Control of plant diseases involves understanding pathogens and the diseases they cause in order to manage them. There are four basic types of disease control: biological, cultural, legislative/regulatory, and chemical. Biological control includes host resistance and use of antagonistic microorganisms. Cultural control manipulates the environment through practices like crop rotation, irrigation management, and sanitation. Legislative controls establish quarantines and inspection programs. Chemical control applies pesticides like fungicides and fumigants. Proper use of different control strategies and fungicide rotation are important to prevent pathogen resistance.

1. CONTROL OF PLANT DISEASES
As plant pathologists, we don't study morphology, life cycles,
and spread of pathogens because it's so interesting;
instead, the main purpose behind understanding
pathogens and the diseases they cause is so diseases can
be controlled.
For most crops, the goal is to save most of the plant population,
not selected individuals. exception: tree crops (citrus, pecan,
timber)
** Images and lecture material were not entirely created by J. Bond.
Some of this material was created by others.**

2. Basic principles of disease control:
Control strategies can be divided into two groups based on their effect on
the development of resistance to the control measure by the
pathogen:
1. Eradicative control measures — designed to eliminate the entire
pathogen population - examples: pesticides, vertical or complete
resistance - These tend to select for resistant variants of the
pathogen. Why? All individuals are affected, so the pathogen must
adapt or die.
2. Management control measures — designed to reduce the pathogen
population by destroying a portion of the population - examples:
horizontal or partial resistance, antagonism, cultural practices,
quarantine - These do not apply heavy selection pressure to the
pathogen. Why? Portions of the pathogen population remain
unaffected, no pressure to adapt.
* Of the two, we prefer to use management strategies.

3. Disease control
There are four basic types of control measures:
a. Biological control
b. Cultural control
c. Legislative and regulatory control
d. Chemical control

4. Biological control
Manipulation of biotic entities; host and antagonistic
microorganisms
1. Host resistance - control based on the genes and the
resistance mechanisms they control
a. Van der Plank described two types of resistance
(1960s; these are the "classics‘’):
i. Vertical resistance — resistance that is effective against
some, but not all, races of a pathogen; decreases the
effective amount of incoming inoculum (avirulent races
can't infect), but does not reduce the rate of disease
development (virulent races are not affected)
ii. Horizontal resistance — resistance that is effective against
all races of the pathogen; decreases the rate of disease
development for all races

6. Biological control
b. Resistance has been defined in many other ways since Van der
Plank, including systems based on: epidemiologic effects,
number of genes involved, how long the resistance lasts
under field conditions; additional terms you should be
familiar with are:
i. tolerance — plants are diseased, but they do not yield less
than healthy plants
ii. induced resistance — a normally susceptible plant treated
with an avirulent strain of a pathogen gives a resistant
reaction when challenged later with a strain that is virulent

7. Biological control
2. Antagonists — control using microorganisms that inhibit the growth,
development, or reproduction of pathogens
Four types of activity:
1. Antibiosis — inhibition of pathogen through antibiotics produced by the
antagonist - examples: streptomycin (antibacterial, from actinomycete),
penicillin (antibacterial, from fungus)
2. Competition — two organisms attempt to utilize the same limiting
factors (nutrients, oxygen); supply not large enough to support both
antagonist and pathogen
3. Amensalism — antagonist makes the environment unsuitable for the
pathogen (modifies pH, temperature, moisture)
4. Parasitism & predation — antagonist directly attacks the pathogen
example: nematode-trapping fungi

8. Biological control
Antagonism frequently operates under natural conditions;
difficult to manipulate due to the modifying effects of the
environment; may be important in suppressive soils — soils in
which the pathogen cannot establish, develop, or survive
example: Queensland avocado grove has been productive for 34
years even though researchers routinely collect a virulent
isolate of Phytophthora from the soil; root rot is common in
nearby groves, but very rare in the grove with suppressive soil

9. Note: Slide is the property of B.M. Pryor, U. Arizona

10. Cultural control
Cultural (physical) control — manipulation of the environment
There are many types of cultural control. Here are few selected
examples:
1. Crop rotation — rotate crops and varieties over seasons to reduce
pathogen inoculum levels * This is probably the most widely
employed control measure in agriculture! example: rotate
soybean with corn to control soybean cyst nematode
2. Selection of planting date or planting location — choose a
time/place favorable for the host, rather than the pathogen: avoid
pathogen or its vector example: (time) plant cotton late to control
damping-off caused by Pythium (warm soil)
3. Seeding rate and canopy density — adjust within-row and
between-row spacing to open the canopy and reduce diseases
that spread in the humid, protected canopy environment

