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MID-ATLANTIC Soybean Disease Scouting Guide

MISSION The mission of the Delaware Soybean Board (DSB), Maryland Soybean Board (MSB), and Virginia Soybean Board (VSB) is to maximize the profitability of soybean farmers in their respective states by investing soybean checkoff funds in targeted domestic and international research, promotion and communication initiatives. The volunteer farmer-leaders who serve on the DSB, MSB and VSB boards of directors invest your checkoff dollars in research to improve soybean production practices to make your farm more profitable and ensure the sustainability of Mid-Atlantic soybean production. This guide provides an in-depth overview of soybean diseases in the Mid-Atlantic region, including appropriate scouting and management procedures for control. DSB, MSB and VSB would like to thank the collaborating researchers from the University of Delaware, University of Maryland and Virginia Tech for contributing information and providing technical editing for this guide. Special thanks to Hillary Mehl, Ph.D., Virginia Tech, for leading the development of this guide. 2

Mid-Atlantic Soybean Disease Scouting Guide

Authors: Hillary L. Mehl1, Tian Zhou1, Nathan Kleczewski2 Virginia Tech Tidewater AREC, Suffolk, VA2

1

University of Delaware, Newark, DE 3

TABLE OF CONTENTS INTRODUCTION 6-13

Disease and Nematode Management in Field Crops FOLIAR DISEASES

4

14-15

Bacterial Diseases

16-17

Cercospora Leaf Blight and Purple Seed Stain

18-19

Downy Mildew

20-21

Frogeye Leaf Spot

22-23

Phomopsis Seed Decay

24-25

Phyllosticta Leaf Spot

26-27

Septoria Brown Spot

28-29

Soybean Mosaic

30-31

Soybean Rust

32-33

Soybean Vein Necrosis

34-35

Target Spot

Mid-Atlantic Soybean Disease Scouting Guide

STEM AND ROOT DISEASES 36-37

Anthracnose Stem Blight

38-39

Brown Stem Rot

40-41

Charcoal Rot

42-43

Fusarium Wilt and Root Rot

44-45

Phytophthora Root and Stem Rot

46-47

Pod and Stem Blight

48-49

Sclerotinia White Mold

50-51

Seedling Diseases

52-53

Stem Canker (Northern and Southern)

55

Sudden Death Syndrome NEMATODES

56-57

Root-Knot Nematode

58-59

Soybean Cyst Nematode

5

INTRODUCTION: DISEASE AND NEMATODE MANAGEMENT IN FIELD CROPS Disease control is an essential component of crop management for maximizing yield potential. However, the need for management and best tactics for disease control vary by crop, field and year, so an integrated approach to disease management is necessary to minimize unnecessary inputs and maximize profits. Integrated disease management combines multiple approaches, including cultivar selection, cultural practices such as crop rotation and judicious use of pesticides based on disease advisories and/or scouting. The following points should be considered when making diseasemanagement decisions:

• Susceptibility of crop variety to disease. Varieties have a high turnover rate so check with your local extension office or seed dealer for current information on which varieties have some level of resistance to diseases in the region. Be aware of the specific disease resistances and susceptibilities of your varieties.

• Yield potential and commodity price. Seed treatments can be a relatively inexpensive line of defense against soybean disease, especially when soybeans are planted into wet soils. However, not all seed treatments are created equal. Use a seed treatment

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Mid-Atlantic Soybean Disease Scouting Guide

that controls a broad range of diseases and addresses the problems of a particular field for best results.

• Previous crop and cropping system (e.g. no till). Many pathogens are able to survive on crop residues. Tillage and rotation to non-host crops helps bury and decompose this residue. Keep in mind that some diseases require a living plant host and must move in from warmer regions each year (e.g. some rusts).

• Crop growth stage and timing of fungicide applications. Diseases are more likely to impact yield when they occur at particular growth stages of the crop (typically during development of the grain) so timing fungicide applications accordingly is key.

• Proper pathogen identification. Field crops can be affected by numerous fungi, bacteria, viruses and nematodes. Each type of pathogen requires a different approach for disease management. Proper disease identification is key to avoiding ineffective or unnecessary chemical applications.

• Disease pressure. Are diseases present at levels that require intervention? Scout fields regularly to maximize profits.

