Disease Management

Paul Vincelli

Diseases of crops can occur whenever Crop rotation is thus one of the most able, disease resistance should be the a disease-causing agent is in contact important disease control practices for foundation for economical disease with a susceptible host plant in an envi- corn production worldwide. control. ronment that is favorable for disease de- Pathogens that are not as effec- No single corn hybrid is resistant velopment. These three fundamental tively controlled by crop rotation in- to all diseases present in Kentucky. ingredients are necessary for a disease clude those that do not survive in the Furthermore, the importance and to develop and are often referred to production field itself. For example, prevalence of corn diseases vary from as the disease triangle (Figure 1). rust fungi that attack corn overwin- one farm to the next and from one Understanding this fundamental ter south of Kentucky and are blown year to the next. These facts can com- relationship helps us understand dis- into our corn fields each season on plicate the hybrid selection process. ease management since all disease wind currents. Pathogens that attack Nevertheless, an informed decision management practices presented in a wide range of field crops are also less can be made by selecting hybrids with this publication affect one or more effectively controlled through rota- resistance to the diseases most likely sides of the disease triangle. For ex- tion. For example, the charcoal rot to be a problem. Resistance to other ample, planting a hybrid with some pathogen can attack corn, soybean, diseases should be considered on a sec- resistance to gray leaf spot targets the and grain sorghum and is not con- ondary basis. host side of the disease triangle. Crop trolled through a corn/soybean rota- When selecting a hybrid, the pro- rotation helps to starve a pathogen (dis- tion. Likewise, pathogens that persist ducer should recognize that there are ease-causing agent) by depriving it of indefinitely in agricultural soils are different levels of disease resistance. its food source; this affects the patho- not effectively controlled through If available, agronomically acceptable gen side of the disease triangle. Delay- crop rotation. An example is the fun- hybrids with high levels of resistance ing planting until soil temperatures gus Pythium ultimum, the most com- usually provide the best protection exceed 50°F reduces the amount of mon cause of damping off of corn. against a serious disease outbreak. seedling damping off by targeting the Hybrids may also exhibit moderate or environment side of the disease triangle. Resistant Hybrids even low levels of resistance to par- ticular diseases. This means that, One of the most practical and eco- while the disease still can develop on Preplant Decisions nomical means of disease control is to these hybrids, lower incidence of dis- That Affect Disease select agronomically suitable hybrids ease can be expected in most circum- with adequate resistance to diseases Development stances than on a fully susceptible of concern on your farm. Unfortu- Most of the agronomic decisions hybrid. For some diseases, low to mod- nately, resistance is not available for corn producers make have some im- erate resistance is all that is available some diseases. However, when avail- pact on disease development. In fact, once a corn field is planted, a producer’s disease management pro- Figure 1. Disease triangle. Disease only develops when three conditions are met: a pathogen gram is essentially in place, for better infects a susceptible host under disease-favorable conditions. or worse. Thus, consider your preplant decisions as disease-management de- cisions also. favorable environment Crop Rotation favorable environment Many corn pathogens survive be- tween crops in the corn residue, and some do not attack other field crops commonly grown in rotation with susceptible pathogen corn. Consequently, rotating to wheat, host (cause) susceptible pathogen soybean, or other crops helps to starve host (cause) certain corn pathogens that survive in the field by depriving them of a food source as the crop residue decomposes. NO DISEASE DISEASE

