Chapter 3 Corn/ Maize diseases

1. Chapter 3 Corn/ Maize diseases

2. Introduction Many corn fields develop disease problems every year that affect yield and quality of the grain crop. As history has shown repeatedly, corn diseases can and do periodically cause significant yield losses in patterns that are difficult to predict in advance. Corn diseases typically cause minimal damage over the entire state, however, some acreage suffers significant disease damage each year.

3. Fortunately, corn has effective genetic resistance to many of the important diseases, however, numerous challenges remain in management of corn diseases. This includes the seed and seedling diseases, leaf diseases, stalk diseases, and ear rots. Introduction

4. The diseases of corn may be classified as parasitic and nonparasitic. Most parasitic (infectious) diseases of corn are caused by fungi, a few by bacteria, and a few by viruses. Nonparasitic disorders result from unfavorable climatic and soil conditions. Deficiencies of nitrogen, phosphorus, or potassium cause some of the most frequently observed nonparasitic disorders of corn. Occasionally, corn may suffer from lack of essential minor elements in the soil. Introduction

5. The major corn diseases can be grouped into four categories: leaf blights, stalk rots, ear rots, and viral diseases. Introduction

6. Ear and kernel rots decrease yield, quality, and feeding value of the grain. Stalk diseases not only lower yield and quality, but also make harvesting difficult. When leaves are damaged by disease, the production of carbohydrates to be stored in the grain is decreased; immature, chaffy ears are the result. Introduction

7. Introduction EAR ROTS DIPLODIA EAR ROT FUSARIUM KERNEL ROT GIBBERELLA EAR ROT GRAY EAR ROT LEAF DISEASES NORTHERN CORN LEAF BLIGHT SOUTHERN CORN LEAF BLIGHT COMMON CORN RUST DOWNEY MILDEW OR CRAZY TOP COMMON SMUT GRAY LEAF SPOT CURVULARIA LEAF SPOT STALK ROTS AND ROOT ROTS DIPLODIA STALK ROT GIBBERELLA STALK ROT CHARCOAL ROT PYTHIUM STALK ROT PYTHIUM ROOT ROT BACTERIAL WILT VIRUS DISEASES MAIZE DWARF MOSAIC MAIZE CHLOROTIC DWARF

8. 3-1 LEAF BLIGHTS Northern Corn Leaf Blight

9. LEAF BLIGHTS The most common are gray leaf spot, Stewart's bacterial leaf blight, and northern corn leaf blight. These diseases can be found in almost any field, depending on the year and susceptibility of the hybrid planted. Some leaf-blight diseases are most often found associated with continuous corn, especially in reduced-tillage, continuous corn fields. These are anthracnose, gray leaf spot, eyespot, and northern leaf spot.

10. All leaf blight diseases cause loss of green leaf tissue, resulting in fewer kernels and lightweight grain. Plants may be predisposed to stalk-rot diseases when leaf damage is severe. LEAF BLIGHTS

11. The amount of yield loss is usually related to the time when the plant's upper leaves become infected. The most severe yield loss occurs when the upper leaves, the ear leaf, and those above the ear, become infected at or soon after tasseling. Yield losses will be minimal if disease does not occur on these leaves until six to eight weeks after tasseling. LEAF BLIGHTS

12. Leaf blight diseases are most effectively controlled by selecting hybrids with genetic resistance. Contact your seed dealer for information on hybrids with resistance to gray leaf spot, Stewart's bacterial leaf blight and other leaf diseases important in your area. A one-to two-year rotation away from corn and destruction of old corn residues by tillage may be helpful if susceptible hybrids must be grown. Fungicides are also available for control of leaf diseases, but are economically viable only under severe disease pressure. LEAF BLIGHTS

13. Significance Northern corn leaf blight (NCLB), caused by the fungus Exserohilum turcicum previously called Helmithosporium turcicum, can cause yield losses in humid areas where corn is grown. NCLB can occur throughout the state but usually does not appear in fields before silking.

14. Significance • This disease rarely causes significant yield losses during dry weather, but during wet weather it may result in losses of over 30% if established on the upper leaves of the plant by the silking stage of development. If leaf damage is only moderate or is delayed until 6 weeks after silking, yield losses are minimal.

