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 Copyright
 Background information
 Examination of plants (symptom...
 Information from experts and choice...
 Reference
 Table 1: Some characteristics of...
 Table 2: Symptoms of fungal and...
 Table 3: Spectrum of some fungicides...
 Table 4: Spectrum of some fungicides...






Group Title: ARC-A research report - Agricultural Research and Education Center - RH-82-8
Title: Diagnosis of foliage plant diseases
CITATION PAGE IMAGE ZOOMABLE PAGE TEXT
Full Citation
STANDARD VIEW MARC VIEW
Permanent Link: http://ufdc.ufl.edu/UF00066480/00001
 Material Information
Title: Diagnosis of foliage plant diseases
Series Title: ARC-A research report
Physical Description: 9 p. : ; 28 cm.
Language: English
Creator: Chase, A. R ( Ann Renee )
Agricultural Research Center (Apopka, Fla.)
Publisher: University of Florida, IFAS, Agricultural Research Center-Apopka
Place of Publication: Apopka FL
Publication Date: 1982
 Subjects
Subject: Foliage plants -- Diseases and pests -- Florida   ( lcsh )
Plant diseases -- Diagnosis -- Florida   ( lcsh )
Genre: government publication (state, provincial, terriorial, dependent)   ( marcgt )
bibliography   ( marcgt )
non-fiction   ( marcgt )
 Notes
Bibliography: Includes bibliographical references (p. 7).
Statement of Responsibility: A.R. Chase.
General Note: Caption title.
Funding: Florida Historical Agriculture and Rural Life
 Record Information
Bibliographic ID: UF00066480
Volume ID: VID00001
Source Institution: Marston Science Library, George A. Smathers Libraries, University of Florida
Holding Location: Florida Agricultural Experiment Station, Florida Cooperative Extension Service, Florida Department of Agriculture and Consumer Services, and the Engineering and Industrial Experiment Station; Institute for Food and Agricultural Services (IFAS), University of Florida
Rights Management: All rights reserved, Board of Trustees of the University of Florida
Resource Identifier: oclc - 71212661

Table of Contents
    Copyright
        Copyright
    Background information
        Page 1
    Examination of plants (symptomatology)
        Page 2
        Page 3
        Page 4
    Information from experts and choice of appropriate controls
        Page 5
        Page 6
    Reference
        Page 7
    Table 1: Some characteristics of fungal leaf spot diseases compared to disorders caused by other factors
        Page 8
    Table 2: Symptoms of fungal and bacterial diseases of foliage plants which generally occur
        Page 8
    Table 3: Spectrum of some fungicides available for use as foliar sprays for control of fungal leaf and stem rot diseases
        Page 9
    Table 4: Spectrum of some fungicides available for use as soil drenches for control of root and stem rot diseases
        Page 9
Full Text





HISTORIC NOTE


The publications in this collection do
not reflect current scientific knowledge
or recommendations. These texts
represent the historic publishing
record of the Institute for Food and
Agricultural Sciences and should be
used only to trace the historic work of
the Institute and its staff. Current IFAS
research may be found on the
Electronic Data Information Source
(EDIS)

site maintained by the Florida
Cooperative Extension Service.






Copyright 2005, Board of Trustees, University
of Florida







DIAGNOSIS OF FOLIAGE PLANT DISEASES


A. R. Chase
University of Florida, IFAS i '
Agricultural Research Center-Apopka
ARC-A Research Report RH-82-8


Diagnosing plant diseases is not a simple procedure. "'Trfat,-in.

many instances accurate disease diagnosis can only be made through culturing

and identification of the pathogen by a trained plant pathologist. There

are,however, some important facts that growers can utilize to diagnose dis-

orders caused by plant pathogens and parasites. Some of the steps involved

in accurate disease diagnosis and control are: 1) background information;

2) examination of plants (symptomatology); 3) information from experts; and

4) choice of appropriate controls.

Background information -

The first step toward an accurate diagnosis of a disorder is determin-

ation of the nature of the disorder. In most cases, the cause is not a

pathogen but rather a nutritional imbalance, phytotoxic response, environ-

mental change, or some other organism such as an insect, mite or nematode.

The type of symptom can help determine whether or not the cause is a pathogen.

