Corn Stunt Disease

Corn stunt is one of the most economically important diseases affecting corn in the Americas and the Caribbean.

• Corn stunt disease is characterized by severely stunted plants that often produce multiple small ears with loose or missing kernels.
• Yield loss associated with corn stunt disease can be severe – over 70% – and major outbreaks have impacted yields in Brazil and Argentina in recent years.
• Corn stunt disease is less known in the U.S. because outbreaks have been rare and confined to the southernmost parts of the country.

The primary causal organism for corn stunt disease is Spiroplasma kunkelii, a bacterial pathogen commonly referred to as corn stunt spiroplasma (CSS).

• S. kunkelii is transmitted by corn leafhoppers (Dalbulus maidis), which spread the organism from diseased plants to healthy plants.
• This bacterial pathogen is transmitted singly or in combination with maize bushy stunt phytoplasma (MBSP), maize rayado fino virus (MRFV), and/or sugarcane mosaic virus.
• Because of the multiple pathogens involved, corn stunt disease is often referred to as a disease complex.

• Foliar symptoms of corn stunt are small chlorotic stripes that develop at the base of the leaves on young plants. Over time, these chlorotic stripes expand and coalesce, and leaves may develop a reddish or reddish-purple color (Figure 1).
• Infected plants can have shortened internodes. Plants infected early in their development may reach a final height of only 5 feet (1.5 m).

• Ears of infected plants are smaller than normal and do not fill properly. Ears often have blank spaces, and kernels that do develop are loosely attached to the cob, a condition sometimes referred to as “loose tooth ears” (Figure 2).
• Symptoms of corn stunt disease observed in the U.S. are generally less severe than those in South America and the Caribbean due to the timing of infection (Figure 3).
• Outbreaks of corn stunt in the U.S. are largely driven by leafhopper populations moving northward from Mexico, which results in infection later in the growing season.

• The most critical factor in the corn stunt disease pathosystem is not the pathogen, but rather the vector – the movement and proliferation of leafhoppers have been shown to drive corn stunt outbreaks.
• D. maidis has a limited host range, feeding only on corn, its wild relatives in the genus Zea and grasses in the closely related genus Tripsacum.
• Other grass crops, such as wheat and sorghum, can serve as a reservoir for leafhopper populations – giving them a place to persist when no corn is available – but reproduction only occurs on corn.
• Outbreaks of corn stunt in the U.S. are likely driven by leafhopper populations moving up from Mexico, where corn is under continuous cultivation.
• Leafhopper populations can move with prevailing winds, sometimes over long distances. Previous outbreaks of corn stunt disease in southern Florida are believed to be due to leafhopper populations carried in with tropical storms.
• The spread of leafhoppers further north into the U.S. is limited by cold temperatures and lack of secondary hosts to provide a year-round source of food.

• D. maidis begins as an egg and then undergoes five nymphal instars before reaching adulthood.
• Corn leafhoppers do not enter any type of overwinter dormancy; populations survive as active adults.
• Under optimal conditions, corn leafhopper adults can survive without reproducing for up to three months.
• The number of corn leafhopper generations per year can vary greatly based on environmental conditions and host availability.
• Temperature has a significant influence on corn leafhopper development and reproduction.
• In the least favorable environments, a minimum of two generations of corn leafhoppers will develop on a single corn crop.
• In areas with favorable temperatures where corn is grown throughout the year – particularly corn under irrigation – corn leafhoppers can go through more than 12 generations per year.
• In areas with year-round corn production, the corn leafhopper maintains breeding populations throughout the year, which can allow populations to grow very large.

• There are no management tools available to combat the pathogen complex that causes corn stunt disease, so management is primarily focused on preventing infection by managing the insect vector.
• Field experience with managing corn leafhoppers thus far is largely from South America where corn stunt disease is a much more persistent and serious threat.
• Insecticides are commonly used in South America to prevent the spread of corn stunt disease by controlling corn leafhoppers.
• The threshold is for foliar insecticide treatment is the presence of corn leafhoppers. A Corteva Agriscience greenhouse study found that as few as two leafhoppers per plant feeding for just one day was enough to compromise corn yield.
• The key factor for corn leafhopper reproduction is the presence of living corn plants, so cultural control practices are largely focused on eliminating the continuous presence of corn through crop rotation, narrowing the planting window, and controlling volunteers.
• Corn hybrids can differ in their resistance to corn leafhopper feeding. Resistance works via reduced feeding preference (antixenosis) or survival (antibiosis), both of which reduce the duration of insect-plant interaction, which reduces the inoculation efficiency of S. kunkelii.
• In countries such as Brazil, where corn stunt disease is a persistent threat, hybrids are rated for their resistance to leafhoppers. Given the infrequency of corn stunt outbreaks in the U.S., no such ratings for corn hybrids have been developed.
• Corn stunt disease is unlikely to post a threat to corn production in the U.S. Corn Belt. Warm temperatures and a living host are both critical factors for corn leafhoppers, neither of which are available year-round in the Corn Belt.