33 among current commercial hybrids, ter during spring under conservation Foliar sprays of fungicides may be even though higher levels of resis- tillage. economical in seed corn fields to pro- tance would be desirable. In these While conservation tillage systems tect against a variety of leaf diseases. cases, use of a hybrid with even a low may favor certain diseases, they can They may also occasionally be justi- level of resistance is usually superior also reduce pressure from certain oth- fied for production of certain specialty to planting a susceptible hybrid. ers. For example, charcoal rot, which corns. However, fungicide sprays typi- Sometimes a moderate level of resis- is favored by high soil temperatures cally do not show justifiable economic tance is acceptable for fields where re- and low soil moisture early in the returns for commercial production of duced disease pressure is expected. growing season, would be expected to dent corn. However, under high disease pressure, be worse in a conventional system low to moderate levels of disease re- than a no-till system. Scouting for Diseases sistance will not provide adequate dis- The possibility of enhancing pres- While it is not possible to know ease control. Such hybrids may sure from certain diseases under con- with complete certainty which dis- require you to pay greater attention servation tillage is not necessarily an eases will develop in a given season, to other disease management strate- argument to return to conventional the disease history of the farm and gies in order to achieve good results. tillage. However, producers should area will indicate the diseases most Hybrids can also be selected for recognize situations when their pro- likely to occur. A disease history for a tolerance—the ability to yield well duction system may enhance disease farm is established by scouting fields even though symptoms develop. In- activity so they can employ other dis- and identifying disease outbreaks formation on disease-tolerant hybrids ease management practices in order when they occur. Your county Exten- is limited, but tolerant hybrids can be to maintain adequate levels of disease sion agent, farm supply dealer, and useful when available. control. crop consultant can also be good It is important to plant more than sources of information. However, one corn hybrid on your farm. Plant- Other Cultural Practices farm-specific information obtained ing one hybrid is like “putting all your Other preplant decisions can also through field scouting is the most re- eggs in one basket.” Should a disease influence disease activity. For ex- liable basis for developing a farm dis- problem develop on that hybrid, your ample, early planting tends to en- ease history. Unless you are absolutely whole crop is at risk. Planting several hance activity of Pythium seedling certain as to the cause of a particular hybrids helps to spread the risk of losses diseases, which are favored by cool, problem, have the condition diag- from disease. wet soils. Conversely, late planting nosed by a reputable field specialist, can enhance pressure from gray leaf or submit the sample to the Univer- Tillage spot, a late-season disease that is more sity of Kentucky Plant Diagnostic Lab. Conservation tillage systems pro- damaging on younger crops than more vide for less soil erosion, less fuel con- mature ones. sumption, savings of time and labor, Plant populations are usually se- Several mycotoxins—toxins pro- moisture conservation, and easier lected on the basis of hybrid charac- duced by fungi—can occasionally be double-cropping. Conservation tillage teristics and yield potential of the field. found in shelled corn. Aflatoxins oc- systems can, however, increase pres- Some diseases can be more severe at cur very infrequently in Kentucky, but sure from certain diseases, especially high plant populations; several of the when they occur, they are often asso- under continuous corn production. stalk rot diseases are examples. A fer- ciated with hot, dry weather during Prime examples are gray leaf spot and tility program that is inadequate or grain fill or with improper storage con- ear rot. Spore levels of the excessive, or in which major nutrients ditions. Fumonisins can also sometimes fungi that cause these diseases are are not in proper balance, may also be found in Kentucky corn, as can higher in fields where previously in- enhance disease activity. vomitoxin (also called deoxynivalenol, fected corn residue is left on the soil or DON), and zearalenone. More in- surface. When residue is tilled into the Fungicides formation on mycotoxins in corn can soil, spores are trapped underground Essentially all corn seed is treated be found in the Extension publications and cannot easily spread to before purchase with fungicides to help Aflatoxins in Corn (ID-59) and Myc- aboveground plant parts. Further- control seed rots and seedling diseases. otoxins in Corn Produced by Fusarium more, buried crop residues decompose This provides inexpensive protection Fungi (ID-121). faster, which reduces pathogen sur- against stand loss, should conditions vival. Activity of seedling diseases can favor these diseases after planting. Un- also be increased in no-till systems treated seed should be treated with fun- because soils remain cooler and wet- gicides before planting.