This subject has been discussed at length (1) and is briefly summarized in

Table 1. Even if a grower determines that his crop is being produced under

optimum conditions, he should become familiarized with diseases which affect

his crop. This information is generally available in articles published by

University personnel (2,3,4,5,6) and through a compilation of past diagnoses


This paper was adapted from a talk presented at the National Tropical

Foliage Short Course, Orlando, FL, January 1982. Use of trade names in this

paper does not constitute an endorsement of these products over others with

similar activities.







by each grower. For instance, schefflera growers may have an outbreak of

Alternaria leaf spot each year and through experience recognize symptoms

of this disease. Another example of this type of information is the time

of year a disease occurs. Dieffenbachia growers should know that

Colletotrichum and Leptosphaeria leaf spots both occur primarily during

the cooler winter months. Thus, a leaf spot occurring during the summer

is not likely to be caused by one of these organisms. In summary, growers

should become knowledgeable about their crops in terms of diseases, nutrition,

and sensitivities to pesticides or they cannot accurately determine the

cause of a problem.

Examination of plants (symptomatology) -

The most important part of successful disease control is identification

of the pathogen, since only then can the most effective control be chosen,

be it cultural, chemical or both. Since control strategies differ substan-

tially according to whether the pathogen is a bacterium or a fungus, growers

should try to distinguish between these two disease types. Differentiating

between bacterial and fungal stem or leaf rots is not always clear cut, but

certain symptoms occur in each type of disease (Table 2).

Bacterial diseases are typified by: 1) water-soaking; 2) slimy texture;

3) fishy or rotten odor; 4) confinement between leaf veins; and 5) tissue

disintegration. Water-soaking frequently occurs in bacterial leaf spot

diseases such as Erwinia blight of dieffenbachias. Leaf spots are not dis-

crete but appear to run in an irregular pattern in the leaf. Holding the

leaf to light usually reveals the water-soaking and in general, bacterial

infections show this characteristic more than fungal infections. The slimy

texture of bacterial stem and leaf rots is caused by a substance which

many bacteria produce and is generally a good indication that a bacterium is

involved in the disorder. The fishy or rotten odor associated with Erwinia







blight of foliage plants. is another good indication of bacterial infection.

Many bacterial leaf spots such as Xanthomonas leaf spot of philodendrons

(also called Red Edge disease) expand until they reach a large leaf vein.

This vein frequently acts as a barrier which inhibits the bacteria from

spreading further. Finally, tissue disintegration is most common in bacterial

diseases. The ability of bacteria (usually Erwinia spp.) to dissolve the

material holding plant cells together results in a complete destruction of

leaf or stem integrity. Some fungi also produce this symptom but not usually

as extensively as Erwinia spp.

Fungal leaf spots and stem rots are also characterized by various symp-

toms: 1) dry texture; 2) concentric rings; 3) discoloration; and 4) fruiting

structures. Fungal leaf spots and stem rots are usually dry or papery. This

is especially true in dry climates but may not always hold true in Florida

due to high temperatures and humidities favoring disease development. The

leaf spots caused by fungi generally have distinct margins and many times are

circular with concentric rings of light and dark brown. The margins of leaf

spots and stem rots can be brightly discolored, such as purple (Fusarium

stem rot) or yellow (Helminthosporium leaf spot), making these symptoms quite

striking.

The most important symptom of a fungal disease is the presence of

fruiting bodies of the fungus itself. These range in size from microscopic

to those easily detected with the naked eye, and are found within the leaf spot

stem rot area. Each type of fungus has its own characteristic structures

which enable plant pathologists to distinguish it from other fungi. The

vast majority of fungal diseases cannot be diagnosed by a grower since the

fungus does not produce structures visible to the naked eye. Root rot

diseases should not be .diagnosed based upon symptomatology or fungal struc-

tures since the roots of a plant attacked by one organism greatly resemble




4 -


those of a plant attacked by a different organism. These organisms do not

usually make any structures visible to the naked eye and only culturing

diseased roots and identifying the organism with a microscope can accurately

determine the identity of the pathogen.

There are some diseases however which can be identified on the basis

of structures formed on the leaf or stem. Among others these include

Sclerotium rolfsii causing Southern blight, Colletotrichum spp. causing leaf

spots, Myrothecium roridum causing leaf spots, and Fusarium solani causing

stem rot. In each case the fungal pathogen forms a characteristic structure.

