Lesson Plan
 Module 1
 Module 2
 Module 3
 Module 4
 Module 5
 Module 6
 Module 7
 Module 8
 Module 9

Title: Plant pathology guidelines for master gardeners
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Permanent Link: http://ufdc.ufl.edu/UF00076835/00001
 Material Information
Title: Plant pathology guidelines for master gardeners
Physical Description: Book
Creator: Pernezny, Ken; Simone, Gary; Collins, Janice; and Richard Lentini
Publisher: University of Florida
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Bibliographic ID: UF00076835
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Table of Contents
    Lesson Plan
        Page 1
    Module 1
        Page 2
        Page 3
        Page 4
        Page 5
    Module 2
        Page 6
        Page 7
        Page 8
        Page 9
    Module 3
        Page 10
        Page 11
        Page 12
        Page 13
        Page 14
        Page 15
        Page 16
        Page 17
        Page 18
        Page 19
        Page 20
        Page 21
    Module 4
        Page 22
        Page 23
        Page 24
        Page 25
        Page 26
    Module 5
        Page 27
        Page 28
    Module 6
        Page 29
        Page 30
        Page 31
    Module 7
        Page 32
        Page 33
        Page 34
        Page 35
        Page 36
    Module 8
        Page 37
        Page 38
        Page 39
        Page 40
        Page 41
        Page 42
    Module 9
        Page 43
        Page 44
        Page 45
        Page 46
        Page 47
        Page 48
        Page 49
        Page 50
        Page 51
        Page 52
Full Text

Plant Pathology Guidelines for Master Gardeners

Plant Pathology Guidelines for Master Gardeners
Ken Pernezny, Gary Simone, Janice Collins, and Richard Lentini


The word pathology comes from two Greek words, "pathos" (suffering) and "logos" (study).
Therefore, plant pathology is the study of the suffering or diseases of plants.

This Web tutorial is a synopsis of a 3-1/2 hour slide/discussion presentation that we have
developed over the years in support of several county extension Master Gardener training
classes. Our unit at the Everglades Research and Education Center is primarily responsible for
extension and research programs aimed at commercial vegetable farmers. Therefore the great
majority of examples and pictures are from commercial vegetable situations. However, the
principles of plant pathology are what is important. The basic concepts presented are applicable
to all plants and all growing conditions, whether they be diseases in 300 acre tomato fields or
individual hibiscus plants in a home landscape.

This section is strictly on plant diseases. Insect problems are covered in another unit of the
Master Gardener training.

This tutorial is divided into modules (1 thru 9) for convenience and ease of learning. To get the
most value out of this site, the modules should be taken in sequence.

Module Title:
1) Non-parasitic Disorders
2) Parasitic Diseases and the Plant Pathogens that Cause Them
3) Symptoms of Plant Diseases
4) Signs of the Pathogen
5) Specific Symptoms and Signs of Bacterial Diseases
6) Symptoms of Virus Diseases
7) Plant Disease Control
8) Chemical Methods for Disease Control
9) Oh No!-- A Quiz

Plant Pathology Guidelines for Master Gardeners

Module 1: Non-parasitic Disorders

Most plant pathologists concentrate on those problems caused by parasitic organisms (primarily
microorganisms). However, it is important to recognize non-parasitic disorders of plants so that
these can be differentiated from plant diseases when Master Gardeners are confronted with
clientele problems.

These non-parasitic disorders include:

nutrient imbalances
temperature extremes
toxic chemicals
mechanical injury
water imbalances
air pollution
genetic defects

Most of the examples used throughout this tutorial will come from our 23 years experience with
diseases of commercial vegetables in Florida. This is our primary area of responsibility.
Therefore, naturally, our collection of photographs reflects this activity in the execution of our daily
tasks. However, the principles of plant disease diagnosis are applicable to all situations, be they a
500-acre commercial tomato field in southern Florida or an annual flowerbed in Pensacola.

Each figure below is associated with a question. See if you can select the correct answer.
Correct answers are at the end.

Fig. 1. What caused this widespread damage to this snap bean field in Homestead (hint: the
damage appeared rapidly on January 13 of the winter vegetable season)?

a.) Spray drift from an adjoining orchard
b.) A lightening strike
c.) A rare genetic disorder
d.) Frost

Plant Pathology Guidelines for Master Gardeners

Fig. 2. What is the common name of this problem on the tomato fruit? (Hint: The damage is
always on the outside of the canopy.)

a.) Phosphorous deficiency
b.) Sunscald
c.) Weather fleck
d.) Scab

Fig. 3. What is the cause of darker green flecks or spots on this immature tomato fruit? (Hint: it is
on all fruit surfaces.)

a.) Sunscald
b.) Unusually dry weather
c.) A genetic disorder
d.) Fertilizer burn

Plant Pathology Guidelines for Master Gardeners

Fig. 4. What is this problem on ficus?

a.) Potassium deficiency
b.) Excess water
c.) Frost injury
d.) Mechanical injury from leaves rubbing in the wind.

