Module 3 Onset 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 Distribution Distribution 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 topographical elements 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 leaf burn

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 leaf burn

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