34 Diseases of Corn Ear and Kernel Rots deoxynivalenol), zearalenone, or both. Cause: Stenocarpella, Gibberella, Key Features of Disease Cycle: Anthracnose Fusarium, Aspergillus, Penicillium Wounds made by birds and insects Cause: Colletotrichum graminicola Symptoms: Moldy growth on ears provide infection sites for these fungi, Symptoms: Tan to brown leaf spots and kernels. Helpful distinguishing although infection may occur in un- surrounded by a yellow halo, usually features: wounded tissues. Other factors that more abundant toward leaf tip. Le- • Diplodia ear rot, caused by can aggravate ear and kernel rots in- sions may coalesce, blighting entire Stenocarpella—white mold growth clude lodging of stalks that brings ears leaves. Early in season, anthracnose between kernels, usually progress- in contact with soil, incomplete cov- symptoms are most common on lower ing from base of ear. erage of ears by husks, and maturation leaves. Late in season, symptoms of • Gibberella—pink to reddish mold of ears in upright position. anthracnose include blighting of up- growth, often progressing from ear Management: For Diplodia ear rot, per leaves and possibly breakage of tip. rotate away from corn when 2 to 3 per- plant tops (see Top Dieback). Anthra- • Penicillium—green or blue-green cent of ears have the disease; break cnose also causes a late-season lower powdery mold on and between ker- up corn residue if practical to enhance stalk rot. Black spines may be visible nels, often at the ear tip. decomposition; and avoid highly sus- in dead leaf spots with a hand lens. • Aspergillus—greenish-yellow mold ceptible hybrids. For all ear and ker- Damage: Early-season leaf symp- on and between kernels. nel rots, choose hybrids in which ears toms usually are not damaging but in- • Fusarium—whitish pink to laven- are well covered by husks and in dicate the need to scout later for stalk der mold growing on individual which ears point downward at matu- rot. Yields can be reduced from leaf kernels or small clusters of kernels. rity. Control insects that feed on ears blighting, although this is uncommon. Damage: Ear and kernel rots reduce in the field. Harvest at about 25 per- The stalk rot phase can cause stalk feed value and marketability. Yield and cent moisture for shelled corn to mini- lodging. test weight may also be reduced. When mize kernel damage and field losses. Key Features of Disease Cycle: The severe, Diplodia ear rot can affect 50 Adjust harvesting equipment for survives in undecomposed corn percent or more of the ears in a field. minimum kernel damage and maxi- residue. Spores are spread by wind- Contamination of grain by mycotox- mum cleaning. Avoid harvesting blown rain and rainsplash. ins from certain ear molds can also re- faster than drying facilities can oper- Management: Use resistant hybrids, duce nutritional value and mar- ate effectively. Dry shelled grain to especially where corn is grown with- ketability of the corn. Aspergillus can below 15.5 percent moisture within out rotation under reduced tillage. Ro- contaminate grain with aflatoxins, al- 24 to 48 hours after harvest. Clean tate away from corn for one to two though this toxin is very uncommon bins before storage and maintain dry years. in Kentucky. storage conditions. Control insect in- (=Fusarium moniliforme) and related festations in storage. Periodically aer- fungi can produce fumonisins in the ate and check for heating, crusting, grain, and Gibberella can contaminate or musty odors. Maintain stored corn the grain with vomitoxin (=DON or uniformly as indicated in Table 1.

Table 1. Recommended temperatures for stored corn. Average Minimum Maximum monthly grain grain temperature temperature temperature Below 40˚ F35˚ F45˚ F 40˚ - 60˚ F Within 5˚ F of Within 5˚ F of average monthly average monthly temperature temperature Above 60˚ F55˚ F65˚ F

Anthracnose leaf symptoms. (R. Stuckey)

35 Diplodia ear rot. (P. Vincelli) Gibberella ear rot. (D. G. White)

Sources of Additional Information: leaves during grain fill. disease is most severe in fields with Principles of Grain Storage (AEN-20), Damage: Yield is reduced through corn following corn under conserva- Aeration, Inspection, and Sampling of shorter ears and smaller kernels. Yield tion tillage. Severe yield loss can oc- Grain in Storage Bins (AEN-45), Afla- losses in the range of 10 to 20 percent cur when leaves become blighted toxins in Corn (ID-59), Mycotoxins in are typical in susceptible hybrids during early grain fill. Corn Produced by Fusarium Fungi grown in Kentucky, although losses of Management: Use resistant hybrids, (ID-121). 50 percent or more may occur under especially when grown without rota- very high disease pressure. Test weight tion under conservation tillage. Rec- Gray Leaf Spot may also be reduced. When leaf ognize that there are no immune Cause: Cercospora zeae-maydis blighting is severe, stalks may weaken hybrids, although hybrids exist with Symptoms: Gray to tan, narrow, and lodge as the plant draws nutrients a wide range of levels of partial resis- 1 rectangular lesions /4 to 2 inches long. from the stalk to fill ears. tance. Typically there is a greater Lesions on some hybrids exhibit a yel- Key Features of Disease Cycle: The choice of resistant hybrids among low border. Lesions are restricted by fungus survives for one to two years mid- and full-season hybrids than veins. Substantial numbers of leaf le- in undecomposed residue of infected among early-maturing hybrids. Con- sions usually do not appear until tas- leaf blades and sheaths. Spores are sider using a hybrid with high levels seling or later. Older leaves are spread by air movement. Leaves be- of resistance in fields where 1) last affected first; severely affected leaves come infected during prolonged peri- year’s crop was corn, or 2) corn was can be killed when lesions coalesce. ods (11 to 14 hours or more) of high grown two years ago and residue cover Weakening and lodging of stalks may relative humidity (>95 percent) and is at least 30 percent, or 3) there is occur if a severe outbreak blights warm temperatures (72° to 86°F). The untilled corn residue within 150 to