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• Weather. Temperature and humidity greatly influence the onset and development of disease. Even if the crop is susceptible and a pathogen is present, the risk of yield loss to disease may be low if environmental conditions are not conducive to pathogen growth and reproduction. Warm, humid conditions are favorable for many foliar diseases in our region. In some cases, the microclimate within a field may be conducive for disease development even when ambient conditions are relatively dry, especially when high plant populations and a dense canopy are present in a field. All of these factors should be considered before making plant disease-management decisions, especially before applying a fungicide that may or may not be needed. Diseases are most effectively managed by integrating several of the following practices:

1. Adapted, disease-resistant cultivars. Produce plants able to resist disease attack. Every variety has a disease-resistance “package” that provides a level of resistance or tolerance to specific diseases. This information can be obtained from seed dealers, commercial companies and university variety trials such as those conducted at Virginia Tech and the University of Delaware.

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Mid-Atlantic Soybean Disease Scouting Guide

Some diseases cannot be managed with resistance, and not every variety has adequate resistance to all diseases. However, farmers who do not consider disease resistance when selecting varieties are taking a considerable, and likely, a costly risk.

2. Rotation. Avoid presence of pathogens by not continually planting soybeans in the same fields year after year. Rotation reduces the amount of residue, and therefore the amount of some pathogens, in fields. Rotation also is essential for reducing populations of some plant parasitic nematodes.

3. Sanitation. Use tillage and weed management. Deep tillage (not disking) buries plant pathogens, favoring their decomposition and reducing their ability to reach the plant and cause damage. However, many pathogens are “regional” and therefore disking one field may not have any impact on disease if nearby fields contain ample residue. In addition, the widespread use of conservation or no-till agriculture to realize environmental benefits eliminates the disease management benefits of tillage.

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4. Planting date. Avoid pathogens altogether or reduce the length of the infection period by avoiding early planting. This is particularly effective in managing seedling diseases.

5. Seedbed preparation and balanced fertility. Provide good seed-to-soil contact and fertility to promote vigorous plant stands. Healthy plants are likely to be less stressed and less prone to some diseases.

6. High-quality, disease-free seed. Promote healthy, vigorous seedling development. In addition, some diseases can move in or on seed. Using clean, certified seed can help reduce the development of these diseases and maximize stands.

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Mid-Atlantic Soybean Disease Scouting Guide

7. Seed-treatment fungicides. Depending on fungicide and situation, these can help protect developing seedlings from a number of pathogens.

8. Seed-treatment nematicides/insecticides. Insecticides can sometimes reduce the spread of plant viruses that are spread by insect vectors. Seed-treatment nematicides are available for some crops and can help minimize nematode damage to seedling roots.

9. Foliar-applied fungicides. These products can protect yield when cultural practices and cultivar resistance fail and prevent economic losses. However, in order to be most economical, these often must be applied preventively and at the appropriate time.

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EARLY SEASON VE #

FOLIAR DISEASES

1

Bacterial Diseases

2

Cercospora Leaf Blight/Purple Seed Stain

3

Downy Mildew

4

Frogeye Leaf Spot

5

Phomopsis Seed Decay

6

Phyllosticta Leaf Spot

7

Septoria Brown Spot

8

Soybean Mosaic

9

Soybean Rust

10

Soybean Vein Necrosis

11

Target Spot STEM AND ROOT DISEASES

12

Anthracnose Stem Blight

13

Brown Stem Rot

14

Charcoal Rot

15

Fusarium Wilt and Root Rot

16

Phytophthora Root and Stem Rot

17

Pod and Stem Blight

18

Sclerotinia White Mold

19

Seedling Diseases

20

Stem Canker

21

Sudden Death Syndrome

22

Root-Knot Nematode

NEMATODES 23 12

Soybean Cyst Nematode Mid-Atlantic Soybean Disease Scouting Guide

VC

V1

V2

MIDSEASON V3

V4

V5-Vn

R1

R2

LATE SEASON R3

R4

R5

R6

R7

R8

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FOLIAR DISEASES

BACTERIAL DISEASES Causal organisms: Pseudomonas savastanoi pv. glycinea (bacterial blight) and Xanthomonas axonopodis pv. glycines (bacterial pustules) Favorable conditions: Cool, wet conditions (bacterial blight); warm, wet conditions (bacterial pustule). When to scout: Late vegetative growth stages through pod filling. Impact on yield: Bacterial diseases rarely cause significant yield loss. Symptoms and signs: • Leaves – small, angular reddish-brown lesions with watersoaked margins surrounded by yellow halos (Fig. 1.1), tattered leaves (bacterial blight); dark brown lesions with raised centers (Fig. 1.2), pustules in the center of lesions on lower leaf surface (Fig. 1.3) (bacterial pustule). • Distribution – mid- to upper canopy, widespread in field. May be confused with: Downy mildew, Septoria brown spot, soybean rust, soybean vein necrosis virus. Management: • Resistant or less susceptible varieties. • Non-host crop rotation (e.g. corn or small grains) and tillage to bury infested crop residues.

