Stewart's wilt is a bacterial disease caused by Pantoea stewartii. The disease affects only corn. It has been known since 1889 in New York. It is common in the eastern U.S. and in parts of Central America, Eastern Europe, Russia, and China. Corn plants can be infected any time during the season, but losses are greatest when the plants are infected while young.
Symptoms: Plants may be killed any time, while seedlings, after tasseling or even later. Plants usually wilt rapidly after infection. When plants are infected young, they are usually stunted severely, wilted, and have long leaf streaks with wavy irregular edges. The streaks are first watersoaked or yellow, and later turn brown. They run parallel with the veins and may run the entire length of the leaf. The streaks can be 1/16" to 1/2" or more in width. When plants are older at the time of infection, the streaks are much smaller and are more numerous. They give the leaves a dead, "fired" appearance. The lower leaves are usually affected the worse. Leaves may wilt. Tassels may grow slowly, and reach maturity late, be bleached in appearance, or may die. Cavities form in the center of the stalks near the soil line.
Vector: This disease is spread from plant to plant by flea beetles (Chaetonema pulicaria). It is also spread by the 12-spotted cucumber beetle, the larvae of the seed-corn maggot, wheat wire-worm, and may beetle. The flea beetle is most important because the bacteria spend the winter in flea beetles.
Identification of disease: Cut through the lower portion of the stem of a plant that is wilted or dying, but has a green stem. Small droplets of yellowish bacterial ooze may appear at the cut surface. Touch a knife blade flat to the cut surface and slowly pull it away. Fine threads of bacterial ooze up to 1/4" long may be drawn from the cut surface.
Similar Diseases: The leaf streaks of plants infected while mature are similar to those caused by nutrient deficiencies, drought, or insect damage. Damage may also look like Northern Corn Leaf Blight, caused by a fungus. In NCLB, the spots are up to 6" long, have straight edges, and may have grayish-black fungal growth inside. In Stewart's Wilt, flea beetles are usually present, the streaks are of a similar shape to one another and are separate from one another.
Prevention: Disease is worse in seasons following a warm winter, which allows the flea beetles to survive the winter better. When the sum of the average temperatures of December, January, and February is above 100o F, disease levels can be expected to be high. When the sum is below 90 oF, disease tends to be low. Intermediate levels of disease can be expected when the sum of the temperatures is 90oF to100oF. Use disease free seed. Resistant varieties are available and should be used. High levels of ammonium nitrogen and phosphorous increase susceptibility to the disease, high levels of calcium and potassium tend to decrease susceptibility. Control flea beetles as soon as they emerge. See current recommendations for chemical control measures.
Maize dwarf mosaic is caused by the maize dwarf mosaic virus, and can affect corn, and other grasses, especially Johnson grass. It causes poor yield in affected plants.
Symptoms: This disease begins in the pre-tassel stage. Early symptoms include a fine mottling of light and dark green on the bases of the youngest leaves. This often progresses to narrow light green or yellowish streaks along the veins. Later the mosaic may appear as dark green "islands" on a yellow background. Internodes on upper portion of the plant are sometimes shortened, giving the plant a "feather duster" appearance. All leaves that grow after infection will usually have symptoms. As plants grow, the mottling may disappear, and leaves may become yellowish green, or may be blotched or streaked with red. Severely affected plants may not set seed.
Vector: The virus is transmitted from one plant to another by more than 20 aphid species, including the Green Peach Aphid (Myzus persicae), the Corn Leaf Aphid (Rhopalosiphum maidis), and the Greenbug (Schizaphis graminum). The virus is transmitted in a non-persistent manner, which means that the aphid acquires the virus from an infected plant after feeding for only a few seconds to a minute, but it is able to infect healthy plants for only a few days to a week.
Prevention: Resistant varieties are available, and are considered to be the most effective line of defense against this disease. Eradicate Johnson grass and other overwintering hosts. Plant early to avoid aphids. Locate later plantings away from peach trees to avoid the most common aphid vector. See current recommendations for chemical control of aphids. Because the virus is transmitted in a non-persistent manner, the use of insecticides may not help control the virus. Mineral oil sprays have not been useful in preventing transmission of this virus in the field.
Common Rust is caused by the fungus Puccinia sorghi, which causes disease only in corn. It was first described in the United States since 1832, and is now found worldwide. It causes a decrease in yield.
Symptoms: Circular to elongated, cinnamon brown pustules can be found on both sides of the leaves as well as on tassels, husks, and necks. As the pustules mature, the surface of the plant breaks open and the cinnamon brown powdery spores dust out. When the pustules are abundant, the leaves and leaf sheaths may turn yellow and die. Late in the season, the pustules may become darker as overwintering spores are produced in them.
Identification of disease: The pustules are very distinct.
Prevention: Some resistant and tolerant varieties are available for this disease. Plow debris under soon after harvest. A crop rotation of at least one year is recommended. See current recommendations for chemical control measures.
Common Smut is caused by the fungus Ustilago maydis, and causes disease only on corn. It is more destructive on sweet corn than on field corn. It is found worldwide, and is favored by dry weather, 80o to 95o F. It causes a decrease in yield and quality. In some parts of the world, the young galls are considered an edible delicacy.
Symptoms: Galls are usually formed on ears, but may also form on leaves, stems, and tassels. This disease is more severe on young, actively growing plants. When young, galls are glossy greenish-gray to silvery-gray in color with a firm to spongy texture. As they mature, they become darker gray and spongier. When the are fully mature, they break open easily. Inside the mature galls are millions of black dusty spores. Galls can be up to several inches in diameter. Galls on leaves usually remain small and hard, and do not break open to release spores.
Identification of disease: Unmistakable due to appearance of galls.
Prevention: Some resistant varieties are available for this disease. A 6 to 8-year crop rotation may help prevent the disease, but the spores are carried long distances by wind. Avoid excess nitrogen levels. Avoid injuring plants. In home gardens, remove and burn galls before they break open and release spores. See current recommendations for chemical control measures.
By Pamela S. Mercure, IPM Program Assistant, University of Connecticut
Sherf, A.F. and A. A. MacNab. 1986. Vegetable Diseases and Their Control. John Wiley and Sons, New York.
Shurtleff, M.C. 1980. Compendium of Corn Diseases. Second Edition. APS press, St. Paul, MN.
Straub, R.W, and B. Emmett. 1992. Pests of Monocotyledon Crops. Chapter 7 in Vegetable Crop Pests, R.G. McKinlay, ed. CRC Press, Inc, Boca Raton, FL.
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