Aerial Transmission of Foliar Bacterial Pathogens by Fungus Gnats and Shore Flies1992 Proposal
Summary
Greenhouse infestations by fungus gnats and shore flies are commonly
regarded as a nuisance by the floricultural and ornamental industry. However,
we recently (1990) documented, for the first time, aerial transmission
of a root-infecting fungus (Pythium aphanidermatum) from infected to healthy
plants and that all plants frequented by internally-infested adult flies
became infected with the fungus. Our results indicate that these insects
constitute a previously unsuspected source of introduction and/or spread
of root pathogens in greenhouses. Our proposed research objectives are
to elucidate the role of these insects as aerial vectors of bacterial pathogens
which cause foliar diseases of greenhouse crops. Insect transmission of
these bacterial pathogens may account for primary infection sites on foliage.
Thus, knowledge of the role of these insects in the epidemiology of foliar
bacterial pathogens will result in recognition of the necessity as well
as the development of strategies, for control of infestations by these
insects.
INTRODUCTION AND LITERATURE REVIEW
Greenhouse floriculture and ornamental horticultural crops are subject
to numerous fungal and bacterial diseases which place constraints upon
production and/or marketability of these crops. Management of these diseases
is based upon preventive as well as curative disease control strategies.
Obviously, preventive strategies, i.e.- the exclusion of the pathogen from
the production area, are more desirable for a number of economic and ecological
reasons. However, preventive strategies are dependent upon knowledge of
the ecology of the particular pathogen in question. Specifically, knowledge
of the source(s) of pathogen introduction and/or spread within a commercial
facility is vital to the development of effective strategies for disease
control. Documented sources of pathogen introduction and/or spread include
pathogen-infested or infected seed and seedlings, pathogen-infested peat,
soil, and irrigation water.
Insects must now be added to this list as a source of pathogen introduction
and spread either into and within greenhouse facilities. In 1990, we discovered,
for the first time, that adult shore flies (3) are capable of aerial transmission
of a soil-borne, root-infecting fungus. We showed that larvae of this insect
feed upon infected roots and ingest oospores of Pythium aphanidermatum.
Ingested oospores of the fungus were demonstrated to be viable upon excretion
and that a portion of the ingested oospores were retained in the intestine
of the insect through pupation. Upon pupation, internally-infested adult
flies were shown to be capable of aerial transmission, via deposition of
infested feces, of this fungus. All plants frequented by these adult flies
became infected by the fungus within 6 days whereas none of the plants
housed in insect-proof cages became infected. Subsequent research (author,
unpublished) has shown that shore flies, as well as fungus gnats, are capable
of aerial transmission of several other soil-borne fungal plant pathogens.
These results indicate that control of these insects, which previously
were considered simply as a nuisance, is now essential for maintenance
of a pathogen-free environment.
In addition to aerial transmission of soil-borne fungi, these insects
(fungus gnats and shore flies) may also be vectors of bacterial plant pathogens.
Among the more economically important bacterial diseases are foliar and
stem disorders caused by species of Erwinia Xanthomonas, and Pseudomonas
(2). Although the ecology of these bacterial diseases is not known with
certainty, it is generally believed that these bacteria are rhizosphere
inhabitants (5) and that these root populations serve as a primary source
of inoculum, via splash dispersal, for stem and foliar diseases. While
splash dispersal, via overhead irrigation, accounts for secondary spread
of these bacteria, the question remains unanswered as to how these bacteria
originally found their way to the foliar portions of the plant, particularly
when canopy density precludes splash dispersal of these bacteria from soil
to the foliage. Our hypothesis is that shore fly and fungus gnat larvae,
by virtue of their root-feeding habits, could ingest these pathogens and,
as shown for root-infecting fungi, the ingested populations could persist
through pupation in the intestine of the insect. Internally infested adult
flies could then function as aerial vectors of the bacterial pathogens.
Feces containing viable populations of these bacterial pathogens would
then be deposited on aerial portions of susceptible plants. Overhead irrigation
and/or misting would provide the moisture levels necessary for subsequent
primary invasion and infection of the plant by these bacteria.
Our hypothesis is not without precedent. Persistence of bacterial plant
pathogens in the intestine of insects through pupation and subsequent transmission
has been reported (1,4) for several diseases: bacterial wilt of cucumber
caused by Erwinia tracheiphilia and vectored by cucumber beetles bacterial
wilt of corn caused by Xanthomonas stewardi and vectored by flea beetles
potato blackleg caused by Erwinia carotovora and vectored by the seed-corn
maggot and fruit flies and olive-knot caused by Pseudomonas and vectored
by the olive fly. In some of the above diseases, pathogen-infested insect
feces function as the primary source of inoculum.
OBJECTIVES
Our overall objective is to elucidate the role of fungus gnats and shore
flies as aerial vectors of bacterial pathogens of floricultural and ornamental
crops. Specific objectives include determination of:
a. the acquisition and intestinal retention of bacterial pathogens by
larval and adult life stages of fungus gnats and shore flies,
b. persistency of bacterial pathogens through pupation,
c. aerial transmission of bacterial pathogens, via deposition of feces
on leaves and/or stems, by adult insects and
d. documentation of foliar disease development originating from infested
insect feces.
