New corn diseases in the 1960’s

​Race T of Helminthosporium maydis, cause of the 1969-1971 southern corn leaf blight epidemic got the attention of everyone involve in corn.  This was not the first ‘new’ corn disease that seemingly suddenly emerged.  Probably occurring in isolated areas of isolated fields these diseases may have been present for some time before gaining enough attention for specialists to make note and study. Several gained notoriety in the 1960’s.

Eyespot, caused by Kabatiella zeae, was first identified in Japan in 1956 and later in northern corn areas of USA, and in Argentina, Brazil, Europe, China, India, and New Zealand.  The fungus is only known to infect corn, overwinters on corn debris.  It is not known to infect seed but apparently it is associated with distribution via seed at least one case (https://www.cabi.org/isc/datasheet/29297).  The disease severity is related to use of susceptible hosts.  In the USA, the wide use of inbreds W64A was associated with the damage from this disease in areas with high humidity and cool summers.  Leaf lesions are found on other genotypes but spread and damage is limited on more resistant genotypes.

Maize dwarf mosaic disease, caused by several strains of a virus named MDMV, gained it earliest attention in the 1960’s as one of its overwintering hosts, Johnson Grass, gained prominence in south central US corn belt.  An aphid vector feeding on this alternate host carried the virus to young corn plants, causing significant damage.  The aphids also would spread the virus to sweet corn planted later in northern corn areas.  Most dent and sweet corn hybrids were (and are) susceptible.  Damage is increased when the plants are infected with another virus as well.  Maize Chlorotic Dwarf Virus (MCDV) was identified in the late 1960’s to also infect Johnson Grass.  MCDV transmitted by a leafhopper (Graminella nigrifrons) to corn also infected with MDMV in young plants can result in 100% yield loss in susceptible, infected plants.  Better control of Johnson Grass, insect control and use of resistant hybrids has led to less damage from this disease.

Goss’s bacterial wilt (caused by Clavibactermichiganensissubspecies nebraskense) was first identified in Nebraska in 1969, it was especially associated with physical damage to corn leaves such as from hail.  It was most notable in the early 1970’s on hybrids using susceptible inbreds such as A632.  Switching to more resistance hybrids greatly reduced the incidence of the disease.  It emerged with minor epidemics outside of Nebraska in recent years again associated with susceptible inbreds.

These fungal, virus and bacterial diseases emerged with significance in the USA during the 60’s.  Continuance of this pattern after that period will be the focus of the next blog.