11. Cultural control
Cultural (physical) control — manipulation of the environment
4. Irrigation
a. Pathogens can be spread in irrigation water or favored by wet soils-
example: late blight (Phytophthora)
b. Pathogens can be controlled by flooding - example: Fusarium wilt on
banana
5. Control insects and weeds — insects vector viruses and other
pathogens; weeds serve as alternate hosts for pathogens or vectors
and increase canopy density
6. Sanitation ~ keep area free of diseased plant material by pruning
diseased branches (fireblight), plowing under or burning debris,
washing and sterilizing harvesting and processing equipment (Rhizopus
soft rot); poor sanitation contributed to the late blight outbreak that
caused the Irish famine
7. Heat or refrigeration -- hot air, hot water, or steam treatments are
used to kill pathogens in seed or propagation materials; harvested
fruits and vegetables are kept refrigerated

12. Legislative and regulatory control
1. Quarantine — detention and associated practices for preventing
the entry of diseased materials or pathogens into an area;
relatively inexpensive; can be at federal or state level (CA citrus)
a. APHIS (Animal and Plant Health Inspection Service) —
agency within USDA that runs:
i. PPQ (Plant Protection and Quarantine) — agency
responsible for federal quarantines -established by the
Plant Quarantine Act (1912), which resulted from
epidemics of chestnut blight and Dutch elm disease
b. Pest and Disease Survey — national database; all pests on major
crops in each state
c. Action programs -- eradicate or contain pests that get past
quarantine worked for: citrus canker (FL); didn't work for; potato
golden nematode on Long Island, NY (birds); sugarcane smut, FL
(hurricanes)

13. Legislative and regulatory control
1. Quarantine —
2. Inspection and certification programs — state level;
plants/seeds grown under conditions unfavorable for
pathogens and are inspected to be sure that pests
are not transported along with packing material
3. Pesticide labeling and applicator certification — these
activities are under the control of the EPA
(Environmental Protection Agency)

14. Note: Slide is the property of B.M. Pryor, U. Arizona

15. Chemical
Application of pesticides
Pesticide — chemical that kills a pest (fungicide, bactericide, nematicide,
etc.); fungicides as examples, since fungi are the largest group of
plant pathogens
Types of fungicides and selected examples:
1. Inorganic
a. Sulfur -- oldest known fungicide
b. Copper — oldest formulated fungicide is the Bordeaux mixture (downy
mildew of grape); still the most widely used copper fungicide in the world
2. Organic
a. Protective fungicides (preventative, contact)-- protect infection court
i. thiram (Thiram, Tersan) — seed and bulb treatment of vegetables
ii. dichloran (Botran) ~ used against Botrytis on vegetables and flowers
iii. Azoxystrobin (Quadris), Pyraclostrobin (Headline/Cabrio/Insignia) -- used
against leaf spots and blights, fruit rots

16. Protective/Contact Fungicides
 Are sprayed onto foliage to form a
protective barrier
 Do not protect emerging plant shoots
 Must be applied frequently
• Usually on a schedule, also based upon
forecasting systems
Note: Slide is the property of B.M. Pryor, U. Arizona

17. Chemical
Types of fungicides and selected examples:
2. Organic
b. Systemic fungicides (curative) — are absorbed
through foliage or roots and are translocated
upward through the xylem; control already
established pathogens and protect against new
infections
i. metalaxyl (Ridomil, Apron) -- controls oomycetes
ii. benomyl (Benlate) — broad-spectrum fungicide
iii. propiconazole (Tilt)/ tebuconazole (Folicur) — broad-
spectrum fungicide
iv. aldicarb (Temik) – broad spectrum – bacteria, nematodes,
etc.

18. Chemical
Types of fungicides and selected examples:
1. Inorganic
2. Organic
Fumigant — highly volatile, small molecular weight compounds
with activity against a wide variety of pathogens (not
limited to fungi); dangerous to humans
example: methyl bromide; currently being pulled from market
due to danger to nontarget organisms, including humans

19. Fungicide Resistance
Fungicide Resistance – the
inheritable modification of a fungus
to a fungicide. Fungi become less
sensitive to a fungicide due to a
mutation.
Note: Slide is the property of B.M. Pryor, U. Arizona

20. Actions That Lead To Resistance
Continuous use of a single fungicide or
fungicides with the same mode of action
Not using tank mixes of multiple fungicides
Not alternating fungicides with different
modes of action
Note: Slide is the property of B.M. Pryor, U. Arizona

21. Examples of Fungi Resistance
Fungicide Pathogen Disease
Tersan 1991 Sclerotinia Dollar Spot
Subdue Pythium Pythium Blight
Chipco 26019 MicrodochiumPink Snow Mold
Bayleton Sclerotinia Dollar Spot
Note: Slide is the property of B.M. Pryor, U. Arizona

22. Fungicide Resistance Management
Strategies
 Alternate or tank mix fungicides with different
modes of action
 Minimize applications of fungicides that are
susceptible to resistance development
 Use label rates
 Integrate other control methods with fungicide
usage
 Use fungicides to prevent disease development
instead of as a curative action
Note: Slide is the property of B.M. Pryor, U. Arizona