Fusarium ear rot. (R. Stuckey) Aspergillus ear rot. (R. Stuckey)

36 Gray leaf spot on a susceptible hybrid. (D. G. White) Northern leaf blight on a susceptible hybrid. (D. G. White)

500 feet of the field to be planted (the Key Features of Disease Cycle: The Key Features of Disease Cycle: Spores later the planting, the further it fungus survives in undecomposed corn of both fungi are carried on springtime should be from untilled corn residue residue. Spores are spread by air cur- winds from southern areas of the if it is a susceptible variety). Fungi- rents. Spores germinate and infect United States. Common rust is active cidal control of gray leaf spot may oc- leaves during wet weather with mod- during cool (60° to 75°F), humid casionally be economically justified in erate (64° to 81°F) temperatures. Se- weather; southern rust is most active certain fields of specialty corns. How- vere yield loss can occur when leaves during warm (80°F), humid condi- ever, fungicide sprays usually do not become blighted during early grain tions. Both fungi infect leaves when show justifiable economic returns for fill. More severe in fields with corn spores are present and leaf surfaces are commercial dent corn production. following corn under reduced tillage. wet. Both are potentially more severe Sources of Additional Information: Also infects sorghum. in late plantings. Greatest yield loss Gray Leaf Spot of Corn (PPA-35). Management: Use resistant hybrids, occurs in susceptible hybrids when out- especially when grown without rota- breaks begin during early grain fill. Northern Leaf Blight tion under conservation tillage. Hy- Management: Most hybrids in Ken- Cause: Setosphaeria turcica (= brids with either single-gene (Ht) or tucky have adequate resistance levels Exserohilum turcicum, = Helmintho- multiple-gene resistance are available. to common rust for our conditions. Re- sporium turcicum) Rotate away from corn and sorghum sistance to southern rust is limited in Symptoms: Elliptical, grayish-green for one to two years. hybrids commonly grown in Kentucky. or tan lesions 1 to 6 inches long with Southern rust outbreaks, when they smooth margins. During damp Rusts occur, develop in late summer. There- weather, greenish-black fungal sporu- Cause: Puccinia sorghi, Puccinia fore, minimize late plantings, which lation is produced in lesions. Older polysora would be at a younger age and there- leaves are affected first. Severely af- Symptoms: Pustules that are circu- fore more subject to yield loss should fected leaves can be killed when le- lar to oval, golden-brown to cinna- an outbreak occur. sions coalesce. On hybrids carrying an mon brown, up to 1/8 inch long.

Ht2 resistance gene, long, yellow to Pustules become brown to black at tan lesions with wavy margins and no harvest. Leaves turn yellow and dry sporulation are observed on leaves in- up when severely infected. Pustules of fected with S. turcica. These resis- common rust (P. sorghi) are common tance-reaction lesions can be easily on both leaf surfaces. Pustules of confused with Stewart’s wilt. southern rust (P. polysora) are densely Damage: Yield and test weight can scattered on upper leaf surface with be substantially reduced in cool, wet few on lower surface. summers, although most hybrids Damage: Common rust rarely grown in Kentucky have adequate re- causes economic loss in field corn in sistance. Kentucky. An aggressive outbreak of southern rust in late-planted crops may reduce stalk strength in a grain Common rust (left) and southern rust (right). crop and quickly desiccate silage corn. (D. G. White)

