Principles of Disease Development
Innovative Tobacco
Grower Program
Kenny Seebold
University of Kentucky
Dept. of Plant Pathology
What is a Plant Disease?
• A series of harmful processes operating over a
period of time…
• A plant disease can be defined as any malfunction
or alteration that negatively affects plant
performance OR reduces the plant’s ability to
occupy its ecological niche.
– Abiotic disorders (non-infectious)
– Biotic agents (infectious)
Common Pathogens
(Know Your Enemy)
• Fungi
– most diverse and most common group of pathogens
• Bacteria
– second only to fungi in abundance
• Viruses
– depend on host to replicate
– require vectors to spread
• Nematodes
– microscopic roundworms
Types of Plant Diseases
FOLIAR
SOILBORNE
Critical Factors for Plant Disease
• Causal agent (PATHOGEN)
• Favorable environment
• Susceptible host
Disease Triangle
The Pathogen
• Facultative parasites
– Rhizoctonia solani
(soreshin)
• Obligate parasites
– Peronospora tabacina
(blue mold)
• Pathogens attack
hosts in a number of
ways (specialized
structures INOCULUM)
le
Bla
1X route
e Cyc
30X route
48 hrs.
Swimming
spores
formed
1,000,000X.
More roots
infected.
if
L
Resting
spores
made from
all parts
ofcolony.
ck Sha
nk
Resting Spore
Swimming
spores
locate
tobacco
roots.
Each spore can form a
colony on/in roots.
Colony makes many
spore-producing structures.
The Host
• Non-hosts
– Pathogen doesn’t affect host
• Susceptible hosts
– Host totally susceptible to pathogen
• Partially resistant hosts
– Host shows some degree of disease
• Resistant hosts
– Host “immune” to pathogen
The Environment
• Temperature
– Each pathogen has an optimal temperature
range
• Moisture
– Pathogens vary in their moisture requirments
• Competition
– Other microorganisms compete with pathogens
What Should You Know?
• Know what types of diseases affect tobacco.
• Disease identification – be aware of the signs and
symptoms of tobacco diseases.
• Understand the ecology of the diseases you face.
• Use this knowledge to find “chinks” in the armor of
your foes!
• Be familiar with all the tools at your disposal for
disease management.
Blue Mold Life Cycle
Plant Disease Epidemiology
• Epidemiology = study of disease in populations
• Integrates knowledge of host, pathogen, and
environment
• Core of effective disease management program!
– Disrupt pathogen cycle at “weak links”
• Direct effect on pathogen
• Manipulate environment
• Utilize host resistance
Principles of Plant Disease
Control
• Avoidance: planting date & location
• Exclusion: eliminate or reduce influx of inoculum
from outside sources
• Eradication: reduce or inactivate initial inoculum
• Protection: create protective barrier against
pathogen
• Resistance: use resistant cultivars to limit inoculum
efficacy
• Therapy: produce pathogen-free seed and stock
using heat, chemicals, tissue culture
Useful Terms
• Latent Period
– Time between infection and appearance of symptoms
• Different for each disease (1-14+ days!)
• Affected by environment
• Gives the impression that disease gets worse after
fungicide applications
• Monocyclic vs. Polycylic diseases
– monocyclic = pathogen produces one infection cycle during
crop season
• Amount of disease at end is related to amount of pathogen
in soil
– polycyclic = multiple infection cycles per season
• No relationship between initial pathogen level and final
disease
Epidemiological Considerations
•Most foliar diseases are “polycyclic”
• Most soilborne fungal diseases are “monocyclic”
•Strategies for management can be different for each
disease type
Epidemiology and Disease Control
Practical Examples
•Soreshin
•Typical soilborne (monocyclic) disease
•One cycle (infection to survival structure) per season.
•Once you see disease, it’s pretty much too late to stop
it (can’t apply a chemical at this point).
• Pathogen (Rhizoctonia solani) survives between crops on
residue and organic matter.
•Most effective control is to reduce or limit buildup of
inoculum
•Rotation
•Fumigation
Epidemiology and Disease Control
Practical Examples
•Blue Mold
•Typical foliar (polycyclic) disease
•Many cycles (infection – spore production) per season.
•Once you see disease, it is possible to stop disease with
fungicides if disease level is low-moderate.
• Pathogen (Peronospora tabacina) is introduced from
southern sources each year.
•Most effective control is to reduce or limit efficacy of
initial inoculum
•Resistance
•Fungicides (prior to infection)
Fungicides
• Literally “fungus killers”; can be applied to soil, seed,
foliage, and fruit
• Contact fungicides (protectants)
– Adhere to surfaces of plants, no penetration
– Fungus must come in contact for activity
– Will not affect established infections – hence they
“protect” only and must be applied prior to arrival
of fungus.
– Must be applied regularly to protect new growth and
replenish weathered material.
– COVERAGE is critical for good efficacy!
– Ex. Dithane®, Kocide®
Fungicides
• Systemic fungicides
– Penetrate plant parts to
some degree
– May be taken up by roots
– True systemics are
redistributed via xylem
and/or phloem.
– Locally systemic compounds
exhibit limited movement in
plants.
– Mesostemic – certain
fungicides move from treated
to untreated side of leaves
(translaminar movement).
Some argue that these
compounds are contact
fungicides.
Mesostemic compounds tend to adhere
to or are absorbed by cuticular wax.
Fungicides
• Systemic fungicides (continued)
– Act as protectants against infection
– Coverage less critical than for protectants
• Also tend to have longer residual activity
– Many can also impact existing infections
• Curative activity – also called eradicant, therapeutic or
kickback activity – will halt or reduce established
infections.
• Extent of curative activity dependent on compound, crop,
and on fungal pathogen
– Ex. Ridomil®, Acrobat®
Spores land
curative
Latent period - time in days between infection and appearance of symptoms
Curative fungicides can stop an established infection before symptoms
appear if applied in time – this depends on the fungicide, crop, and
environment
Late-Season Diseases
• Blue mold
• Black shank
• Frogeye leaf spot & Target spot
Blue Mold
• Not in U.S. at this time
• Monitor crops and weather
– Blue mold historically shows up by the end of June
– Get on preventive program, particularly if status or
weather changes.
• Several controls registered
– Actigard (0.5 oz/A / 3 apps max after 18 in.)
– Dithane (1.5-2 lb/A)
– Acrobat (2-8 oz/A) – tank mix with Dithane
• Quadris situation
– Application for Section 18 made in early June
– Decision pending
Black Shank
• Increasing as dry, hot weather prevails
• Resistant varieties may not perform well if droughtstressed
• Mefenoxam products can be used
– Ridomil Gold – 1 pt/A (1st cultivation / layby)
– Ultra Flourish – 1 qt/A (1st cultivation / layby)
– Incorporate by irrigation (1 in.)
• Cultivation may stress diseased plants
• Clean equipment and feet after working infested fields
Frogeye Leaf Spot
Target Spot
• May show up if weather turns wet later in season
• Proper nitrogen fertilization decreases likelihood of
problems
• No fungicides labeled at this time
• Once registered, Quadris should be effective against these
diseases