Fig. 5. In what plant organ did this problem originate?

a.) Leaves
b.) Fruit
c.) Roots

Plant Pathology Guidelines for Master Gardeners

Fig. 6. Examine this picture. These potato plants showed this brown flecking on leaves almost
overnight. The entire field is uniformly affected. The location of the field is in Homestead, Florida
in southeast Miami-Dade County. It occurred in early November.

Select the most probable cause for this problem.

a.) Frost injury
b.) Air pollution
c.) Metribuzin herbicide injury
d.) Excess of boron


Figure 1: If you selected "d" (frost injury), you are correct! An important point to note is
that the damage is quite uniform throughout the field. Most environmentally induced
problems tend to be uniform, whereas parasitic diseases may begin as spots throughout
the field. Of course, you can check the weather and see that frost did indeed occur just
prior to the widespread appearance of this problem.
Figure 2: If you selected "b", you are correct. Fruit exposed to long periods of bright, hot
sunlight may show "scalded", shriveled, and dried tissue, known as "sunscald" or
sometimes "sunburn". It appears only on the side of fruit facing to the outside of the
canopy, where the tissue is exposed to the ravages of the sun.
Figure 3: The correct answer is "c". This is a genetic disorder called "gold fleck". It is
caused by a "bad" gene that has been carried in this tomato line and is expressed in
some fruit under certain conditions.
Figure 4: "a" is the correct answer. There are a myriad of symptoms associated with
mineral deficiencies and excesses. Proper identification usually requires considerable
experience with specific plants.
Figure 5: If you answered "c", give yourself a pat on the back. This is metribuzin herbicide
injury to tomato. The herbicide entered the tomato roots and was transported to the main
veins of the leaf where it destroyed the chlorophyll, causing the "vein-clearing" seen in
the picture.
Figure 6: "b" is the correct answer. Automobile-based air pollutants, such as ozone,
sweep down into south Miami-Dade county from Miami with the first substantial north-to-
south cold fronts of late fall. Potatoes are extremely susceptible to these air pollutants.
This malady is sometimes referred to by growers as "weather fleck."

Plant Pathology Guidelines for Master Gardeners

Module 2: Parasitic Diseases and the Plant Pathogens
that Cause Them

The bulk of this program concentrates on those plant health problems that are caused by
pathogenic microorganisms. These organisms include fungi, bacteria, and viruses.

About 85% of all plant diseases are caused by fungi. Therefore, on a statistical basis alone, you
are likely to encounter fungal diseases much more often than those caused by other types of
pathogens. We will now proceed to a description of the main characteristics of fungi.

Fungi include the molds and mildews that we are all familiar with in Florida.

At one time fungi were considered to be types of plants. Indeed, mycology, the scientific study of
fungi, is still done today in botany departments. However, in modern biology, fungi are not
considered plants. They are placed in their own Kingdom (Mycota, for the serious biologists out
there), with equivalent status to the familiar Animal and Plant Kingdoms.

Sometimes, growth of fungi is so profuse that a large enough mass (mycelia -- multicelled
microscopic strands) will accumulate to be seen with the naked eye. A good example is the
growth of the target spot fungus on this ripe tomato fruit.

Fig. 7. Target spot on ripe tomato fruit.

Most of the time, however, careful examination with a microscope is needed to see fungi and
ultimately identify them. Fungi consist of multi-celled microscopic strands. Often, spores, or the
reproductive structures of fungi are readily visible, as seen in this photomicrograph of the fungus

Plant Pathology Guidelines for Master Gardeners

It is the peculiar size, shape, coloration, etc. of these spores that are most useful in identification
of fungi, including those that cause plant disease.

Sometimes, we encounter important pathogenic fungi that do not readily form spores. A good
example is the root-infecting fungus, Rhizoctonia, shown here.

We take note of the distinctive right-angle branching of the fungal threads (mycelia) in making an
identification of Rhizoctonia.

Plant Pathogenic Bacteria

Our next group of pathogens is the bacteria. These are smaller than fungi. Though fungi cause
more diseases than bacteria, bacterial diseases are generally more difficult to control. Again,
bacteria are not plants. They are one-celled microorganisms, requiring good, powerful light
microscopes to be seen. Though some bacteria produce resistant spores, no plant pathogenic
bacteria do so. Details of bacteria are best seen at the very high magnifications of electron

ly. o. Duuyuis.

Plant Pathology Guidelines for Master Gardeners


Fig. 10. Note the whip-like appendages

E bacteria in this photomicrograph.

Bacteria depend on outside agents for dispersal. They do not spread on the wind as many fungi
do. Splashing water is the number one means by which bacteria are disseminated, as in this
picture showing overhead sprinkler irrigation of a farmer's tomato plants.

Many bacterial diseases can be spread readily simply by touching an infected plant and then by
touching a healthy plant. Like human bacteria, plant pathogenic bacteria are extremely
contagious. Bacteria cannot penetrate the cuticle of plants but must enter the plant through a
wound or natural opening.

Plant Viruses

Viruses are by far the smallest of the pathogens considered in this program. The term
"organism" may not be appropriate for a virus. It is simply a strand of genetic material (DNA or
RNA) that is enclosed in a coat or wrapping of protein. They must have a living host in order to
reproduce (replicate).

