Fungi and stalk rot

​As harvest season approaches, standability in the corn field becomes a primary interest to the grower.  Appearance of the lower stalk is an indication of the plant’s vulnerability to lodging from stalk rot.  Those stalks that remain green 60 days after pollination are likely to have solid interiors of the stalk and will not break before harvest.  These plants did not have premature wilting and thus the cells within the stalk did not withdraw from the rind.  One can judge this by observation alone in plots. Plants with deteriorating stalks show discoloration beginning with a slight pale yellow, and then a darkening yellow that eventually turns to brown. 
 
Various fungi invade the dead interior tissue of such a corn stalk.  Multiple species can be present in such stalks although a few get the most attention because of their distinct features on the stalk.  Colletotrichum graminicola, the fungus associated with anthracnose is distinguished by narrow black streaks on outside of rind but is also present inside the infected stalk.  
 
Gibberella zeae, sexual stage of the fungus Fusarium graminearum, shows distinct black ‘perithecia’ on the outside of the rind near the stalk nodes.  Ease of rubbing these off the stalk is diagnostic of Gibberella stalk rot.  Inside such a stalk usually has a pink discoloration caused by the asexual stage of this fungus that is known as Fusarium graminearum.
 
Another common fungus found in rotted stalks is Diplodia maydis, also known as 
Stenocarpella maydis. This fungus frequently is found on the outside of the stalk nodes as a black sticky structures known as pycnidia.  The fungus in only known to be present on corn and overwinters in the soil.
 
Nearly all corn plants are exposed to another Fusarium species, Fusarium verticillioides (also known as Fusarium moniliforme.  Some think it exists in corn plants mostly as a nearly saprophytic occupant rather than an aggressive pathogen, but it is easily found in nearly all rotted stalks. 
 
There are several other fungi that have been isolated from deteriorated stalks, but we like to name a stalk rot as caused by the most easily identified.  Ultimately, however, the actual cause is the physiology of that plant.  That plant had insufficient carbohydrate to complete the fill of grain and maintain root life.  The plant wilted, cells in stalk die, withdraw from rind (reducing strength by 1/3) and lose resistance to invading fungi.  It is most important to analyze the cause of this lack of the photosynthetic stress and translocation balance in these rotted plants