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Mid-Atlantic Soybean Disease Scouting Guide

Figure 1.1 Bacterial blight lesions on soybean leaves. Photo credit: N. Kleczewski, University of Delaware

Figure 1.2 Soybean leaf with bacterial pustule lesions. Photo credit: D. Mueller, Iowa State University, Bugwood.org

Figure 1.3 Bacterial pustules on the underside of a soybean leaf. Photo credit: P. Bachi, University of Kentucky Research and Education Center, Bugwood.org

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FOLIAR DISEASES

CERCOSPORA LEAF BLIGHT/ PURPLE SEED STAIN Causal organism: Cercospora kikuchii Favorable conditions: Wet or humid weather and warm temperatures (above 75°F). When to scout: Infections may occur early in the season, but foliar symptoms are usually observed during R3-R6 growth stages. Impact on yield: Estimated 15% or greater yield loss, purple seed stain can reduce seed quality. Symptoms: • L eaves – light to dark purple discoloration in spots or patches on upper surfaces (Fig. 2.1), may become leathery, defoliation of uppermost leaves may occur with petioles remaining attached. • P etioles – elongated, dark red lesions on petioles in upper canopy. • Seeds – pinkish to purple discoloration of seed coat (Fig. 2.2). • Distribution – upper canopy, widespread in field. May be confused with: Soybean rust, soybean vein necrosis virus, herbicide injury, sunscald. Management: • Resistant varieties (leaf blight only; resistant varieties are not completely immune and the level of resistance varies by location). • High-quality, disease-free seeds. • Non-host crop rotation (e.g. corn or small grains) and tillage to bury infested crop residue. •Foliar fungicides (may be resistant to some fungicide chemistries). 16

Mid-Atlantic Soybean Disease Scouting Guide

Figure 2.1 Downy mildew lesions on upper and lower surfaces of soybean leaves. Photo credit: N. Kleczewski, University of Delaware

Figure 2.2 Fuzzy fungal growth on the underside of a soybean leaf infected with downy mildew. Photo credit: N. Kleczewski, University of Delaware

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FOLIAR DISEASES

DOWNY MILDEW Causal organism: Peronospora manshurica Favorable conditions: Long periods of leaf wetness and moderate temperatures (50-80°F). When to scout: Late vegetative stages through pod filling (R6). Impact on yield: Little to no impact on yield. Symptoms: • L eaves – small, irregular pale-green to yellow spots on the upper leaf surface (Fig. 3.1); newer lesions are reddish-brown in the center with yellow border; older lesions turn grayish brown to dark brown; pale yellow fuzzy structures growing from the spots on the lower leaf surfaces (Fig. 3.2). • S eeds – may be partially or completely covered with white, fluffy fungal growth and be smaller and lighter than healthy seed. • Distribution – upper canopy, widespread in field. May be confused with: Bacterial blight, Phomopsis seed decay, powdery mildew, Septoria brown spot, soybean rust, soybean vein necrosis virus. Management: • Resistant varieties. • Crop rotation and tillage to bury infested crop residue.

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Mid-Atlantic Soybean Disease Scouting Guide

Figure 3.1 Downy mildew lesions on upper and lower surfaces of soybean leaves. Photo credit: N. Gregory, University of Delaware

Figure 3.2 Fuzzy fungal growth on the underside of a soybean leaf infected with downy mildew. Photo credit: N. Kleczewski, University of Delaware

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FOLIAR DISEASES

FROGEYE LEAF SPOT Causal organism: Cercospora sojina Favorable conditions: Warm, wet or humid weather. Young leaves are more susceptible to infection than older leaves. When to scout: R1 to R6 growth stages. Impact on yield: Yield reduction can range from 15% to 30%. Symptoms: • L eaves – irregular-shaped spots on leaf surface; usually tan or light brown in the center with a reddish-brown to purple margin (Fig. 4.1); spores may be observed in the center of the lesions on the lower surface of the leaves. • P ods and stems – narrow, reddish-brown to light gray lesions with dark margins on stems; similarly colored circular to oval lesions on pods. • S eeds – symptomless or light to dark gray blotches and cracked seed coat. • Distribution – upper canopy, small to large patches throughout field. May be confused with: Phyllosticta leaf spot, herbicide injury. Management: • Resistant varieties. • High-quality, disease-free seed. • Non-host crop rotation (e.g. corn or small grains) and tillage to bury infested crop residue. • Foliar fungicides (may be resistant to some fungicide chemistries).

