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Module 3Onset of the Problem
• Sometimes an observant, conscientious grower will know just when a particular concern appeared
• Sometimes indicators pinpoint when a problem began to exhibit symptoms
Onset of the Problem
• Growth stage• Location and movement
in the field• Relationship to specific
event or application
During what growth stage did the problem or symptom first appear?
Symptom Distribution
Distribution of the observed plant symptoms will be your key to identifying the potential of a biological threat to the crop.
In this section of the module, you will learn how symptom distribution can direct your field diagnosis.
Symptom DistributionDistribution of plant symptoms can be described as:
Uniform: symptoms observed across the entire field, plant or leaf, and also may be affecting noncrop species
In some cases, pattern can be identified (e.g., equipment traffic path)
Nonuniform: scattered damage in the field or on the plant
Localized in patches or centered on topographicalelements in the field
Uniform Symptom Distribution
Uniform damage patterns over a large area generally indicate abiotic or nonliving factors as causes, such as chemical or mechanical damage.
Some biotic or living causes of damage may have uniform field distribution, but nonuniform symptom distribution on the plant and/or leaves.
Non-uniform Symptom Distribution
Non-uniform damage patterns often indicate causes are from biotic or living factors, such as insects, pathogens or animals.
Determining if other non-uniform parameters (e.g., soil variation) are associated with the symptoms is important.
Symptom Distribution
Distribution of plant symptoms can be divided into three scales, which are: Field
Plant
Leaf
Symptom Distribution
Distribution of plant symptoms can be divided into three scales, which are: Field
Plant
Leaf
Symptom Distribution
Distribution of plant symptoms can be divided into three scales, which are: Field
Plant
Leaf
Field Symptom Distribution
Determining the distribution of plant symptoms in the field is the first step. Examples of uniform distribution include: Chemical
misapplications
Airborne foliar
diseases
Field distribution for chemical misapplications may have different types of uniform patterns as affected by: Application methods
Chemical formulation
Soil type variation
Field Symptom Distribution
Drift event Overlap
Poor product mixing
Overlap or faulty
nozzles
Determining distribution of plant symptoms in the field is the first step. Examples of uniform distribution include: Chemical
misapplications
Airborne foliar diseases
Field Symptom Distribution
Examples of uniformly distributed airborne foliar diseases include many residue-borne foliar pathogens and rusts.
Field Symptom Distribution
Grey leaf spot on corn Southern rust on corn
Field Symptom Distribution
Most crop maladies of biotic cause will have a nonuniform field distribution.
Field Symptom Distribution
Examples of non-uniform distribution include:
Insect infestations
Soil-borne diseases
Soil type variations glassy-winged sharpshooter Photo by Reyes Garcia III
Field Symptom Distribution
Brown stem rot on soybeans
Examples of non-uniform distribution include:
Insect infestations
Soil-borne diseases
Soil type variations
Field Symptom Distribution
Examples of non-uniform distribution include:
Insect infestations
Soil-borne diseases
Soil type variations
Photo by Stephen Ausmus
Once you have identified the field distribution, you should focus on the plant. Field
Plant
Leaf
Symptom Distribution
The location or zone of the symptoms on the plant will help you identify the cause.
Plant Symptom Distribution
Most biotic causes of
plant damage will
affect specific plant
parts.
Damage to specific zones of the plant can be an indicator of the cause of the symptoms.
Plant Symptom Distribution
Distribution of the
damage generally will not
be uniform over the
entire plant.
Damage to the lower portion and roots of the plant can indicate:
Plant Symptom Distribution
Soil-borne fungal
diseases
Root and stem feeding
insect injury
Herbicide injury
In most cases, injury to this zone will be nonuniform across the field and may be related to:
Plant Symptom Distribution
Field topography
Soil type variation
Examples of injury to this zone that may be uniform across the field would be:
Plant Symptom Distribution
Cultivation injury
Fertilizer application
(e.g., soil-applied
ammonia)
As we move up the plant, the field distribution may change as the causes for the symptoms also may change.
Plant Symptom Distribution
Injury to the lower leaves are indicative of:
Plant Symptom Distribution
Airborne foliar diseases
Translocatable nutrient
deficiencies
Some insect injury
Many foliar diseases and some insect species will start in the lower canopy first and move
upward over time.
Plant Symptom Distribution
Symptoms on the upper leaves with no or little damage to the lower leaves can indicate:
Plant Symptom Distribution
Insect damage
Contact herbicide
misapplication
Viral diseases
Few foliar pathogens will start on the upper canopy leaves, as most require free moisture and the lower canopy region creates a more favorable environment.
Plant Symptom Distribution
Symptoms confined to the grain portion of the crop can be an indicator of:
Plant Symptom Distribution
Herbicide misapplication
Grain molds
Grain-feeding insects
Bird/animal injury
Symptom Distribution
By combining the distribution on the plant with the field distribution, you may start to build clues about the cause of the observed symptoms.
Once you have identified the zone or part of the plant being affected, you should focus on the leaves.
What is the pattern
of symptoms on the
leaves?
Symptom Distribution
Just like field symptoms, distribution of leaf symptoms is a good indicator of the cause. Leaf symptoms will appear as:
Uniform
Nonuniform
Leaf Symptom Distribution
Just like field symptoms, distribution of leaf symptoms is a good indicator of the cause. Leaf symptoms will appear as:
Uniform
Nonuniform
Leaf Symptom Distribution
Leaf mottling
Leaf spotting
Marginal leafburn
Examples of uniform leaf symptoms are associated most commonly with herbicide misapplication.
Leaf Symptom Distribution
Sorghum with gramoxone drift injury
Photo: Loren J Giesler
While some foliar diseases may appear uniform initially, careful inspection often will result in finding them more nonuniform.
Leaf Symptom Distribution
Oak wilt disease
Photo: Fred Baker, forestryimages.org
Several factors can cause nonuniform leaf symptoms. Insect feeding injury or foliar disease typically causes leaf spotting.
Leaf Symptom Distribution
Leaf spotting
Leaf mottling can be caused by:
Viral diseases
Insect feeding
Chemical misapplication
Nutrient deficiencies
Leaf Symptom Distribution
Leaf mottling
Marginal leaf burning can be caused by:
Root and/or stem damage
Chemical misapplication
Nutrient deficiencies
Leaf Symptom Distribution
Marginal leafburn
By assessing symptom distribution at the three scales, determine if you need a sample to further identify the cause of the symptoms.
Symptom Distribution