The most widely distributed and important virus diseases of corn in Kentucky are maize dwarf mosaic and maize chlorotic dwarf. Maize dwarf mosaic is caused by Maize Dwarf Mosaic Virus (MDMV), while maize chlorotic dwarf is caused by Maize Chlorotic Dwarf Virus (MCDV). These two viruses are usually found together and the disease they cause is often referred to as the "virus complex" of corn. Both field corn and sweet corn are susceptible to the virus complex.
MDMV and MCDV overwinter in rhizomes of johnsongrass and are spread (vectored) to corn by various insects. These viruses can cause sporadic but serious yield loss, depending on the corn hybrid planted, time of infection, amount of johnsongrass, and other factors.
MDMV infects over 250 species of grasses, including both annual and perennial species. Sorghum is also an important host of MDMV. The principal hosts of MCDV are corn, johnsongrass, and sorghum, the last two being symptomless hosts.
Symptoms of the corn virus complex can be quite variable under actual field conditions, making diagnosis a problem. Similar symptoms may be caused by other factors such as herbicides, nutrient deficiencies, genetic aberrations, or an adverse environment. Disease diagnosis is even more difficult when plants are infected by both viruses, or when several strains of each virus are present in the field. Consequently, a combination of careful field observations and laboratory tests is the most reliable way to diagnose virus diseases of corn.
Corn plants infected with MDMV often exhibit irregular, light and dark green mottled or mosaic patterns in the leaves, particularly at the base of the youngest leaves. Symptoms of maize dwarf mosaic may be present on all leaves, leaf sheaths, and husks that develop following infection. In hot weather, the mosaic symptoms may disappear, with young leaves exhibiting only a general yellowing. Infected plants are sometimes stunted, and grain production can be reduced. The symptoms and effects of maize dwarf mosaic are usually most severe in and around areas where rhizome johnsongrass is present.
Corn plants infected with MCDV often exhibit shortening of the upper internodes, yellowing of the youngest leaves, reddening of leaf margins, and, occasionally, leaf uttering. Such symptoms may also be caused by factors other than MCDV. The only diagnostic symptom of maize chlorotic dwarf is a chlorosis of tertiary veins (veinbanding). However, recognition of veinbanding takes considerable training, and definitive diagnosis is best accompanied by laboratory analysis. Severely affected plants may produce little or no grain. As with maize dwarf mosaic, symptoms of maize chlorotic dwarf in corn are typically most severe in and around areas of rhizome johnsongrass.
Corn plants infected by both viruses can have a range of symptoms, but affected plants often exhibit severe stunting and reddening and may lack ears.
In johnsongrass, MDMV produces a very faint mosaic symptom which may only be evident at the base of the youngest leaf blades. MCDV produces no symptoms in johnsongrass.
Both MDMV and MCDV infect johnsongrass and can survive between corn crops within the underground rhizomes of this weed. Infected johnsongrass rhizomes sprout and produce infected shoots in the spring or early summer. The viruses are then transmitted to corn plants by the feeding activity of several insect vectors (listed below). As the viruses multiply within newly developing tissues, symptoms eventually develop in a susceptible corn plant. Infected corn plants can then serve as a source of virus for new infections to both corn and seedling johnsongrass.
MDMV is transmitted by several aphid species, most commonly the corn leaf aphid, the greenbug, and the green peach aphid. These aphids typically acquire MDMV very quickly during feeding activity, often as quickly as 15 to 60 seconds. MDMV is spread when viruliferous aphids (aphids carrying the virus) fly or are carried by wind to a healthy plant. Viruliferous aphids are capable of infecting a plant after feeding for less than one minute. Outbreaks of maize dwarf mosaic usually are most severe around areas of the field with infestations of infected rhizome johnsongrass. However, some studies have shown that viruliferous aphids may be blown some distance from infected johnsongrass, so the potential exists for outbreaks even in areas of a field with no johnsongrass. MDMV can be seedborne at very low levels, but infected seeds are not considered to be an important source of inoculum in Kentucky.
MCDV is transmitted by several leafhopper species, most commonly the black-faced leafhopper. Usually, several hours of feeding are necessary before leafhoppers acquire or transmit MCDV. Outbreaks of maize chlorotic dwarf typically are limited to areas of the field with infestations of rhizome johnsongrass. MCDV is not known to be seedborne.
Outbreaks of the corn virus complex are not always predictable and will occur only if the crop is susceptible to infection, the viruses and insect vectors are present, and environmental conditions favor virus spread and symptom development. If one of these factors is missing, then little or no disease will develop. However, when these factors come together, unexpected outbreaks of virus diseases can occur.
Several factors can influence development of virus diseases. Corn hybrids differ greatly in their susceptibility or tolerance to the virus complex, so hybrid selection can influence disease development. Weather and other factors that affect the number and activity of insect vectors can also have an impact on the virus complex.
Late-planted corn is more susceptible to the virus complex. Compared to corn planted on time, late-planted corn is at an earlier stage of crop development during periods of peak activity of insect vectors. Earlier infection of corn usually results in more severe symptoms of the virus complex and greater yield loss. Also, studies by UK entomologists have shown that populations of leafhopper vectors can be higher in late-planted corn.
The amount of johnsongrass present in the field can also affect disease development, as fields highly infested with rhizome johnsongrass usually are highly infested with virus. Even in fields where rhizomes have been eradicated, johnsongrass infestations in the fence rows can serve as a source of virus. This "border effect" is particularly important in small fields or long narrow fields.
Serious outbreaks of the virus complex have occurred in some corn fields treated with postemergence grass herbicides. There is some concern that these herbicide applications may occasionally increase the severity of the virus complex. Some research shows increased virus pressure where these herbicides are used. Other research--including our own in 1993--shows no effect of the herbicides.
Like most plant diseases, the corn virus complex is best managed by integrating several control practices. A recommended management program includes using virus-tolerant hybrids, controlling johnsongrass, and avoiding late planting. Insecticides are rarely effective and are not recommended.
A virus-tolerant hybrid of corn is recommended for any field with a significant infestation of rhizome johnsongrass. A number of high-yielding, virus-tolerant hybrids of field corn are available for use on Kentucky farms. With a virus-tolerant hybrid, yield loss will be minimal should environmental conditions favor virus spread and disease development.
Destroying johnsongrass rhizomes in and around a field eliminates the overwintering host of corn viruses. Therefore, controlling johnsongrass greatly reduces the potential for outbreaks of the corn virus complex. See the UK Extension publication "Chemical Control of Weeds in Kentucky Farm Crops" (AGR-6) for current recommendations on cultural and chemical practices to control johnsongrass.
Outbreaks of the corn virus complex are most severe in late-planted crops, This is because of increased susceptibility of corn to the viruses, as well as greater activity of insect vectors in late planted corn. Thus, avoid planting corn late to reduce the risk of the disease.
Insecticides are not recommended for use in corn to control the virus complex. Studies have shown that insecticides usually provide no protection against MDMV outbreaks. One reason for this is that aphids can acquire and transmit MDMV within minutes, well before many insecticides can act. Studies with MCDV occasionally have shown some yield benefit using insecticides. However, equal or better disease control can be obtained using virus-tolerant hybrids of field corn without the extra cost of treating with insecticides. Furthermore, MDMV, which is not controlled by the insecticide, is usually present with MCDV.
Reviews by E. Stromberg, J. Hartman,and D. Hershman are gratefully acknowledged.