Electron microscopes are needed to see viruses. Even the best light microscopes are not good
enough to see such tiny particles. This is a picture of the rod-shaped virus particles of the
tobacco mosaic virus, the first plant-infecting virus discovered.


Plant Pathology Guidelines for Master Gardeners

Viruses usually are vectored or carried from infected to healthy plants by insects. Can you identify
the insect in this picture?

Fig. 12. The above image is of a(r

a.) hornworm
b.) stink bug
c.) flying ant
d.) aphid


Figure 12: "d" is the correct answer. Aphids (and in recent years, whiteflies) are the most
important vectors of viruses in Florida.

Plant Pathology Guidelines for Master Gardeners

Module 3: Symptoms of Plant Diseases
Symptoms are abnormal states that indicate a bodily disorder. It is important that all concerned,
Master Gardeners, Master Gardener coordinators, county agents, and UF plant pathologists use
the same terminology when describing disease symptoms. Fortunately, the terminology used for
description of plant symptoms is really quite simple and straightforward, not at all like that of, say,
human medicine.

Observe Figure 13 (below) carefully. It is a schematic representation of the basic functions in a
plant (left) and of the interference with these functions (right) caused by some common types of
plant diseases.

Proteins synthesized

........ Proeein
and minerals synthesized

Fig. 13. Basic functions in a plant (left) and interference with those functions (right). (Modified
from Agrios, G.N. 1997. Plant Pathology (4th ed.). Academic Press, NY, NY.)

Many of the symptom classes are illustrated here. A "spot" is a relatively small, distinct lesion,
with definite borders. Most times, we indicate the plant organ affected when describing a plant
disease symptom. For example, if the spot is on leaves, it is called a "leaf spot". If the spots are
on fruit, it is naturally a "fruit spot".

As spots grow and coalesce, the symptoms may well be described as a "blight". There are
gradations from spots to blights and the better term to use may not always be clear. Galls or
tumors are masses of undifferentiated growth (similar to cancerous growths in humans). They are
usually associated with the woody growth of stems and branches. Cankers, again, found mostly
on stems and branches, are sunken lesions. Wilts occur when plants droop, indicating problems
with water uptake. Rots occur when tissue breaks down. Often rots lead to a slimy, wet "mush".

Plant Pathology Guidelines for Master Gardeners

However, dry rots can occur. It is important that you get used to using these terms, so that
everyone is on the same page when describing symptoms.

Examine the following set of pictures. Name the symptom type (e.g., leaf spot, wilt) that
best fits each malady.

1-ig. -14. -)ympiom iype (

Plant Pathology Guidelines for Master Gardeners

Fig. 16. Symptom type?

Fig 17. Symptom type?

. oyIliptuIII typer

Plant Pathology Guidelines for Master Gardeners

Fig. 19. Symptom type?

Fig. 20. Symptom type?

Plant Pathology Guidelines for Master Gardeners

Fig. 21. Symptom type?

oYI I IPU II Lpty f


Fig. 23. Symptom type'?

Plant Pathology Guidelines for Master Gardeners

Symptom type?

Symptom type?

Plant Pathology Guidelines for Master Gardeners


Fig. 27. Symptom type?

Fig. 28. Symptom type?

What is the symptom seen here on mango?

r-ig. zu. sympiom typer-

What is the symptom seen here on strawberry?

Plant Pathology Guidelines for Master Gardeners

rig. ,u. rympiom type (

What type of symptom is seen on this papaya fruit? Where did the causal agent come from?

Fig. 31. Symptom type?

What is the symptom seen on these tomato plants?

Fig. 32. Symptom type?

What is wrong with this watermelon plant?

Plant Pathology Guidelines for Master Gardeners

symptom type'

rnal tissues of this tomato stem?

eympiom type .

Plant Pathology Guidelines for Master Gardeners

Fig. 36. Symptom type?

Fig. 37. Symptom type?

What would you call this fungus-caused problem on snap bean?