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Mid-Atlantic Soybean Disease Scouting Guide

Figure 3.1 Frogeye leaf spot lesions on soybean leaves. Photo credit: H. Mehl, Virginia Tech Tidewater AREC

21

FOLIAR DISEASES

PHOMOPSIS SEED DECAY Causal organism: Diaporthe longicolla (syn. Phomopsis longicolla) Favorable conditions: Extended wet and warm (above 68°F) periods during the pod-development and maturation stages. When to scout: Full pod through maturity. Impact on yield: Seed decay can cause yield reduction from 10% to 50% and a reduction in seed quality. Symptoms: • S tems – thin, irregular black “zone lines” on the internal lower portions of the stem. • S eeds – cracked, dry and shriveled with white colored mold on surface (Fig. 5.1). • Distribution – widespread in field. May be confused with: Downy mildew, white mold (symptoms on seed). Management: • Resistant varieties. • High-quality, disease-free seeds. • Non-host crop rotation (e.g. corn or small grains) and tillage to bury infested crop residue. • Foliar fungicides (an R5 application protects seed quality but does not impact yield).

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Mid-Atlantic Soybean Disease Scouting Guide

Figure 5.1 Signs and symptoms of Phomopsis seed decay. Photo credit: A. Sisson, Iowa State University, Bugwood.org

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FOLIAR DISEASES

PHYLLOSTICTA LEAF SPOT Causal organism: Pleosphaerulina sojicola (syn. Phyllosticta sojicola) Favorable conditions: Cool, moist weather. When to scout: Late vegetative stages through pod fill (R6). Impact on yield: Rarely affects yield. Symptoms and signs: • Leaves – lesions can be circular, irregular or V-shaped; lesions often grayish to tan with narrow dark margins; black specks (pycnidia) may be visible in the lesions. • Distribution – patches throughout field. May be confused with: Frogeye leaf spot, Septoria brown spot, soybean vein necrosis virus, nutrient deficiency, herbicide injury. Management: • Management generally not needed for this disease. • High-quality, disease-free seeds. • Non-host crop rotation and tillage to bury infested crop residue.

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Mid-Atlantic Soybean Disease Scouting Guide

Figure 6.1 Lesions of Phyllosticta leaf spot on soybean. Photo credit: N. Kleczewski, University of Delaware

25

FOLIAR DISEASES

SEPTORIA BROWN SPOT Causal organism: Septoria glycines Favorable conditions: Wet and warm weather (80s°F). When to scout: Vegetative growth stages through pod fill (R6). Impact on yield: Little impact on yield when in the lower canopy. Symptoms: • Leaves – small, irregularly shaped brown lesions (Fig. 7.1); spots can coalesce to form larger lesions; infected leaves turn yellow and drop (Fig. 7.2). • Stems, petioles and pods – less common than foliar lesions; brown and irregular. • Distribution – begins in lower canopy and moves up the plant; widespread within a field. May be confused with: Bacterial blight, bacterial pustule, downy mildew, Phyllosticta leaf spot, soybean rust, soybean vein necrosis, target spot, herbicide injury. Management: • Non-host crop rotation (e.g. corn or small grains) and tillage to bury infested crop residue. • Foliar fungicides are rarely economical for management of this disease.

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Mid-Atlantic Soybean Disease Scouting Guide

Figure 7.1 Septoria brown spot lesions. Photo credit: N. Kleczewski, University of Delaware

Figure 7.2 Yellowing in the lower canopy caused by Septoria brown spot. Photo credit: D. Mueller, Iowa State University, Bugwood.org

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FOLIAR DISEASES

SOYBEAN MOSAIC Causal organism: Soybean mosaic virus (SMV) (Potyvirus) Favorable conditions: Cool temperatures and aphid activity. When to scout: Flowering through pod filling. Impact on yield: Potential yield loss and reduced seed quality. Symptoms: • General – stunting. • Leaves – mosaic pattern of light and dark green, chlorosis, stunting, crinkling (Fig. 8.1); symptoms most severe on youngest leaves. • Pods – small, flat and reduced in number. • Seeds – mottling and dark discoloration, smaller size, reduced germination. • Distribution – single or clusters of plants in a field. May be confused with: Other viruses, nutrient deficiency, herbicide injury. Management: • High-quality, disease-free seeds. • Avoid late planting (increases risk for SMV infection).