15. Develop on lower leaves first and progress up the plant under favorable weather conditions ( temperatures of 20－25℃ and high relative humidity). Several types of lesions may occur on leaves and husks. The type of lesion present is dependent on host resistance genes. Symptoms

16. The typical symptoms seen on a susceptible host are long elliptical spots up to 15 cm in length. Spots are grayish-green to tan in color. Spore produced in the lesions are olive-green to black and may be produced in concentric rings giving the spot a target like appearance. Spores from the primary lesions reinfect the host producing secondary cycles of the disease. Symptoms

18. Lesions produced on hybrids with polygenic (quantitative) resistance are long and narrow resembling those of Stewart's wilt. These lesions may extend the entire length of the leaf. Fewer lesions are produced on these hybrids and their size, in term of surface area affected, is less than on susceptible hybrids. Lesions produced on hybrids with monogenic resistance are characterized as small necrotic spots that are surrounded by a chlorotic halo. Spore is greatly reduced or absent in these lesions. Symptoms

19. NCLB is caused by the fungus Exserohilum turcicum, teleomorph Setosphaeria turcica. Both the common name and causal organism have several synonyms. Conidiospores of E. turcicum have a slightly protruding hilum which aids in identification of the fungus. Pathogen

20. Host of E. turcicum include corn, sorghum, Sudangrass, Johnsongrass, gamagrass (鸭茅状摩擦禾，产于美洲，饲料草) and teosinte(墨西哥类蜀黍). Exserohilum turcicum is divided into 5 races and infection of hosts from different genera and species is dependent on the race. In addition, two biotypes have been identified from maize. Pathogen

23. The fungus causing NCLB overwinters as mycelia and conidia on corn residues left on the soil surface. The conidia are transformed into thick-walled resting spores called chlamydospores. During warm, moist weather in early summer, new conidia are produced on the old corn residue, and the conidia are carried by the wind or rain to lower leaves of young corn plants. Disease cycle

24. Infection by germinating conidia occurs when free water is present on the leaf surface for 6-18 hours and the temperature is between 18-27℃. Lesions develop within 7-12 days. Secondary spread within fields occurs by conidia produced on the leaf tissues. The conidiospores germinate and penetrate leaf tissue directly or through stomata. Infection occurs when free moisture is present on the leaf surface. Disease cycle

25. 1. Resistant Hybrids Planting resistant hybrids is the most effective method for control of NCLB. Hybrids are available with both monogenic and polygenic resistance and should be used wheneverpossible. Control

26. At least two types of resistance to NCLB are known: small lesion size and few lesions (controlled by multiple genes) and chlorotic lesions with little or no sporulation and a yellowish halo (controlled by a single gene). Thus, even where resistant hybrids are planted, leaves may show some flecking or small lesions, but no economic damage occurs. Resistant hybrids should be planted in all commercial dent corn production fields. Control

27. 2. Residue Management Severe outbreaks of northern corn leaf blight are sporadic but the potential for a major outbreak is present where corn is being continuously cropped in a conservation tillage system. Since the fungus that causes the disease survives between seasons on crop residue, reduction of the residue should reduce the amount of inoculum present in the spring. Control

28. A one- to two-year rotation away from corn and destruction of old corn residues by tillage may be helpful in controlling the disease if susceptible hybrids must be grown. In this case a grower may find it useful to rotate to an unrelated (non-host) crop such as soybeans. Control

29. 3. Fungicide Application Several fungicides are labeled for control of northern corn leaf blight. Mancozeb (代森锰锌) and propiconazole（丙环唑） are labeled for field corn, popcorn and sweet corn. Fungicide sprays are recommended only for fresh market sweet corn and hybrid seed production fields. The spray schedule should start when the first lesions appear on the leaf below the ear. Control

31. 小结 发生概况：分布？ 危害？产量损失轻病害识别：为害？发病时期？症状特点？ 病原：分类地位、形态特点、生理分化 病害发生发展规律：越冬、传播、入侵； 发病及其影响因素：品种抗性（抗病类型与抗病机制）、气候、栽培管理 综合防治：抗病品种；改进栽培技术，减少菌源；药剂防治（只适用于留种田）

32. 3-2 STALK ROT

33. Stalk rots are the most important and common diseases of corn. Annual losses are estimated at 5 to 10 percent. There are several stalk-rot diseases, but Gibberella stalk rot and Anthracnose stalk rot currently are the most prevalent. Both are fungal diseases that result in premature ripening, chaffy ears. The interior of the stalk becomes rotted, tissues break down, and the stalk is easily broken. STALK ROT

34. Anthracnose stalk rot is usually associated with continuous corn and is recognized by the blackening of the outer surface of the stalk late in the season. Stalks with Gibberella stalk rot can be found in nearly any field. Affected stalks often have pink to reddish discolored internal tissues. STALK ROT

