corn ear rots

There are many reports of ear rot this year in the US corn production.  Ear rot is caused by several fungal species, each influenced by the environment at the time of infection.  In most cases, infection occurred at pollination time.  Environments that were unfavorable to successful pollination when the silks extended beyond the ear husks but were favorable to a fungus with potential for feeding on the silks is linked to infection.
 
Rainy days influence rapid extension of the silks, exposing them to pollen.  However, release of pollen grains from anthers involves desiccation of the anther sac and dry air influences the movement of corn pollen.  Consequently, rain and extremely high humidity allows the unpollinated silk to be exposed to fungal spores for a prolonged time.  Pollen normally grows rapidly down the silk channel towards the ovule, and the silk collapses behind it, leaving a dry mass of lignified tissue not easily and quickly penetrated by fungi.  Delay in pollination results in a more nutritious substrate for fungi such as Fusarium verticilloides, Gibberella zeae and Stenocarpella(Diplodia) maydis. 
 
Aspergillus flavus and the smut fungus Ustilago maydis are favored by an extreme dry pollination period.  This stress can cause delay of silk extension.  Pollen maturity and release is not as affected by drought and consequently may be mostly spread before the silks are available for pollination. The drier environments favor the spread of these two fungal species.
 
The fungi grow on the surface of the silk to the ovules.  If the infected silk channel does later support pollination or if adjacent ovules are pollenated, the mycelium spreads to adjacent, newly-formed kernels.  Some ears may have infection limited to a few kernels, but others may have mycelial growth covering the whole ear.  
 
The smut fungus often grows within the infected kernel, replacing it with a gall of mycelium and spores.  These spores (teliospores) overwinter in the soil, germinate the following summer to produce basidiospores that infect plants of the next season.  Infection of leaf tissue can result in gall formation on leaves or stems, providing inoculum for infection of ears. 
 
Infection of a few ears in a corn field by any of these fungi is not unusual as each plant’s environment is slightly different.  Genetic susceptibility evaluation is difficult because it involves tendencies for silking and pollen production during stressful conditions.  This is especially confusing in small plots with multiple hybrids.  A hybrid that has silk emergence timing different than most other entries may have more vulnerability to fungal infection.
 
Ear rots are result of host and pathogen biology interacting with environment variability.  Timing is everything concerning corn ear rots.