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Mid-Atlantic Soybean Disease Scouting Guide

Figure 8.1 Foliar symptoms of soybean mosaic. Photo credit: D. Mueller, Iowa State University, Bugwood.org

29

FOLIAR DISEASES

SOYBEAN RUST Causal organism: Phakopsora pachyrhizi Favorable conditions: Long periods of leaf wetness and daily average temperature of less than 82°F; spores must be blown in by wind from areas where the fungus survives the winter (southern states). When to scout: Between R1 and R6 (flowering to full-pod stage). Impact on yield: Severe yield loss can occur if soybean rust occurs in a field during early pod development and conditions favor disease development and spread. In the Mid-Atlantic, soybean rust is typically found late in the season (if at all), and in a typical year, it is unlikely to impact yield. Symptoms and signs: • Leaves – small, gray spots later become tan or reddish-brown (Fig. 9.1); raised pustules (bumps) typically form on the underside of the leaves (Fig. 9.2); pustules produce orange spores (Fig. 9.3); heavy infection results in defoliation. • Distribution – starts in lower canopy in areas of fields with high moisture (e.g. near the tree line where it is shaded, near bodies of water, etc.) May be confused with: Bacterial blight, bacterial pustule, Cercospora leaf blight, downy mildew, Septoria brown spot, target spot. Management: • Foliar fungicides (only needed if soybean rust is present in a region, and should only be applied before or soon after initial infections occur).

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Mid-Atlantic Soybean Disease Scouting Guide

Figure 9.1 Soybean rust lesions on the upper leaf surface. Photo credit: N. Kleczewski, University of Delaware

Figure 9.2 Soybean rust lesions on the lower leaf surface. Photo credit: N. Kleczewski, University of Delaware

Figure 9.3 Soybean rust pustules with spores on lower leaf surface. Photo credit: N. Kleczewski, University of Delaware

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FOLIAR DISEASES

SOYBEAN VEIN NECROSIS Causal organism: Soybean vein necrosis virus (SVNV) (Tospovirus) Favorable conditions: Transmitted by thrips. When to scout: Flowering through pod filling. Impact on yield: Unknown. Symptoms and signs: • Leaves – small yellowish lesions scattered randomly along the veins (Fig 10.1); over time, the lesions will develop into larger, irregular-shaped lesions that are peppered with reddish-brown (Fig. 10.2); veins may be yellow or dark brown on the underside of the leaves. • Distribution – randomly distributed in field May be confused with: Bacterial blight, bacterial pustule, Cercospora leaf blight, downy mildew, Phyllosticta leaf spot, Septoria brown spot, nutrient deficiency, herbicide injury. Management: • No management strategies have been established for SVNV.

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Mid-Atlantic Soybean Disease Scouting Guide

Figure 10.1 Early SVNV lesions. Photo credit: N. Kleczewski, University of Delaware

Figure 10.2 Late SVNV lesions. Photo credit: N. Gregory, University of Delaware

33

FOLIAR DISEASES

TARGET SPOT Causal organism: Corynespora cassiicola Favorable conditions: Wet weather and cool soil temperatures (60-65°F). When to scout: Vegetative growth stages through pod fill (R6). Impact on yield: Little to none. Symptoms and signs: • Leaves – reddish-brown lesions often surrounded by yellow halos; concentric lesions may develop (Fig. 11.1); the center and the margin of the lesion is usually dark brown in color, while the circles in-between varies from tan to light brown; severe infection can result in leaf drop. • Stems and petioles – tiny spots or elongated dark areas. • Pods and seeds – small, sunken, dark colored with lighter margin; lesions can develop on seeds. • Distribution – starts in lower canopy, in patches within field. Management: • Resistant varieties. • High-quality, disease-free seeds. • Foliar fungicides (rarely economical).

