Fungal species names

​Most people working with corn concentrate on the name of the disease while leaving the nomenclature of the pathogen up to specialists.  It is confusing when one sees pathogen names change but the disease name remains the same. Name changes usually occur as taxonomists attempt to clarify the relationships among species of fungi. Further investigations often discern variants within a species especially affecting their pathogenicity.
 
Many corn leaf pathogenic fungi are part of a group of fungi called Ascomycetes.  The sexual reproduction state of these fungi occurs after the fusion of individuals mating types, forming diploid nuclei which undergo miosis and produce haploid spores within a sac called an ascus. These spores germinate to produced hyphae that asexually reproduce by spores called conidia.  The ascus stage is rarely found in nature because the prominent pathogenic stage is linked to the asexually produced spores. The formal ‘rule’ for fungal taxonomists is to name the fungus by the sexual name.  Consequently, the formal name of the Northern leaf blight is Setosphaeria turcicabecause the sexual stage belongs to the genus Setosphaeria. However, the fungus was mostly known as Helminthosporium turcicumuntil more recent research distinguished it from others previously named Helminthosporium such as those causing southern corn leaf blight. Now the most widely used name is Exserohilum turcicumbecause of the shape of the asexual conidia.  Compendium of Corn Diseases fourth edition lists the causal organism of northern leaf blight as “Setosphaeria turcica(syns. Bipolaris turcica, Drechslera turcica, Exserohilum turcicum, Helminthosporium turcicum and Trichometashaeria turcica). Only one fungus species but with different names.
 
Southern leaf blight fungus currently commonly named as Bipolaris maydis, has a similar nomenclature record. Compendium of Corn Diseases, fourth edition, lists the pathogen as Cochliobolus heterostrophus(syns: Bipolaris maydis, Drechslera maydis, Helminthosporium maydisand Ophiobolus heterostrophus).  Again, one fungus but different names as sexual stage as recognized and relationships with other fungi is acknowledged.
 
Another aspect of pathogen names is distinction of variants that result in different pathogenesis. Some of these are probably simple genetic differences within a species and are often related to specific genes for resistance within corn varieties.  Race 1 of E. turcicumovercomes the Ht1 gene for resistance in some corn varieties.  Race T of B. maydisovercomes corn varieties with the T cytoplasm for male sterility.  In some cases, the pathogen variant is described as a subspecies such as the bacteria causing Goss Wilt (Clavibacter michiganensissubsp. nebraskense). Bacterial leaf streak cause is Xanthomonas vasicola pv. vasculorum.
 
Taxonomists attempt to name pathogens according to the latest research.  Pathologist attempt to use the most meaningful terminology to communicate information about the disease. Growers need to concentrate on the dynamics of the disease and try not to be confused with the pathogen names.