Fungus-caused prc

Plant Pathology Guidelines for Master Gardeners

Figure 14: Leaf spot is correct. This is bacterial spot of tomato.
Figure 15: Leaf spot is the right answer. These are older lesions of bacterial leaf spot of
malanga (Xanthosoma), an edible aroid.
Figure 16: Another leaf spot. This is rust on snap bean.
Figure 17: This is a transition state from leaf spot to leaf blight, as the individual lesions
begin to coalesce. Remember that gradations across symptom classes occur all the time.
The name of the disease is bacterial spot of pepper, irrespective of the symptoms seen.
Figure 18: Symptoms best described as both leaf spot (upper portion of image) and leaf
blight (lower portion of image) are both plainly evident. This combination of symptoms
occurs all the time in nature. This example is downy mildew of yellow squash.
Figure 19: Blight best fits this symptom on tomato leaves. This is the famous late blight of
tomato, the causal fungus of which also attacks potato.
Figure 20: Leaf blight is the most reasonable choice. This is common bacterial blight of
southern (blackeye) pea.
Figure 21: This is an example of canker on tomato stem. The lesions are sunken and
may eventually girdle the stem, leading to wilt and death. The symptoms are part of the
disease known as early blight.
Figure 22: These impressive growths are galls on oleander. The disease is called
Sphaeropsis gall and is common on bottlebrush trees as well.
Figure 23: If you look carefully, you can see a gall on the leaf of this croton. This is
caused by a fungus.
Figure 24: This symptom on mango is best described as leaf blight. The disease is
anthracnose, by far the most common disease of mango in Florida. The causal fungus
attacks all parts of the mango tree, damaging leaves, fruit, and flowers.
Figure 25: This mushy, soft stem is best labeled as a stem rot. This is bacterial soft rot or
hollow stem of potato.
Figure 26: This is a dry stem rot of celery, known as brown stem.
Figure 27: The symptom here is fruit spot. The lesions are described as "scabby"; indeed,
in Israel, the common name of the disease is bacterial scab. The disease is actually
bacterial spot of pepper. You have seen pictures previously of this disease on pepper
Figure 28: These cucumber fruit are soft and almost watery. The causal fungus can be
seen as a white growth covering portions of the rotted fruit. The symptom is obviously
fruit rot, and the disease is known as Pythium fruit rot of cucumber. Local growers call it
"leak", because as the fruit break down, they literally leak fluid out of packing boxes.
Figure 29: Fruit spot is the symptom seen. As might be expected, this is another
manifestation of anthracnose disease.
Figure 30: This is a fruit rot. Even though the tissue is dry, fruit rot is still the best term for
describing the symptoms. This disease is anthracnose of strawberry, although the fungus
that causes the disease is not the same one that causes anthracnose of mango.
Figure 31: The symptom is fruit rot. This is Phytophthora fruit rot of papaya. The
pathogen came from the soil, probably splashed up onto the lowest fruit during a rain
Figure 32: The plants exhibit wilt. How do you know if the problem is truly caused by a
pathogen and not by a general lack of watering? If you noticed that surrounding plants
appear healthy, you're a good detective! A problem with underwatering would be
manifested throughout the planting. A disease, such as the bacterial wilt seen in this
picture, attacks in "hot spots" more-or-less randomly distributed about the field.
Figure 33: Obviously, it is showing symptoms of wilt. This is an isolated plant among
many healthy-looking ones in a large watermelon patch. Therefore, it is likely to be a
disease rather than merely a lack of water. This is one of the most important diseases of
watermelon in Florida Fusarium wilt (a fungus disease).

Plant Pathology Guidelines for Master Gardeners

* Figure 34: This view of the interior of a tomato stem illustrates quite nicely a sure-bet
diagnostic feature of true, pathogen-induced wilts. A longitudinal cut has been made in
the lower stem. Note that the vascular (water-conducting) tissue at the bottom of the
picture is brown (the vascular tissue is toward the outside of the stem, parallel to the
outer surface). This is abnormal. The tissue should be white to light-green as seen in the
top of the picture. This appearance of brown vascular tissue when stems are cut in this
manner, is definitive proof of a wilt disease.
* Figure 35: This is a special symptom known as damping-off. Sometimes seeds planted in
soil will rot before emergence, or young seedlings such as those pictured here will rot at
the soilline and topple over. These are examples of pre- and post-emergence damping-
off, respectively. All damping-off problems are caused by pathogens, soilborne fungi such
as Pythium and Rhizoctonia.
* Figure 36: This is an example of damping-off of watermelon seedlings caused by
Rhizoctonia. The thinning and twisting of the young stems leads to the common name of
this problem among growers wirestem.
* Figure 37: This is a soft, mushy root rot of cassava caused by a bacterium, Erwinia
carotovora. This followed borer insect injury.
* Figure 38: If your botany background is up to speed, you may very well have described
this as a fruit rot, knowing that the bean pods are the fruit of this plant. Pod rot would also
be a workable description, telling your extension colleagues and clientele exactly what
you're observing.

Plant Pathology Guidelines for Master Gardeners

Module 4: Signs of the Pathogen

As you probably already are aware, symptoms associated with many different plant health
problems can look quite similar. Indeed, professional plant pathologists can often struggle with a
diagnosis based on symptoms, even after years of experience. Therefore, protocols for correct
diagnosis are based on signs of the pathogen!

Signs are actual physical evidence of the occurrence of the pathogen in association with the
unhealthy plant material. These include:

1. mycelium or mold growth, under some conditions, readily visible to the naked eye
2. conks and mushrooms the familiar structures of some fungi that are formed by some
pathogenic fungi
3. fruiting bodies reproductive structures of some fungi that are embedded in diseased
tissue, often requiring a hand lens to see
4. sclerotia hard resistant structures of some fungi
5. rusts
6. bacterial ooze and specific odors associated with tissue macerations by certain

Rest assured, you must become familiar with signs of pathogens and the techniques needed to
detect and identify them. If you try to diagnose plant diseases on the basis of symptoms alone,
you will be wrong too many times to be of very much benefit to your clients.

Fig. 39. Note the talcum-powder like growth on the underside of this yellow squash leaf. The top
of the leaf is yellow. Yellowing of squash leaves is common and may be the result of quite a few
causes. However, only one cause, the powdery mildew fungus, produces this characteristic
growth on the under surface of the leaves. This is the actual fungus that you are observing. It is a
sign of the pathogen.

