Head Smut

 ​Less common in the USA corn belt but occasionally regionally destructive is Head Smut.  It is not easily distinguished from common smut, but it has distinctive characters and resistance is more common. This smut-causing fungus is Sphacelotheca reiliana.  This smut shows mostly in the complete destruction of the ear and the tassel of corn.  This species also attacks sorghum, Johnson grass and Teosinte.  The diploid teliospores of this fungus, shed from the smutted ears and tassel remain dormant in the soils for many years but when activated by the root exudates from susceptible corn seedling, the spore undergoes meiosis, producing short hyphae and then haploid basidiospores.  These spores germinate to form hyphae that fuse with compatible haploid hyphae, resulting the cells with two haploid nuclei.  From this point the fungus infects root cells, apparently growing between the cell wall and the cell membrane of the affected cell.
 
Absorbing the nutrients from the infected, live cell, the fungus continues growing cell to cell towards the meristems of the seedling.  From there it continues to advance without killing corn cells as they undergo cell division and eventually producing the ear and tassel.  Disease symptoms from the infection are absent as the fungus absorbs nutrients but does not kill tissue nor penetrate the cell membranes. 
 
After the host plant’s meristematic cells at the upper tip of the stalk and ear change to producing pollen mother cells and ovules, the fungal cells also change.  The two compatible nuclei in each fungal cell fuse to form diploid single cells.  These fungal diploid cells fill and destroy the host cells in the tassel and ears before pollination.  The individual, diploid fungal cells develop thick walls resulting in the first sign of the disease as a loose powder forms instead of ears on the corn plants.  These teliospores are then spread by the wind, the thick wall giving them survival until another host plant is found for repetition of the cycle.
 
Fortunately, high levels of resistance to head smut is more common than susceptibility.  Some of the resistance is thought to be from differences in the root exudates in the initial induction of teliospore germination.  Some comes from the successful recognition of the initial invasion, triggering the response by the host of producing enzymes to keep the pathogen from reaching the meristems. Perhaps it involves the speed of the plants cell divisions, escaping the fungus while infected cells produce anti-fungal chemistry.  Regardless of the method, most corn hybrids are reasonably resistant.
 
The problem historically has been with popular, susceptible hybrids being planted for consecutive years, allowing an increasing concentration of teliospores, until the right environment such as warm, light soils at time of corn seed germination results in noticeable high infection rate.  Switching to more resistant hybrids in future seasons reduces the occurrence of head smut.
 
Biology of corn includes the biology of pathogens.