35. Control of stalk rot diseases is based on reducing plant stress from factors such as lack of moisture, leaf diseases, insect injury, and nutritional stress. Select hybrids with good stand ability and resistance to leaf blight diseases. Adjust soil fertility to recommendations based on a soil test. Avoid excessive rates of nitrogen in relation to potassium. Follow a one- to three-year rotation away from corn. Soybeans, forage legumes, and small grains are acceptable in the rotation. The longer the rotation away from corn the better. STALK ROT

36. Plant at populations recommended for the hybrid grown. Overplanting leads to increased moisture, light and nutrient competition, and more plant stress. Harvest fields with the greatest level of rotted stalks first to avoid lost ears on lodged plants. Control insects, particularly root worms and stalk borer. Insects cause injuries to plant roots and stalks permitting stalk rot fungi to enter the plant. STALK ROT

37. 小结 发生概况：分布？ 危害？产量损失轻病害识别：为害？发病时期？症状特点？ 病原：有性态： 无性态： 病害发生发展规律：越冬、传播、入侵、发病及其影响因素(寄主抗病性、气候、栽培管理、预测预报)? 综合防治：抗病品种；种子处理；田间水肥管理；化学防治；生物防治

38. 3-3 Ear Rot

39. Gibberella, Fusarium, and Diplodia ear rot diseases occur in the world, but Gibberella ear rot is the most important. The Gibberella ear rot fungus is the same fungus that causes Gibberella stalk-rot disease. Gibberella enters from the silk end of the ear when cool, wet weather persists for several weeks through late silking of the crop. Ear Rot

40. The occurrence of a whitish to pinkish mold on the ear tip is diagnostic, but extensive mold growth may not occur. On shelled grain, the symptoms may be seen as a pinkish coloration in some of the kernels. Even though extensive rotting does not always occur, the disease is serious because the fungus frequently produces toxins that makes the corn unfit for feeding. Hogs are particularly sensitive to the toxins produced in moldy grain and may refuse to eat it even when hungry. Some corn hybrids are less susceptible than others to Gibberella ear rot. Ear Rot

41. Diplodia ear rot appears to be more common in continuous corn under reduced tillage. Ears affected by Diplodia are covered with a thick mat of white fungal growth. Fusarium ear rot is common, but only individual kernels are affected on ears. Ear Rot

42. 小结 发生概况：分布？ 危害？产量损失轻病害识别：为害？发病时期？症状特点？ 病原：有性态： 无性态： 病害发生发展规律：越冬、传播、入侵、发病及其影响因素(寄主抗病性、气候、栽培管理、预测预报)? 综合防治：抗病品种；种子处理；田间水肥管理；化学防治；生物防治

43. 3-4 Virus Diseases

44. Maize dwarf mosaic and maize chlorotic dwarf, are potentially destructive diseases where johnsongrass is established. The two viruses that cause these diseases are able to survive in this perennial weed grass. Aphids and leafhoppers feeding on johnsongrass in the spring pick up the virus and inoculate nearby corn. Control is achieved by planting resistant or tolerant hybrids. Efforts also should be made to eradicate johnsongrass. Virus Diseases

45. 3-5 CORN SMUT

46. CORN SMUT Corn smut occurs wherever corn is grown. It is more prevalent, however, in warm and moderately dry areas. Corn smut damages plants and reduces yields by forming galls on the aboveground parts of plants, including ears, tassels, stalks, and leaves. The number, size, and location of smut galls on the plant affect the amount of yield loss.

47. Galls on the ear usually destroy it to a large extent, whereas large galls above the ear cause much greater reduction in yield than galls below the ear. Losses from corn smut range from a trace up to 10% or more in localized areas. Some individual fields of sweet corn may show losses approaching 100% from corn smut. Generally, however, over large areas and with the use of resistant varieties, losses in grain yields average about 2%. CORN SMUT

48. When young corn seedlings are infected, minute galls form on the leaves and stems, and the seedling may remain stunted or may be killed. On older plants, infections occur on the young, actively growing tissues of axillary buds, individual flowers of the ear and tassel, leaves, and stalks. Infected areas are permeated by the fungus mycelium, which stimulates the host cells to divide and enlarge, thus forming galls. Symptoms

49. Galls are first covered with a greenish white membrane. Later, as the galls mature, they reach a size from 1 to 15 centimeters in diameter, and their interior darkens and turns into a mass of powdery, dark olive-brown spores. The silvery gray membrane then ruptures and exposes the millions of sooty teliospores, which are released into the air. Galls on leaves frequently remain very small (about 1-2 cm in diameter), hard, dry, and do not rupture. Symptoms