomonas diseases o01
ornamentals. However, when a test was completed on hibiscus inoculated with Xanthomona.
the use of Aliette appeared to increase disease severity. In the same trial, cupric hydroxide wa,
ineffective against Xanthomonas leaf spot but did not increase disease severity.
Another serious disease of hibiscus cuttings is caused by a variety of bacteria. Since this
disease occurs on cuttings rooted under mist the use of water treatment compound such as
1. Chase, A. R. 1986. Comparisons of three bacterial leaf spots of Hibiscus rosa-sinensis.
Plant Disease 70:334-336.
2. Chase, A. R., L. S. Osborne, J. M. F. Yuen and B. C. Raju. 1987. Effects of growth
regulator chlormequat chloride on severity of three bacterial diseases of 10 cultivars of
Hibiscus rosa-sinensis. Plant Disease 71:186-187.
3. Chase, A. R. 1989. Effect of fertilizer rate on susceptibility of Hibiscus rosa-sinensis
to Xanthomonas campestris pv. malvacearwn. Biological & Cultural Tests for Control
of Plant Diseases 4:83.
4. Chase, A. R. 1989. Aliette 80WP and bacterial disease control I. Xanthomonas.
Foliage Digest 12(11): 1-3.
5. Chase, A. R. 1990. Control of some bacterial diseases of ornamentals with Agribrom.
Proc. Fla. State Hort. Soc. 103: (In Press).
6. Jones, J. B., A. R. Chase, B. C. Raju and J. W. Miller. 1986. Bacterial leaf spot of
Hibiscus rosa-sinensis incited by Pseudomonas syringae pv. hibisci. Plant Disease
7. Lockhart, B. E. L. 1987. Evidence for identity of plant rhabdoviruses causing vein-
yellowing diseases of tomato and Hibiscus rosa-sinensis. Plant Disease 71:731-733.
8. Semer, C. R. IV. and B. C. Raju. 1985. Phytophthora leaf spot of Hibiscus, a new
disease caused by Phytophthora parasitica. Plant Disease 69:1005-1006
9. Uchida, J. Y. and P. S. Yahata. 1989. Hibiscus blight caused by Athelia. Hawaii
Coop. Ext. Serv. Commodity Fact Sheet HIB-4(A). 2pp.
Table 1. Potential for use of integrated methods for control of pathogens of hibiscus.
Strategy Fungi Bacteria
Exclusion moderate high
Sanitation high high
Growing media high moderate
Irrigation high high
Biological none at present none at present
Pathogen-free plants high high
Temperature not known high
Host nutrition not known moderate to high
Host resistance moderate moderate
Table 2. Optimal temperatures for development of some diseases of hibiscus.
Pathogen C F
Erivinia spp. 28 34 77 94
P'hyioplholra parasitlca 20- 22 8 72
Pseudomonas cichorii 21 -27 70-81
Pseudomonas syringae pv. hibisci 15 18 60 65
Rhizoctonia solani 20 35 68 95
Xanthomonas campestris pv. malvacearum 24 33 75 92
Table 3. Degree of susceptibility of some hibiscus cultivars to certain pathogens.
Cultivar Phytophthora Ps. cichorii Ps. syringae Xanthomonas
American Beauty low low low high
Brilliant Red moderate low high low
Butterfly not tested low low moderate
Euterpe not tested low moderate low
Holiday not tested low high not tested
Painted Lady high low high moderate
Pink Versicolor high low low high
President not tested low low moderate
Senorita moderate low moderate high
White Red Eye not tested low low moderate
Figure 1. The effect of fertilizer rate on top quality of Hibiscus rosa-sinensis and severity
of Xanthomonas leaf spot caused by X. campestris pv. malvacearum.
2.5 5 7.5 10 12.5 15 17.5 20-
g Osmocote 19-6-12 per 5 inch pot
M Number spots E Top Quality
Top quality was
0-100 with high
rated on a scale of
quality plants above
Figure 2. The effect of Cycocel on severity of three bacterial diseases of Hibiscus rosa-
M Water M Cycocel