Plant Pathology Guidelines for Master Gardeners

Fig. 40. Note the sign of the pathogen seen here causing the disease of squash fruit called wet
rot. This fungus growth on the tip of the squash fruit is most readily seen early in the morning or
when dews and humidity are high.

There are lots of reasons why the tips of the young squash fruit become brown and withered.
Most notable is a lack of sufficient bee activity to properly pollinate female flowers. However, the
"whiskery" growth of the wet rot fungus is a sure sign that the browning and withering is indeed a
wet rot disease problem.

Fig. 41. There are many causes of leaf spots on tomatoes. However, if you look closely at the
image, you will note an olive-green/gray growth of a fungus within a yellowish circular spot. What
you are seeing is a sign of the fungal pathogen causing a disease known as leaf mold. There may
be as many as 30 causes of yellow leaf spots of tomato leaves, but only one, the leaf mold
fungus, produces this characteristic sign of olive green mold growth. So you see, by looking for
the sign of the pathogen, you have gone from 30 possibilities to the correct diagnosis.

Plant Pathology Guidelines for Master Gardeners

Fig. 42. This tomato stem was split open after noting that the plant was wilted. Note the hard
black sclerotia inside the stem. These are a definitive sign of the pathogen causing this wilt,
Sclerotinia sclerotinium. The disease is commonly called white mold. Many causes, including
simply a lack of adequate irrigation, can cause tomato plants to wilt. But one, and only one, is
associated with formation of these black sclerotia. You can see how important it is to see and
recognize signs of pathogens when properly diagnosing diseases.

Fig. 43. On this tomato fruit, note the signs of fungus growth and formation of very young
sclerotia (the structures that look like seeds forming in the middle of the fungus mat) at the stem
end. These sclerotia identify the fruit-rotting symptoms as caused by the southern blight fungus.

Plant Pathology Guidelines for Master Gardeners

Fig. 44. You might recognize this disease as bean rust. The distinct yellow leaf spots with the
brown centers are relatively easy to identify. But these brown centers are predominantly a mass
of rust-colored spores of the rust fungus. These are observable as a sign of the pathogen when
carefully examined with a hand lens. What you see through the hand lens is an actual massive
accumulation of fungus spores. So what you see is a sign of the pathogen.

Fig. 45. This is a photomicrograph at 400x magnification of the spores taken from the rust
lesions. We might do this for confirmation of the diagnosis in our labs, but you can do a fairly
good job of hazarding a diagnosis with careful observation and a hand lens.

Plant Pathology Guidelines for Master Gardeners

Fig. 46. This used to be an ear of corn. Now what you see is this grotesque growth of the corn
smut fungus which has taken over. What you are observing in the black interior is a mass of
billions of smut spores. (By the way, this fungus growth is supposedly edible, with a typical
mushroom flavor -- but we are not tempted at all!)

Fig. 47. This is a sign of a fungus pathogen. The sign is a mushroom or conk. The disease of turf
grass it causes is called fairy ring because the conks are arranged more or less in a circle.
Obviously, there are lots of reasons for lawns to decline, but only one decline is associated with
this telltale sign of fairy ring.

Plant Pathology Guidelines for Master Gardeners

Module 5: Specific Symptoms and Signs of Bacterial

So far, we have talked primarily about signs of fungi that cause plant disease. But we now come
to specific symptoms and signs associated with bacterial diseases. While fungi cause about 85%
of plant diseases, bacteria cause some that are the most difficult to control. This is especially true
in Florida, because bacterial diseases are most intense in warm, humid, rainy environments.

Fig. 48. This is the underside of a tomato leaf. The symptom you see is called watersoaking. The
spots look like drops of water and are caused by water accumulation in tissues as a direct result
of bacterial attack. Watersoaking is a very common symptom of many bacterial diseases. It
occurs early in the development of bacterial diseases.

Fig. 49. This is an image of a much later stage in the development of a bacterial disease
(bacterial spot of malanga). Now we are at the stage where bacterial lesions are often described
as "greasy looking". There just is not a better way to describe the late stage of many bacterial

Plant Pathology Guidelines for Master Gardeners

Fig. 50. A sign of the bacterial spot of malanga pathogen is evident here. It is the bacterial "ooze"
or exudate seen coming out of watersoaked lesions (see arrows). The "ooze" forms in the readily
seen droplets. These droplets are a sign of the pathogen, being composed mostly of bacterial

Fig. 51. Here is another sign of a bacterial disease. It is bacterial streaming. This tomato plant
was wilted. To properly diagnose bacterial wilt, a horizontal cut was made in the lower stem and
the cut stem immersed partway in water. Within a few minutes, copious amounts of bacterial
exudate emerged from the cut end, forming the white streamers you see in the water. This only
occurs with bacterial wilt and not with any other type of pathogen or abiotic cause. It is virtually
foolproof and should be in the diagnostic arsenal of every master gardener.

Plant Pathology Guidelines for Master Gardeners

Module 6: Symptoms of Virus Diseases

Of all the diseases, those caused by viruses are the most difficult to diagnose. This should
not be surprising viruses produce no telltale signs that can be readily observed. What's more,
symptoms are often quite subtle, often easily confused with nutrient deficiencies and herbicide
injury. In many cases, as a Master Gardener, you will need assistance from county agents and
the regional Plant Disease Clinics in properly diagnosing suspected virus problems.