MATERIALS AND METHODS
Insect colonies: Pathogen-free colonies of fungus gnats and shore flies
will be established and maintained in individual growth chambers.
Bacterial pathogens: The following bacterial genera will be employed
in our investigation: Erwinia carotovora (Jones) Dye and Pseudomonas cichorii
(Swingle) Stapp. These bacterial pathogens were selected for investigation
because they cause either stem and/or foliar diseases on a wide range of
floricultural and ornamental plants. To facilitate achievement of the specific
objectives (as stated above), spontaneous mutant strains of each bacterium,
resistant to rifampicin, will be used in some of the experiments.
Host plant: Syngonium podophyllum Schott will be employed as the host
plant. This plant was selected for investigation because it is highly susceptible
to both bacterial pathogens and the light-colored portions of the leaves
of this plant will enable us to readily locate insect feces deposited by
adult insects. Feces of these insects frequently contain green-algal cells.
Treatments
There will be 13 treatments (listed below) and each treatment will be conducted
in
a separate growth chamber.
1. Syngonium
2. Syngonium + Pseudomonas - misting
3. Syngonium + Pseudomonas + misting
4. Syngonium + Erwinia - misting
5. Syngonium + Erwinia + misting
6. Syngonium + Pseudomonas + fungus gnats - misting
7. Syngonium + Pseudomonas + fungus gnats + misting
8. Syngonium + Pseudomonas + shore flies - misting
9. Syngonium + Pseudomonas + shore flies + misting
10. Syngonium + Erwinia + fungus gnats - misting
11. Syngonium + Erwinia + fungus gnats + misting
12. Syngonium + Erwinia + shore flies - misting
13. Syngonium + Erwinia + shore flies + misting
General procedure: A cell suspension of each bacterium will be drenched
onto the root system of Syngonium plants in appropriate treatments. Subsequent
to verification (via isolation on selective medium) of root colonization
by each bacterium, eggs of the two insects will be placed in respective
growth chambers and incubated until a population (suitable for sampling)
of larvae and adults are established in each chamber. Internal, as well
as external, bacterial infestation of the various life stages of each insect,
as well as feces produced by appropriate life stages, will be quantitatively
assessed by isolation on appropriate selective media. Additionally, the
occurrence of these bacterial pathogens on aerial portions of the plants
in each chamber will be quantitatively assessed by isolation on selective
media. Pathogen infestation of insects and aerial portions of plants will
be conducted at weekly intervals. Once we have ascertained that these bacterial
pathogens are present on the aerial portions of the plants, host penetration
and infection will be induced by frequent misting of the foliage. A film
of free-water is necessary for host penetration by these bacteria. All
precautions will be taken to prevent splash dispersal of the pathogens
from the soil-root surface to the above-ground portions of the host plants.
Development of disease symptoms, resulting from deposits of insect feces,
etc., will be monitored over time and recorded. All experiments will be
repeated and data subjected to statistical analyses.
FACILITIES AND EQUIPMENT
All experiments will be conducted in greenhouse and laboratory facilities
located at the University of Arizona, Department of Plant Pathology, Tucson.
LITERATURE CITED
1. Carter, W. 1962. Insects in relation to plant diseases. Wiley and
Sons, 705 pages.
2. Chase, A. R. 1987. Compendium of Ornamental Foliage Plant Diseases.
APS Press, 92 pages.
3. Goldberg, N.P.and M.E. Stanghellini. 1990. Ingestion-egestion and
aerial transmission of Pythium aphanidermatum by shore flies (Ephydrinae:
Scatella stagnalis). Phytopathology 80:1244-1246.
4. Kloepper, J.W., J.W. Brewer, and M.D. Harrison. 1981. Insect transmission
of Erwinia carotovora var. carotovora and Erwinia carotovora var. atroseptica
to potato plants in the field. Am Potato J. 58:165-175.
5. Stangbellini, M.E. 1982. Soft-rotting bacteria in the rhizosphere.
in “Phytopathogenic Prokaryotes”, Vol.1 (Mount, M.S. and Lacy, G.H.,eds),
pp 249-261, Academic Press, Inc.
BUDGET
Wages (Graduate Student)
$11,823
Fringe Benefits (1.5%)
177
Capital Equipment (Phototrons)
2,500
Expendable Equipment (Plants & Media,
Glassware, Petri Dishes, Chemicals, etc.)
2,500
Travel and Publication Costs
1,500
——————-
Total
$18,500
As indicated in the proposal, we will need 13 growth chambers. We currently
have 8 chambers (Phototrons). Each unit costs ca. $500.00 and we will need
5 additional chambers to conduct the proposed research. Individual chambers
are necessary to insure containment of the insects.
Project Leader Qualifications
Dr. Michael Stanghellini is a Professor of Plant Pathology at the University
of Arizona, Tucson, Az. He has authored or coauthored over 100 scientific
publications and is recognized nationally and internationally as an authority
on the ecology and epidemiology of soil-borne diseases caused by Pythium
and Erwinia as well as diseases of hydroponically- grown vegetable crops.
During his 22 year career at the Univeristy of Arizona, he has trained
23 graduate students and teaches the beginning course in Plant Pathology
and a graduate course on the ecology and epidemiology of soil-borne pathogens.