37 Seed Rot and Damping Off Smut, Common Southern Leaf Blight Cause: Principally Pythium ul- Cause: Ustilago maydis Cause: Cochliobolus heterostrophus timum, but also Stenocarpella, Symptoms: Greenish-white or sil- (=Bipolaris maydis, =Helminthosporium Fusarium, Penicillium, Rhizoctonia very galls, or swellings, up to 6 inches maydis) Symptoms: Rotting of seed before in diameter. Galls can occur on any Symptoms: Elliptical, tan to light 1 1 or after germination. Yellowing, wilt- aboveground plant part. As galls age brown, small lesions ( /8 to /4 inch by 1 3 ing, and death of leaves of emerged (except those on leaves), the interior /4 to /4 inch), often with somewhat plants. Soft rot of stem tissues. Rot- darkens and turns into masses of pow- parallel sides, and sometimes with a ting of roots, which may appear dery, dark olive to black spores. Galls brown border. Older leaves are af- brown, watersoaked and grayish, on leaves usually remain small (0.5 fected first; severely affected leaves faintly pink, or greenish-blue. May inch or less) and become hard and dry can be killed when lesions coalesce. result in uneven stand height later in without rupturing. Plants with galls on Damage: Yield and test weight can season. the lower stalks may be barren or pro- be reduced, although most hybrids Damage: Stand establishment and duce small ears. have adequate resistance for our con- early-season vigor can be reduced, Damage: Hybrids of field corn ditions. leading to lower yields. grown in Kentucky typically have ad- Key Features of Disease Cycle: The Key Features of Disease Cycle: These equate resistance, and consequently, fungus survives in corn residue. Spores pathogens are common fungi in Ken- yield losses typically are minimal (2 are spread by air currents. Spores ger- tucky soils. They usually do not limit percent or less) to nonsignificant. minate and infect leaves during warm stands but can do so when seedlings Key Features of Disease Cycle: The (68° to 90°F), wet weather. More se- are stressed. Common stresses include fungus survives for several years as vere in fields with corn following corn planting in cool, wet soils and chemi- spores in corn residue and in soil. under reduced tillage. Greatest yield cal injury. Early planting dates pre- Spores can infect any actively grow- loss can occur when leaves become ferred by many farmers tend to ing aboveground plant part. Wound- blighted during early grain fill. enhance these diseases. ing (stinkbugs or other forms of Management: Plant resistant hy- Management: Use high-quality, vig- injury) enhances infection substan- brids, especially when grown without orous seed treated with fungicide. tially. Once infection occurs, galls de- rotation under reduced tillage. Rotate Plant in warm (above 50°F), moist velop, enlarge, turn powdery, and away from corn for one to two years. soils; measure soil temperature at a 2- rupture to release spores. inch depth after sunrise. Place herbi- Management: Use hybrids with ad- cide, fertilizer, insecticide, and seed equate resistance. properly to avoid stress or injury to seedling.

Symptoms of Pythium root rot. The outer root cylinder appears watersoaked and easily Southern leaf blight. (D. G. White) is pulled away from vascular tissue. (D. G. White)

Common smut. (R. Stuckey)

38 Stalk Rot • Charcoal Rot (Macrophomina)— susceptible than full-season hybrids. Cause: Stenocarpella maydis Pith contains many very tiny black Several stalk-rot fungi also cause ear (= Diplodia maydis), fungal structures, giving charred and kernel rots. Colletotrichum also (= Fusarium graminearum), Fusarium appearance. Roots rotted and black. causes anthracnose of leaves, as well verticillioides (= Fusarium moniliforme), • Anthracnose (Colletotrichum)— as top dieback. Macrophomina also in- Macrophomina phaseolina, Colleto- Dark brown to black discoloration fects sorghum and soybean. trichum graminicola on exterior of lower stalk. Dark Management: Use hybrids resistant Symptoms: Lower stalk is spongy and spines may be visible with hand to stalk rots and important leaf diseases internal tissue (pith) shredded and of- lens, especially near soil line. Pith like gray leaf spot. Avoid excessive ten discolored. Stalks weaken and light to dark brown. plant populations. Maintain balanced lodge. Plants sometimes turn grayish- Damage: Plants lodge and become soil fertility and adequate but not ex- green and dry prematurely during grain difficult or impossible to harvest. In cessive nitrogen. Control insects that fill. Helpful distinguishing features: severe cases, grain yield may be re- feed on leaves, stalks, and roots. Scout • Diplodia stalk rot, caused by duced. for stalk rots by either pinching the Stenocarpella—Stalk and pith light Key Features of Disease Cycle: These lower two or three stalk internodes or brown. Small, dark-brown to black fungi survive on corn residues. All but by pushing stalks 8 to 12 inches from pimple-like fruiting structures de- charcoal rot are favored by warm, wet vertical to check for lodging. Harvest velop just below epidermis near weather during grain fill. Charcoal rot early if 10 to 15 percent show disease. base of stalk. is favored by hot, dry weather during Avoid growing continuous corn. Con- • Gibberella—Pith pink to reddish. grain fill. Other aggravating factors: sider avoiding soybean and sorghum Small black pimple-like fruiting • High plant populations. following severe outbreaks of charcoal structures develop superficially on • Loss of leaves from disease, insects, rot. stalk near nodes and can be easily or hail. Sources of Additional Information: scraped off with fingernail. • Excessive nitrogen, especially when Corn Stalk Rots (PPA-26). • Fusarium—Pith whitish-pink to combined with low potash. salmon-colored. Roots often rot- Early-season hybrids are often more ted. Difficult to distinguish from Gibberella.

