Corn ear smut

​Multiple stresses during the 2020 corn growing season likely produced environments encouraging ear smut. The biology of the fungus and the corn plant are closely related to the occurrence of corn ear smut.
 
Ustilago maydis is the fungal pathogen of corn causing common smut disease.  Its biology includes a fungal version of sex, a portion of which involves corn.  The fungus produces thick walled spores called teliospores that overwinter in the soil for many years.  Teliospores have diploid nuclei (2 sets of chromosomes).  When moistened, the teliospore nuclei undergo meiosis, resulting in 4 individual cells, each with haploid (monoploid) with one set of chromosomes.  These cells are called sporidia.  Sporidia that land on corn tissue may germinate but cannot enter the corn plant until they are united with sporidia of a different mating type.  After the two mating types fuse, the two nuclei stay separate but the fungus forms special structures to enter the corn plant cells.  Often it is through wounds but also directly through corn cells as they are elongating, as the combination of host cells and fungus form galls.  Within this mass the fungal cells, having two monoploid nuclei per cell, now fuse the nuclei, forming the diploid single nucleus stage of the fungus life cycle.  These diploid cells form the thick cell walls as they become teliospores to be released as the corn plants are harvested.
 
Smut galls may form on leaves, tassels, aborted ear shoots at the leaf nodes and the main ear shoots. The ear shoots are the most damaging to the grain yield, of course.
 
Corn silks that are not pollinated soon after emergence are vulnerable to infection by Ustilago maydis.  The fungus grows down the silk channel infecting the cells surrounding the ovule. If pollen successfully reaches the ovule before the fungus, the smut fungus is inhibited and no gall will form.  Subsequently, the timing of pollen release and silk emergence becomes a significant factor in formation of galls on corn ears.  Rainy days may encourage silk emergence, but delay pollen release or drought may delay silk emergence until there is limited pollen available.  Complete absence of kernels but ears full of smut are a sign that the ear failed to receive pollen. These factors contribute also to the occurrence of smut at the tips of ears as these are from the last silks to emerge, perhaps when no pollen was available.