The first slide you see is of malanga infected with dasheen mosaic virus.

Fig. 52. Infected malanga.
The disease is called dasheen mosaic. You can see that the leaf is a pattern of off colors, some
darker, some lighter than normal. These alternating darker, lighter patterns are referred to as
mottling. This is a quite common symptom associated with virus diseases. Obviously, you must
be familiar with what a healthy malanga leaf looks like in order to detect this subtle symptom
pattern in the dasheen mosaic virus-infected malanga.

Plant Pathology Guidelines for Master Gardeners

I ..
Fig. 54. Symptom type?

What would you describe as the major symptom of the diseased tomato plant in the middle of the

Fig. 55. Major symptom?

What is abnormal about these p

miy. 00. OYMPLU111 L rypr

Plant Pathology Guidelines for Master Gardeners

Fig. 57. The disease triangle.

Most of us realize that disease development requires a susceptible host plant and a virulent
pathogen. This image shows the disease triangle where the third essential factor, a favorable
environment, is included. Diseases usually have relatively specific conditions of temperature,
relative humidity, free moisture, etc. for the symptoms to be expressed or the severity to increase.
Knowing what time of the year a specific disease likely occurs can affect the choice of probable
causes of a plant malady.

Figure 53: You should have readily noted the marked mottling (dark green/yellow) of the
leaf. In addition, if you are familiar with the shape of normal squash leaves, you can see
that the one in the picture is narrowed and distorted. All these symptoms are associated
with papaya ringspot virus infection of squash.
Figure 54: This tomato plant has a disease caused by tobacco etch virus (TEV). Note that
the internodes (space between branches on the stems) are shortened, giving the plant a
bushy appearance. Several of the leaflets show browning (necrosis). This virus was
transmitted by aphids.
Figure 55: The obvious symptom here is stunting. The infected plant is considerably
shorter than adjacent healthy plants. Stunting occurs frequently in virus-infected plants.
This disease is tomato mottle, caused by the tomato mottle virus (TmoV), vectored by
Figure 56: These fruit are covered by circular and C-shaped markings. These marks give
the virus that causes them its name papaya ring spot virus. Mottling and distortion of
leaves also occurs. Fruit sometimes have reduced sugar levels, so flavor can be affected.
This is the most common and serious disease of papaya in Florida.

Plant Pathology Guidelines for Master Gardeners

Module 7: Plant Disease Control

It is very important to remember that a correct diagnosis is the most important step in the
eventual control of a plant disease. Most diseases have a fairly well established control protocol.
Most often, failure to control the disease happens because the problem was misdiagnosed in the
first place.

This is a list of the most important general strategies for management of plant diseases:
1. Crop resistance (should be first line of defense whenever possible).
2. Cultural methods
3. Physical methods
4. Pesticides
5. Regulation

These methods will be discussed further with examples.

Cultural methods for disease control refer to those growing methods that reduce pathogen levels
or reduce the rate of disease development. These include:
1. Sanitation
2. Crop rotation
3. Host eradication
4. Improvement of crop environment

Fig. 58. How are the tomatoes is in this photo being watered? How does this help to reduce

Fig. 59. Note the tall sugarcane planted between rows of much shorter pepper. The sugarcane
serves as a windbreak to reduce windblown sand injury that might create wounds for entry of

Plant Pathology Guidelines for Master Gardeners

pepper pathogens. The cane also traps winged aphids that might harbor and transmit several
important pepper viruses. This is another excellent example of a cultural control.

Fia. 60. What is the obvious cultural control in this Dicture?

at control method is being conveyed in this picture?

Plant Pathology Guidelines for Master Gardeners

Fig. 62. Do these cabbage seedlings, bought at a suburban garden center look completely

Crop resistance

The following terms are important in a discussion of crop resistance:
1. Immunity
2. Resistance
3. Tolerance
Immunity is the rule in the plant kingdom; most plants are immune to most pathogens. Therefore,
one does not have to worry that the black spot on roses will appear next year on the garden

Resistance, strictly speaking, refers to the lower disease levels seen in some cultivars or varieties
of a particular crop species compared to other susceptible cultivars. Always choose resistant
varieties when available.

Tolerance refers to varieties or cultivars that appear to be just as heavily diseased as standard
varieties, but which manage to be more productive or vigorous.

1 O
6.Cekoe.i r a mti

L ~

63. Check out these seed packets.

iere is disease resistance mention

Plant Pathology Guidelines for Master Gardeners

Fig. 64. Note this cucumber variety. How many diseases is it resistant to?

Create an environment unfavorable to the pathogen and favorable to the crop.

Fig. 65. There are many theories and ideas on the best time of the day to water. From a plant
pathology point of view, it is preferable to water late morning, after the dew dries from leaves, but
early enough to allow leaves to dry before evening. Of course, ideally, you would water only the
soil surface because the roots are the water-absorbing organs.