Sclerotium rolfsii makes small seed-like sclerotia which are white to brown

depending upon their age. They are also frequently accompanied by white

mycelium which appears as a cottony mass at the plant base. Colletotrichum

leaf spot of dieffenbachia generally can be identified by the tiny black

structures which form in concentric rings in the leaf spot. However,

Leptosphaeria leaf spot of the same plant appears the same and one cannot

determine which is the cause of disease without culturing. In this case,

control strategies are the same and knowing which fungal pathogen is respon-

sible is not absolutely necessary. Myrothecium roridum also produces black

fruiting bodies in leaf spots but these are larger, irregularly shaped,

fringed with white and usually found on the lower leaf surface unlike those

formed by Colletotrichum or Leptosphaeria spp. Fusarium stem rot of

dieffenbachia is accompanied by the bright red globose perithecia produced at

the plant base. These are smaller than those produced by Sclerotium rolfsii

but they too are easily seen with the naked eye. Many other fungi produce

similar structures on infected plants and these should be used as clues to

the cause of a disease. However the presence of a fungal fruiting structure

cannot be taken as assurance of its role in a disorder since many fungi

which do not cause disease live in damaged tissue.








Since none of the symptoms discussed above characterize only bacterial

or fungal diseases, a diagnosis cannot be based upon a single symptom. In

fact, diagnosis on this basis usually leads to faulty conclusions. For

example, Myrothecium leaf spot is caused by a fungus but the leaf spot is

frequently water-soaked and mushy which is characteristic of bacterial

diseases. Another example of such an exception is Dasheen mosaic virus

of dieffenbachia which can appear as leaf spots confined to areas between

leaf veins. This symptom also is generally an indication of bacterial

disease.

Information from experts -

Consulting county agents, USDA personnel and various publications is

also helpful. Some of the publications which carry information relating to

foliage plant diseases include: Florists' Review, Foliage Digest, Florida

Foliage, Florida Nurseryman, Southern Florist and Nurseryman and American

Nurseryman. In addition, most state agencies publish newsletters which

contain important information. When the disease is not easily recognized

and symptoms are confusing, the best step to take is toward a trained plant

pathologist who can culture the pathogen or otherwise identify the cause of

disease. There are usually diagnostic laboratories privately or publicly

run which can provide this service in each state.

Choice of appropriate controls -

One might ask at this point why not just treat all of the plants with a

broad spectrum pesticide. There are several good reasons why application of

pesticides indiscriminately is not advisable. The first is that no matter

how broad the spectrum of activity of a pesticide it does not include all of

the pathogens which occur on foliage plants. Mixing numerous pesticides to

overcome this problem can lead to serious phytotoxicity reactions. In








addition, applying the wrong pesticide can intensify the problem by de-

creasing the population of other organisms which may be acting as antagonists

to the pathogen. When they are removed the pathogen can develop without

interference and disease may become more severe. Finally, applying the wrong

pesticide is a waste of money.

At this point if a pesticide is needed to control the disease it may

be recommended by state agencies (7) or by a consultant. Although many

pesticides may be efficaceous in controlling a disease one may be superior

in control of a certain disease. The spectrum of activity of some of the

fungicides available for use as foliar sprays (Table 3) or soil drenches

(Table 4) indicate many such situations. For example all of the chemicals

listed (Table 3) control Botrytis blight but only Rovral gives superior

control. In the case of a root rot disease many growers use Banrot in

general but all of the other chemicals listed provide superior control of

certain fungi (Table 4). Knowing the pathogen which causes a root rot

disease is therefore very important since it is not necessary to use Banrot

if only Pythium sp. is causing the disease. Information on chemical use

and efficacy is available through the sources listed earlier and is also

the result of personal experience which only conscientious growers accumulate.

Diagnosing diseases of foliage plants is no easy matter, but exper-

ienced alert growers can eliminate many other possibilities and identify the

more common diseases of their crops through symptomatology. In many in-

stances, only a qualified plant pathologist can diagnose and recommend control

measures and growers should always consult these people when a question of

diagnosis arises.


J *>








Literature cited

1. Chase, A. R. 1981.


Leaf spots Disease, diet or drought. Foliage


Digest 4(3):3-6.


2. Chase, A. R.

Foliage Digest

3. Chase, A. R.

Foliage Digest

4. Chase, A. R.

Foliage Digest

5. Chase, A. R.

Foliage Digest

6. Chase, A. R.


1981. Common fungal leaf spot diseases of foliage plants.


4(5):7-12.

1981. Common

4(6):3-4.

1981. Common

4(7):15-16.

198.1. Common

4(8):10-12.


fungal stem rot diseases of foliage plants.


fungal root rot diseases of foliage plants.


bacterial diseases of foliage plants.


1981. Common viral diseases of foliage plants. Foliage


Digest 4(9):9-11.

7. Simone, G. W. 1982. Disease control pesticides for foliage production -

1982. Ext. Plant Path. Rept. No. 30 (Revision 1). Plant Protection

Pointer, Univ. of Fla.








Table 1. Some characteristics of fungal leaf spot diseases compared to
disorders caused by other factors.

