Unit 4: Plant Disease
Management for Field
Crops
Unit 4: Plant Disease
Management for Field Crops

Unit 4 Objectives:
– Discuss some common plant disease problems
of common field crops in IL
– Awareness of possible prevention/treatment
methods
– Understanding of the types of chemicals or
treatments necessary for control
Unit 4: Plant Disease
Management for Field Crops
Crop yield and quality is affected by
disease each year in IL
 Disease management strategies should be
part of your IPM plan

– Resistant or tolerant varieties of crop
– Crop rotations
– Fungicides
 Use in conjunction w/ other practices or as a last
resort
– Appropriate agronomic practices
Unit 4: Plant Disease
Management for Field Crops

Success often depends on amount of
scouting, stage of identification, and
accuracy of diagnosis
– Proper scouting will ensure that
controls/treatments are used at the most
effective times
– Can help prevent an economic loss
Unit 4: Plant Disease
Management for Field Crops

Disease Diagnosis
– Accurate diagnosis is critical first step for
control
– Many diseases or nutrient deficiencies can
cause the same symptoms
– Must correctly diagnose in order to prevent
unnecessary applications
– Plants should be collected for identification as
soon as disease is suspected
 Is this always realistic?
Unit 4: Plant Disease
Management for Field Crops
– Best to try to identify the disease while the
plant is still alive
– Samples can either be sent to the Plant Clinic
at U of I or be taken to the local Extension
Office

Read the Label
– Make sure you understand all restrictions
associated w/ each chemical
– These chemicals may not be used as
frequently as herbicides or insecticides, so
you may not be as up to date w/ them
Unit 4: Plant Disease
Management for Field Crops
– Must be purchased and applied by a licensed
applicator

Fungicide Guidelines
– Seed Treatments
 Greatest benefits found when:
– Seed is low quality due to damage or fungal infestation
– Seedbed is cool and wet delaying germination and
emergence
– Low seeding rates
 Should not be used as a substitute for good quality
seed
Unit 4: Plant Disease
Management for Field Crops
 Low yields and economic losses will still be realized
w/ low quality or damaged seed
 Selection is crucial
– Some treatments may only control specific pathogens
– Foliar Treatments
 May reduce losses in corn, soybeans, and small
grains
 Should only be used on fields w/ an expected
disease severity
 If the disease is diagnosed early, and fungicide
applied correctly, should have maximum benefit
Unit 4: Plant Disease
Management for Field Crops
 Fungicides applied to corn usually reserved for
seed-production fields
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Leaf blights
Gray leaf spot
Rust diseases
Generally justified only when disease is prevalent within
1-2 weeks of tasseling
– Infections occurring 1-2 weeks or later after tasseling
usually not economically controlled
 Foliar fungicides applied to small grains
– Rusts
– Septoria
Unit 4: Plant Disease
Management for Field Crops
– Leaf blights
– Tan spot
– These tend to occur after inadequate rotations and
during damp weather
 Most damage occurs from emergence to flag
leaf/early milk stage
– Fungicides can increase yields, seed weight, and quality
– Decisions to spray should be based on the amount of
disease at flag leaf emergence
 Use of adjuvants is recommended (especially for
corn and small grains)
Unit 4: Plant Disease
Management for Field Crops

Nematicide Applications
– Granular forms for corn and sorghum should
be applied as band treatments
– Should only be used when soil analysis shows
high numbers of parasitic nematodes
– Not designed to replace crop rotation or use
of resistant varieties
Unit 4: Plant Disease
Management for Field Crops
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Alfalfa Seedling Blight
– Caused by soil-borne fungi
– Often occurs under wet conditions
– Usually the first disease stress alfalfa may
encounter
– Often leads to poor or stunted stands
 May survive, but yields will be reduced
Unit 4: Plant Disease
Management for Field Crops
– Easy way to suspect is if alfalfa stand
emerges, but weeds grow quicker
 Alfalfa usually grows very aggressively and crowds
out weeds
Unit 4: Plant Disease
Management for Field Crops

Alfalfa Root rots
– Phytophthora, Fusarium wilt, Aphanomyces
– Causes poor seedling stands, as well as, root
damage
– Identified by lesions on the taproot,
subsequent disintegration of root tissue, and
death
– Control w/ resistant varieties
Unit 4: Plant Disease
Management for Field Crops