Figure 58: Drip irrigation is used here. The water moves out into the field in the blue pipes
which, in turn, feed water to small emitter lines that deliver water under the plastic mulch
right at the base of plants. Because the foliage doesn't get wet, development and spread
of fungi and bacteria are much reduced.
Figure 60: Mulching is the obvious answer. Of course, there are many benefits to be
gained from mulching, including weed control, soil moisture optimization, and soil
temperature moderation. But mulches can serve as a barrier between above-ground
plant parts and pathogens in the soil. Also, by reducing weeds and alternate hosts for
pathogens, such as several viruses, mulches help in the battle against diseases.
Figure 61: This photo shows pruning shears being disinfected (a better word is
disinfested) by immersion in a chemical. A good disinfestant is 10% Clorox or bleach (10
ml household bleach to 90 ml water). Isopropyl alcohol from the drugstore also works
well. When pruning out diseased plant material, it is best to disinfest after each pruning

Plant Pathology Guidelines for Master Gardeners

operation. Also, make cuts several inches beyond diseased tissue in healthy tissue to
make sure you get all of the pathogens.
* Figure 62: These seedlings have typical "v-shaped" lesions characteristic of black rot, a
serious cabbage disease. These could easily be sold to unsuspecting customers (but not
well-informed Master Gardeners). Be aware that diseased planting material is out there.
Obviously, one of the best ways to manage diseases is to keep them out in the first
place. Inspect all planting material and be a discerning buyer!
* Figure 63: You can see rust resistant snapdragons, VFN (Verticillim wilt, Fusarium wilt,
nematode resistant) tomato and mildew-resistant (powdery mildew) zinnia.
* Figure 64: Four is correct: downy mildew, powdery mildew, anthracnose, and angular leaf

Plant Pathology Guidelines for Master Gardeners

Module 8: Chemical Methods for Disease Control

As a last resort, judicious use of chemical applications may be used to mitigate plant disease
losses. There are three major classes of chemical treatments:

1. soil treatments
2. seed treatments
3. foliar sprays

Seed treatments consist of dusts or slurries applied to seed to protect primarily against damping-
off caused by soil borne pathogenic in fungi.

Fig. 66. In this picture the fungicide coating of the seed has been activated by the
moisture in the soil and now forms a protective chemical barrier around the seed.

Some thin s that must be considered when choosing and using fungicides are:

Fig. 67. Use only the recommended fungicide (not what is on sale, is left over from last
season, etc.).

Plant Pathology Guidelines for Master Gardeners

f~g. 68. Use only the recommended amount.

Fig. 69. Time the spray effectively. This often means starting earlier in the season than
one might initially expect.


........... .. ii. .

Fig. 70. Time to look more closely at those materials and approaches needed to help in
your diagnostic effort and ways to seek assistance.

Plant Pathology Guidelines for Master Gardeners

Fig. 71. Try to gather as many "disease books" for reference as you can, especially those
with color photographs.

Fig. 72. Many county offices will have library or file copies of these valuable books and

Plant Pathology Guidelines for Master Gardeners

*-. :'

Fig. 73. An extremely useful source of diagnostic information on specific plant diseases
is the collection of fact sheets and plant protection pointers put out by the plant pathology
departmental of the University of Florida. Unfortunately, many of the color fact sheets
are out of print. Some of these fact sheets are now available on the University of Florida,
IFAS, Dept. of Plant Pathology Web site. Just click on extension publications when you
get to the departmental home page.

When a client comes in with (or even phones about) a plant health problem, it is
important that you gather as much information as possible about the situation. This
additional information can be critical in making the correct diagnosis. And remember to
ask -- clients very often will not think to offer such information unsolicited.

Things to ask include:

1. no. of plants affected
2. pattern of development in the field or garden
3. the relative severity of the problem
4. recent cultural practices (e.g., pesticide sprays) and localized weather conditions

There are times when you simply cannot figure out what the problem is. In consultation
with the Master Gardener coordinator, you may decide to send a sample to one of the
regional University of Florida plant disease clinics. When you do so, a form as shown
will accompany the sample here:

Plant Pathology Guidelines for Master Gardeners


W.P.fl. h0a
mama ~ ~ T P- .- m

rrdo"is Def ftW Fdbw m hw"*

Fig. 74. Form to accompany sample sent to a plant disease clinic. It is important to the
clinic personnel that you fill in as much information as possible. This will suggest
avenues to pursue for the highest likelihood of success.

Su estions for sam le collection and submission
.... -

Fig. 75. This picture illustrates the best way to submit plant samples.

Moisten the root ball (soil or soil mix around roots) and enclose only the root ball in a
plastic bag that is secured with a rubber band or similar method. Do not enclose the
foliage in the plastic bag, as it will rot by the time it gets to the clinic. Then the entire
plant can be put in a paper bag.

Plant Pathology Guidelines for Master Gardeners

Fig. 76. Fruits can be wrapped in soft paper and place to in a cardboard box.
Fig. 76. Fruits can be wrapped in soft paper and place to in a cardboard box.

Plant Pathology Guidelines for Master Gardeners

Module 9: Oh No! ... A Quiz
Now it is time to see how much you have learned.

In the following pictures, you need to: describe the symptom, and determine if this is -
an abiotic or non-pathogen-induced problem

Remember that control measures are routinely available for many diseases and once a problem
has been pigeon-holed into one of these four categories listed above, similar control measures
exist for many problems in that category.