Cause of the leaf spot
Characteristic Fungal Other
Distribution of Mainly random, but can Uniform near fans, doors,
affected plants start near pads or doors heaters
and spread
Distribution of Usually on newer or older May be on newer or older
leaf spots on leaves, rarely on all leaves, frequently on all
plants
Pattern of leaf Generally irregularly May be confined to tips,
spots on leaf scattered margins or between veins,
uniform pattern
Development of Generally longer than Phytotoxicity can be al-
symptoms for other causes most overnight, nutrit-
ional or environmental
longer
Fungal structures Common Uncommon
Nature of leaf Dry or wet,with or with- Usually dry without a
spot out a halo, various sizes halo, more or less uni-
form size




Table 2. Symptoms of fungal and bacterial diseases of foliage plants
which generally occur.
Cause of the disease
Characteristic Fungal Bacterial

Water-soaking Not as common Common
Texture Dryish,papery Slimy
Odor Usually none Fishy, rotten
Pattern Circular, may have Irregular or confined
concentric rings to between veins
Disintegration Uncommon Common
Color changes Common, red, yellow Uncommon
and purple








Table 1. Some characteristics of fungal leaf spot diseases compared to
disorders caused by other factors.

Cause of the leaf spot
Characteristic Fungal Other
Distribution of Mainly random, but can Uniform near fans, doors,
affected plants start near pads or doors heaters
and spread
Distribution of Usually on newer or older May be on newer or older
leaf spots on leaves, rarely on all leaves, frequently on all
plants
Pattern of leaf Generally irregularly May be confined to tips,
spots on leaf scattered margins or between veins,
uniform pattern
Development of Generally longer than Phytotoxicity can be al-
symptoms for other causes most overnight, nutrit-
ional or environmental
longer
Fungal structures Common Uncommon
Nature of leaf Dry or wet,with or with- Usually dry without a
spot out a halo, various sizes halo, more or less uni-
form size




Table 2. Symptoms of fungal and bacterial diseases of foliage plants
which generally occur.
Cause of the disease
Characteristic Fungal Bacterial

Water-soaking Not as common Common
Texture Dryish,papery Slimy
Odor Usually none Fishy, rotten
Pattern Circular, may have Irregular or confined
concentric rings to between veins
Disintegration Uncommon Common
Color changes Common, red, yellow Uncommon
and purple









Table 3. Spectrum of some fungicides available for use as foliar sprays
for control of fungal leaf and stem rot diseases.

Fungicidesa
Pathogen Benlate Daconil Manzate 200 Rovral

Alternaria xxb xx
Botrytis x x x xxx
Cercospora xxx xx xx
Colletrotrichum xx xx xxx
Fusarium xxx xxx xx x
Helminthosporium xx xx x

aThis table is not complete in terms of either fungicides or pathogens
listed.
bThe number of x's denotes the degree of control: no x's = no control;
x = poor control; xx = moderate control; and xxx = excellent control.


Table 4. Spectrum of some fungicides available for use as soil drenches
for control of root and stem rot diseases.

Fungicidesa
Pathogen Banrot Benlate Subdue Truban

Cylindrocladium xx xxx
Fusarium xx xxx
Phytophthora x xxx xx
Pythium xx xxx xxx
Rhizoctonia xx xxx

aThis table is not complete in terms either of fungicides or pathogens
listed.
bThe number of x's denotes the degree of control; no x's = no control;
x = poor control; xx = moderate control; and xxx = excellent control.









Table 3. Spectrum of some fungicides available for use as foliar sprays
for control of fungal leaf and stem rot diseases.

Fungicidesa
Pathogen Benlate Daconil Manzate 200 Rovral

Alternaria xxb xx
Botrytis x x x xxx
Cercospora xxx xx xx
Colletrotrichum xx xx xxx
Fusarium xxx xxx xx x
Helminthosporium xx xx x

aThis table is not complete in terms of either fungicides or pathogens
listed.
bThe number of x's denotes the degree of control: no x's = no control;
x = poor control; xx = moderate control; and xxx = excellent control.


Table 4. Spectrum of some fungicides available for use as soil drenches
for control of root and stem rot diseases.

Fungicidesa
Pathogen Banrot Benlate Subdue Truban

Cylindrocladium xx xxx
Fusarium xx xxx
Phytophthora x xxx xx
Pythium xx xxx xxx
Rhizoctonia xx xxx

aThis table is not complete in terms either of fungicides or pathogens
listed.
bThe number of x's denotes the degree of control; no x's = no control;
x = poor control; xx = moderate control; and xxx = excellent control.




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