Module 3 Onset of the Problem Sometimes an observant, conscientious grower will know just when a...

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.


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


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


Examples of uniformly distributed airborne foliar diseases include many residue-borne foliar pathogens and rusts.


Grey leaf spot on corn Southern rust on corn


Most crop maladies of biotic cause will have a nonuniform field distribution.


Examples of non-uniform distribution include:

Insect infestations

Soil-borne diseases

Soil type variations glassy-winged sharpshooter Photo by Reyes Garcia III


Brown stem rot on soybeans


Insect infestations

Soil-borne diseases

Soil type variations



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


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.


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:


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:


Field topography

Soil type variation

Examples of injury to this zone that may be uniform across the field would be:


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.


Injury to the lower leaves are indicative of:


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.


Symptoms on the upper leaves with no or little damage to the lower leaves can indicate:


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.


Symptoms confined to the grain portion of the crop can be an indicator of:


Herbicide misapplication

Grain molds

Grain-feeding insects

Bird/animal injury


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?


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 mottling

Leaf spotting

Marginal leafburn

Examples of uniform leaf symptoms are associated most commonly with herbicide misapplication.


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.


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 spotting

Leaf mottling can be caused by:

Viral diseases

Insect feeding

Chemical misapplication

Nutrient deficiencies


Leaf mottling

Marginal leaf burning can be caused by:

Root and/or stem damage

Chemical misapplication

Nutrient deficiencies


Marginal leafburn

By assessing symptom distribution at the three scales, determine if you need a sample to further identify the cause of the symptoms.


Module 3 Onset of the Problem Sometimes an observant, conscientious grower will know just when a...

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