Corn Gray Leaf Spot
– Early symptoms are signified by 1/16” olive spots on
the leaves w/ a yellow halo
– At two weeks the spots are their signature gray or tan
color
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¾ to 2 ½” long
Run parallel to leaf veins
Begins on lower leaves and can spread upward
Entire leaf may appear gray under extreme conditions
– Moist conditions and 70-85 degrees are ideal
Unit 4: Plant Disease
Management for Field Crops
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Corn Stalk Rot
– Fusarium
 Leaves turn from healthy green to dull green
 Lower stalk yellows
 Stalk easily collapses and will cause lodging
 Inside of the stalk may be pink
 Favorable conditions
– Dry early
– Above avg. rain midseason
– Temps 80-100 degrees
– Many other types of stalk rot are known
Unit 4: Plant Disease
Management for Field Crops
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Soybean Brown Stem Rot
– Soil borne disease that becomes visible late in
the season
– Survives on plant material that may be buried
>1’ deep
 Infects the roots and stem early in life
 Grows w/ the plant
 Often infects the xylem tissue and restricts nutrient
and water passage as the plant matures
Unit 4: Plant Disease
Management for Field Crops
– Symptoms
 Typically not visible until late reproductive stages
 May not be visible at all
 Internal browning of vascular tissue in stem and/or
leaves
 Leaves may have appearance similar to high
temperature scorching
 May occur in circular patterns
– Favorable weather conditions
 Cool wet conditions either in early development or
at reproductive stages
 Development inhibited above 80º
Unit 4: Plant Disease
Management for Field Crops

Soybean Phytophthora Root Rot
– Spores reside in the soil
 Germinate under wet conditions to form more
spores
 Mobile w/ soil water
 Encysts on soybean roots
– Symptoms
 Stand reduction either pre or postemergence
 Leaves may wilt
 Water soaked lesions on stem and/or roots
Unit 4: Plant Disease
Management for Field Crops
– Favorable Conditions
 >60º temps
 Very wet soils
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Poorly drained
Low areas
Compacted soils
High clay soils
Not restricted to these areas only
Unit 4: Plant Disease
Management for Field Crops
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Soybean Cyst Nematode
– Very similar symptoms to many other diseases
 Can go undetected as the cause of economic loss
for many years
– Will often occur in circular/oval shaped
patterns
 Infected plants will be less vigorous and somewhat
yellow
 May be slow to fill foliage
Unit 4: Plant Disease
Management for Field Crops
– Severity of symptoms can vary greatly from
nonexistent to very visible
– Infections cause dwarfed and stunted root
systems
 Can reduce the number of nitrogen-fixing nodes
 Roots may be susceptible to other diseases
– True identification can only be done by root
examination
 White/yellow oval shaped eggs on the roots
 Nodes are much smaller than the nitrogen nodes
Unit 4: Plant Disease
Management for Field Crops
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Asian Soybean Rust
– Severe foliar damage
 Destroys photosynthetic tissue
 Early defoliation
 Early maturation
– Yield losses can be 10-80%
– 6 hours of wet conditions and temps 59-82º
ideal for germination
 Dry conditions will restrict it
Unit 4: Plant Disease
Management for Field Crops
– Spores penetrate cells directly instead of wait
for another opening in the leaf
 9-10d from initial infection to next stage of spore
production
– Common hosts include kudzu, vetch, yellow
sweet clover
– Spreads w/ wind patterns
Unit 4: Plant Disease
Management for Field Crops

Wheat Septoria Leaf Blotch
– Recognized by oval shaped lesions on the
leaves
 Centers of the lesions may be grayish
 May ooze when squeezed under high humidity
conditions
– Spores only mobile by splashing raindrops
– Also infect stems and head
 Head become purple-streaked
Unit 4: Plant Disease
Management for Field Crops
– Can result in low test weight and shriveled
seeds
– Usually survives on straw and inoculates the
following crop year
– Favorable conditions:
 Splashing rains
 Temps between 68-82º
Unit 4: Plant Disease
Management for Field Crops

Wheat Fusarium Head Blight (Scab)
– Caused reduced yield and grain quality
– May also contaminate grain w/ mycotoxins
 Vomitoxin – not extremely toxic when fed, but will
decrease consumption and feed efficiency (non
ruminants most sensitive)
– Favorable conditions:
 Humid during flowering
– Identified by bleached spikelets of the head
 May even kill the developing seed in the dough
stage
Unit 4: Plant Disease
Management for Field Crops

Unit 4 Assignment:
– Create a treatment protocol for each of the
discussed diseases
 Product and active ingredient
 Application rate
 Application methods
 Time of application
– Bring an article on a plant disease
 One page written summary
– 20 points -- Due next time!