Stalk rots are one of the most significant set of diseases on corn.  These diseases are insidious, and often growers are unaware of their effects until harvest.  Low levels of stalk rot occur in nearly every corn field, and severity and incidence varies from year to year.  Stalk rots can cause breakage and lodging of stalks and plant death.  In many cases the internal tissues of the stalk, or pith is compromised. 

Initial symptoms of stalk rots are noticed by premature wilting and ear drop.  As the disease progresses stalks senescence rapidly, turning brown to gray.  Stalk rots decay the central pith, which weakens the stalk.  One can envision a healthy stalk being a solid rod of plant tissue.  A rotted stalk is no more than a tube with decaying pith loosely packed inside. 

Several factors determine the effects of stalk rots.  Some corn hybrids are more susceptible to stalk rots than others.  Factors that limit photosynthesis, such as foliar disease, cloud cover, insect damage, and planting density can significantly impact stalk rot.  In addition, cob size, kernel number, and  moisture availability play roles in the severity and incidence of stalk rots.  The aforementioned factors likely impact the carbon budget of corn plants.  When plants grow they produce carbohydrates via photosynthesis.  These carbohydrates are used to support the basic physiological properties of plant cells and organ development.  During ear development carbohydrates are preferably allocated to support grain fill.  If carbohydrate demands during grain fill are not met by photosynthesis (i.e. foliage is damaged or levels of photosynthesis are reduced) then stored carbohydrates, located in the roots and stalks, are utilized for this purpose.  Preferential allocation from roots and stalks limits the performance of cells in these organs, increasing their susceptibility to stalk rotting pathogens.  For this reason many of the stalk rotting pathogens are opportunistic, and caused by whatever organism happens to be in the vicinity when plant senescence occurs.

Research indicates that fertility can have significant impacts on stalk rots.  Although there is a great deal of variability in the literature in regards to the effects of nutrients on specific stalk rots, there is evidence that stalk rot increases if nutrients, particularly nitrogen, are lost during the growing season.

Management of stalk rot should include the use of: 1) hybrids with resistance to stalk rot pathogens and/or high ratings of stalk strength and stay green characteristics; 2) a balanced fertility program based on the environment, population, and hybrid; 3) programs to control foliar diseases and insects; 4) irrigation to avoid drought stress. 

Fields can be scouted for stalk rot before harvest by scouting 10 sites for every 10 acres of the field.  At each site pinch the base of ten plants to assess stalk integrity.  Alternatively, push plants 30-40 degrees from vertical.  If more than 10-15% of tested plants in a field show lodging potential (i.e. easily pinched or fall over using push test) schedule that field for early harvest.

Identification of Stalk Rots

There are a few common stalk rots in Delaware.  Characteristics of these stalk rots are listed below. 

Disease

Organism

External symptoms

Internal symptoms

Environment

Anthracnose stalk rot

Colletotrichum graminicola

Black shiny areas covering outer stalk.  Typically located near base of plant

Lower nodes contain macerated pith.  Tan to brown discoloration

Extended periods of hot, cloudy, humid weather

Diplodia stalk rot

Stenocarpella maydis

Small black structures embedded within lower stalk

Lower nodes contain macerated pith.  Tan to brown discoloration

Dry and warm conditions early in the season, followed by wet weather after silking

Charcoal rot

Macrophomina phaseolina

 

Black pin head or smaller structures peppered throughout macerated pith.  Tend to be located near soil line.

Hot, dry conditions during grain fill

Gibberella stalk rot

Gibberella zeae

Small, round, bluish/black structures attached superficially to nodes.    

Lower nodes contain macerated pith. 

Pink / red discoloration

Dry and warm conditions early in the season, followed by wet weather after silking

Fusarium stalk rot

Fusarium moniliforme and others.

 White, fuzzy growth near nodes

Lower nodes contain macerated pith. 

Pink / purple discoloration

Dry and warm conditions early in the season, followed by wet weather after silking

Bacterial stalk rot

Erwinia chrysanthemi (bacterium)

Sweet, foul odor.  Watersoaking at internodes.

Slimy rot of internal tissue

Hot, wet early in the season