Gibberella stalk rot. Note pink to reddish discoloration in pith. (D. G. White)

Gibberella stalk rot. Note fruiting structures Diplodia stalk rot. (R. Stuckey) on thumbnail. (D. G. White)

39 Charcoal rot. (D. G. White) Anthracnose stalk rot. (D. G. White)

Stewart’s Wilt Damage: Yield and test weight can have not been commonly reported in Cause: Pantoea stewartii (=Erwinia be reduced in susceptible hybrids, al- Kentucky in recent years. Stalk lodg- stewartii) though most hybrids of dent corn ing may occur. Symptoms: Long (2 to 10 inches), have adequate resistance. In a highly Key Features of Disease Cycle: The linear (1/8 to 1 inch wide) leaf lesions susceptible variety, stands may be re- fungus survives in corn residue. Spores with very wavy margins. At first, le- duced if infected as seedlings. are spread by windblown rain and sions are pale green to yellow, but they Key Features of Disease Cycle: Over- rainsplash. Also causes early-season become light brown when they dry. winters in body of corn flea beetle, anthracnose on leaves, as well as an- Severely affected leaves are killed. which also spreads the bacterium. Dis- thracnose stalk rot. Lesions of Stewart’s wilt are easily ease pressure is usually high in Ken- Management: Use resistant hybrids, confused with lesions on hybrids car- tucky but can be low following a very especially when grown without rota-

rying an Ht2 resistance gene to north- cold winter, which kills overwinter- tion under reduced tillage. Rotate ern leaf blight. To aid field diagnosis, ing flea beetles. away from corn for one to two years. hold leaves to light and look in le- Management: Plant resistant hy- sions for scratch-like feeding marks of brids. Control of the disease in field flea beetle; if uncertain, submit corn through application of insecti- samples to the University of Kentucky cides targeting the flea beetle is un- Plant Diagnostic Labs. Infection of economical. seedlings causes rapid wilt and death. Sources of Additional Information: Stewart’s Wilt of Corn (PPA-33).

Top Dieback (Upper Stalk Rot) Cause: Colletotrichum graminicola Symptoms: Plants turn yellow or red from top downward during grain fill. Leaves at ear level remain green. Lodging and breakage of stalks occur when severe. Internal stalk tissue has brown discoloration. Be sure to rule out stalk injury from European corn borer. Damage: Yield and test weight can be reduced, although serious out- breaks of this phase of anthracnose Stewart’s wilt. (D. G. White) Top dieback. (R. Stuckey)

40 Virus Complex pared to corn planted on time, late- Cause: Dwarf Mosaic Virus planted corn is at an earlier stage of (MDMV), Maize Chlorotic Dwarf Vi- crop development when insect vec- rus (MCDV) tors become active. Earlier infection Symptoms: Symptoms can be vari- usually results in more severe symp- able. MDMV typically causes stunt- toms. MDMV also causes a disease of ing and irregular, light and dark green sorghum. mosaic patterns in the leaves, espe- Management: Use virus-tolerant cially the youngest leaves. MCDV hybrids in fields with heavy infesta- typically causes stunting, yellowing, tions of johnsongrass rhizomes. Elimi- and sometimes reddening of the nate johnsongrass rhizomes to reduce youngest leaves and sometimes causes disease pressure. Avoid late planting leaf tattering. Usually most severe since the younger a crop is when an around areas of fields highly infested outbreak occurs, the more yield loss with johnsongrass rhizomes. is possible. Damage: Yield and test weight can Sources of Additional Information: be reduced substantially in localized Virus Diseases of Corn in Kentucky outbreaks in and around areas with (PPA-40). rhizome johnsongrass. Key Features of Disease Cycle: Both Acknowledgment Virus complex. (P. Vincelli) viruses overwinter in johnsongrass Thanks to Donald Hershman for re- rhizomes. MDMV is spread by certain viewing a previous draft of this chap- aphids; MCDV, by certain leafhop- ter and to Donald G. White and pers. Late-planted fields have greater Richard Stuckey for providing photos. risk of serious disease outbreaks. Com-

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