Fig. 77. Host-- snap bean.

my. (0. nMUo-- LuuuII1ueI.

Plant Pathology Guidelines for Master Gardeners

nu1L -- IIIdilyU.

Fig. 81. Host-- tomato.

r-ig. tv. riosi-- iomaio.

Plant Pathology Guidelines for Master Gardeners

Fig. 82. This strawberry plant is in the middle of a patch that shows a definite damage gradient
from East to West (most severe damage at eastern end, no damage at western end). What
caused it?

Fig. 84. Bean pods.

Plant Pathology Guidelines for Master Gardeners

Host -- tomato. See Fig. 87, below, for a hint.

Plant Pathology Guidelines for Master Gardeners

Fig. 87. A cut stem from one of the above plants placed in water.

Plant Pathology Guidelines for Master Gardeners


Fig. 89. What may have caused this problem on avocado fruit?

Plant Pathology Guidelines for Master Gardeners

Fig. 90. What is the peculiar symptom seen at the base of this potted sunflower?


What is the symptom evident on the mango flower cluster on the right? What is the

Plant Pathology Guidelines for Master Gardeners

This is the last quiz question!

Fig. 92. Note the more or less circular to oval area of dead plants in the middle of a tomato field.
This problem appeared literally almost overnight around October 1st in southern Florida. The
area is not low. All the other plants are perfectly healthy. All cultural practices are the same as
those normally used.

Hopefully, this Web tutorial has sharpened your plant disease diagnostic skills and made you
more aware of the role of plant diseases in the framework of plant health.

Plant Pathology Guidelines for Master Gardeners


Figure 77: Symptom: Stem rot. Fungus (see sign of fungus growing in stems). This is
white mold of snap beans.
Figure 78: Symptom: leaf spot (or possibly leaf blight). Bacterium (note watersoaking of
underside of leaf). This is angular leaf spot of cucumber.
Figure 79: Symptom: stunting, off color, mottling. Virus. This is tomato mottle virus
affecting stake tomato.
Figure 80: Symptom: leaf spot. Fungus. Leaf spots appear fairly dry. Fungus may be
the best answer simply on the basis of the 85% rule. Since the leaf is from a mango tree,
this is, not surprisingly, mango anthracnose.
Figure 81: Symptom: wilt; when the lower stem is cut across horizontally and the cut end
immersed in water, no cloudy streaming is seen. Fungus. This is Fusarium crown rot of
Figure 82: This is herbicide injury. Herbicide applied in a walkway just east of the
strawberry patch drifted in the prevailing easterly wind over the patch, creating the
gradient in severity of the damage.
Figure 83: The symptom seen here is a leaf spot. It is bacterial. Although it doesn't look
particularly watersoaked or greasy, the clumped nature of the spots suggest bacterial
origin. This is because bacteria are primarily dispersed in splashing rain so that a group
of bacterial cells all land with a water drop in approximately the same area. This is
bacterial blight of snap bean.
Figure 84: The symptom, evident if you look closely, is a fruit spot. There are tiny, slightly
raised black pimples on these bean pods (fruit). This is fungal. The disease is Alternaria
spot of snap bean.
Figure 85: The symptoms are mottling, stunting, and misshapen leaves of tomato. The
problem is definitely viral. The disease is tomato yellows, an aphid transmitted virus.
Figure 86-7: The symptoms here are primarily wilting with some necrosis (browning) of
the tomato foliage. Fig. 87 shows the famous bacterial streaming, so the problem is, of
course, bacterial. The disease is bacterial wilt of tomato.
Figure 88: Don't ignore the obvious! Someone ran over these poor tomatoes. We refer to
this facetiously as "tire blight".
Figure 89: The symptom is probably best described as a fruit blight. But the name of the
disease is more descriptive -- avocado scab. It is fungal. This conclusion may best be
reached by the process of the elimination. It is certainly not typical of either a bacterial or
viral infection and remember that about 85 percent of all plant diseases are caused by
Figure 90: This is a gall or tumor. It happens to be a bacterial gall. The disease is crown
gall of sunflower.
Figure 91: Using the strictly correct botanical nomenclature, the symptom is panicle blight
(the flower cluster is a panicle). Flower blight would be fine. The disease is fungal.
Because it is on mango, the disease is -- you guessed it --anthracnose.
Figure 92: The problem is a lightning strike! This is actually what happened in this
commercial tomato field. The stems of the plants near the center of the strike are hollow
due to the extreme heat of the electricity.

Plant Pathology Guidelines for Master Gardeners

We welcome your questions or comments.

Ken Pernezny, Ph.D.
Plant Pathologist
University of Florida
Everglades Research and Education Center
P.O. Box 8003
Belle Glade, FL 33430
(561) 993-1561
Suncom 233-1561

Richard Lentini, Webmaster
Sr. Biological Scientist
University of Florida
Everglades Research and Education Center
(561) 993-1563
Suncom 233-1563

Janice Collins
Biological Scientist
University of Florida
Everglades Research and Education Center
(561) 993-1562
Suncom 233-1562

Gary Simone, Ph.D.
Plant Pathologist (Retired)
Univeristy of Florida

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