Maize dwarf mosaic of maize in the United States is caused by either of two major viruses: Maize dwarf mosaic virus (MDMV), or Sugarcane mosaic virus (SCMV). This is one of the most widely distributed and important corn virus diseases in the U.S. It has been reported in 37 states located in most regions of the country, including Hawaii. Major losses due to MDMV or SCMV have not been reported for several years, but the viruses remain ubiquitous in distribution. It is transmitted by several species of aphids. Experimentally, losses up to 54% and 40% have been determined in inoculated susceptible dent and sweet corn hybrids, respectively.
MDMV and SCMV Symptoms
|Fig. 1. Sweet corn plant showing typical symptoms MDMV in Ohio.|
Infected plants show mosaic patches of light and dark green color on leaves (Fig. 1). Plants show chlorosis and stunting after infection is established. Initial leaf symptoms of susceptible plants are pale chlorotic spots and short streaks ranging from 0.5 to 1.5 mm in width. As the chlorotic spots become more abundant, a typical yellowing mosaic or mottle pattern is more evident. Flecks and rings may be observed on newly formed leaves.
Infected plants are often stunted (Fig. 2), barren and predisposed to root rot and other secondary infections. In late infections, disease symptoms are found only on the upper leaves of mature plants. In tolerant or resistant hybrids, short chlorotic spots or streaks develop on only one or two leaves. Disease symptoms may vary according to the hybrid and the stage of the plant at the time of infection.
MDMV overwinters mainly in rhizomes of Johnsongrass (Sorghum halepense), but its hosts includes 82 genera and over 193 species of the grass family (Gramineae or Poaceae). Early in the growing season, MDMV is non-persistently transmitted by at least 15 aphid species. The aphids acquire the virus by feeding on an infected plant. One minute of feeding is more than enough to acquire and transmit the virus. Aphids can retain the virus for up to two hours.
Vector aphid genera include: Acyrthosiphon, Aphis, Brevicoryne, Macrosiphum, Myzus, Rhopalosiphum, Schizaphis, Sitobion, and Therioaphids. The corn leaf aphid (Ropalosiphum maidis) and the green peach aphid (Myzus persicae) are the most common vectors of MDMV in Ohio (Fig. 3). Uredospores of corn rust (Puccinia sorghi) originated from infected plants can also transmit the disease. Seed transmission of the virus is very unlikely (less than 0.5%). SCMV (formerly known as MDMV-B or the “non-Johnsongrass-infecting strain of MDMV”) is also aphid transmitted, is found in northern Ohio and in the United States, and causes symptoms similar to MDMV, but its major overwintering host is not known.
|Fig. 2. Symptoms of MDMV in corn. (A) Close-up of a corn leaf showing the typical mosaic pattern of MDMV, including chlorotic spots, yellow mosaic, and streaks on a susceptible dent corn plant. (B) Light yellowing and moderate stunting of a plant infected with MDMV in Ohio.|
Several related and some unrelated viruses cause similar mosaic symptoms in corn. Identification of the virus based solely on plant symptoms is difficult. Samples of symptomatic leaves should be tested in a laboratory to confirm the presence of the virus. Proper identification of the virus is usually done by ELISA, or reverse-transcriptase Polymerase chain reaction (PCR), and may be confirmed by electron microscopy. Bioassays by virus inoculation of a series of plants, including maize, sorghum, and wheat can also be performed to differentiate between MDMV, SCMV, and related viruses. For the bioassay, seven-day-old plants are rub-inoculated every other day 3 times. Symptoms on susceptible plants should appear around 7 days after rubbing (Table 1).
- The use of resistant hybrids should be the first option to prevent the Dent and sweet corn hybrids with adequate levels of resistance to MDMV and SCMV are available. Look for them on the fact sheets or promotional tables of your favorite seed company.
- Apply herbicide to eradicate Johnsongrass within and adjacent to maize fields early in the season.
- CIMMYT, Maize Doctor. http://maizedoctor.cimmyt.org/index.php?option=com_content&task=view&id=238
- University of Florida IFAS Extension Factsheet: SSAGR209. http://edis.ifas.ufl.edu/pdffiles/SC/SC00900.pdf
- University of Illinois, Maize Dwarf Mosaic Virus. http://cropdisease.cropsci.illinois.edu/corn/maizedwarfmosaicvirus.html
- Cornell University, Department of Plant Pathology. http://vegetablemdonline.ppath.cornell.edu/NewsArticles/CornVirus/Sweet%20Corn%20Virus%202001.pdf
Laboratories testing MSMV and SCMV
- Agdia: https://orders.agdia.com/InventoryD.asp?loc=IN&collection=TSE%20MDMV&attribute_Size=not%20applicable
Fig. 3. Maize dwarf mosaic virus (MDMV) cycle. (A) Johnsongrass showing symptoms of MDMV, (B) a close-up of corn aphids, and (C) a corn tassel severely infested with the corn leaf aphid.