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Mid-Atlantic Soybean Disease Scouting Guide

Figure 11.1 Target spot lesions. Photo credit: N. Kleczewski, University of Delaware

35

STEM AND ROOT DISEASES

ANTHRACNOSE STEM BLIGHT Causal organism: Colletotrichum truncatum Favorable conditions: Long periods of wet, warm weather. When to scout: Early pod development through maturity. Impact on yield: Minor impact to 20% yield reduction. Symptoms and signs: • Leaves – reddish discoloration of veins, leaf rolling, premature defoliation. • Stems and petioles – red to dark brown blotches during early reproductive stages; petiole infection can result in a shepherd’s crook (Fig. 12.1); near plant maturity, black fruiting bodies with black spines and spores are formed (Fig. 12.2). • Pods and seeds – early pod infections can result in poor (or no) seed development, later pod infection can cause seed infection; symptoms on seed include brown discoloration and small, gray areas covered with black specks. • Distribution – in small or large patches in the field. Management: • Resistant varieties. • High-quality, disease-free seeds. • Foliar fungicides (rarely economical).

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Mid-Atlantic Soybean Disease Scouting Guide

Figure 12.1 Shepherd’s crook caused by anthracnose stem blight. Photo credit: D. Mueller, Iowa State University, Bugwood.org

Figure 12.2 Stem with symptoms of anthracnose stem blight. Photo credit: D. Mueller, Iowa State University, Bugwood.org

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STEM AND ROOT DISEASES

BROWN STEM ROT Causal organism: Cadophora gregata (synonym: Phialophora gregata) Favorable conditions: Wet and cool conditions during early vegetative growth stages favor infection; foliar symptoms more severe when hot, dry conditions occur during late reproductive growth stages. When to scout: Flowering through pod fill. Impact on yield: Reduction in number and size of seeds can cause losses of up to 30%. Symptoms and signs: • Leaves – foliar symptoms occur during pod development; interveinal yellowing and browning (Fig. 13.1), leaf curling and leaf death. • Stem – externally, stems look healthy; when the stem is split longitudinally, the pith and vascular tissues display a brown discoloration, especially in the lower parts of the stem (Fig. 13.2). • Distribution – in small or large patches in the field. May be confused with: Charcoal rot, Phytophthora root and stem rot, stem canker, sudden death syndrome, herbicide injury, fungicide injury. Management: • Resistant varieties. • Non-host crop rotation (e.g. corn or small grains) and tillage to bury infested crop residue.

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Mid-Atlantic Soybean Disease Scouting Guide

INSERT FIG. 13.1, 13.2 Figure 13.1 Foliar symptoms of brown stem rot. Photo credit: H. Mehl, Virginia Tech Tidewater AREC

Figure 13.2 Browning of the pith caused by brown stem rot. Photo credit: M. Draper, USDA-NIFA, Bugwood.org

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STEM AND ROOT DISEASES

CHARCOAL ROT Causal organism: Macrophomina phaseolina Favorable conditions: Hot and dry weather, especially when the crop is drought-stressed during reproductive growth stages; charcoal rot is more severe in fields with soybean cyst nematode. When to scout: Beginning pod development (R3) through maturity. Impact on yield: Variable (highly dependent on weather conditions and soil type). Symptoms and signs: •General – reduced vigor, yellowing, wilting, premature death with leaves still attached (Fig. 14.1). • Stems and roots – tiny black specks called microsclerotia that resemble charcoal dust can be found in root and lower stem tissues (Fig. 14.2); microsclerotia may not be visible until plant maturity or death. • Distribution – patchy in field (driest parts of field, e.g. sandy areas, edges, compacted soils). May be confused with: Anthracnose stem blight, brown stem rot, Phytophthora root and stem rot, pod and stem blight, soybean cyst nematode, stem canker, sudden death syndrome, drought stress. Management: • Resistant varieties (partial resistance). • Minimize drought stress. • Non-host crop rotation (wheat; corn is a host for the charcoal rot fungus). • Manage soybean cyst nematode populations.

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Mid-Atlantic Soybean Disease Scouting Guide

INSERT FIG. 13.1, 13.2 Figure 14.1 Wilting caused by charcoal rot. Photo credit: H. Mehl, Virginia Tech Tidewater AREC

Figure 14.2 Microsclerotia in lower stem tissue. Photo credit: P. Bachi, University of Kentucky Research and Education Center, Bugwood.org

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STEM AND ROOT DISEASES

FUSARIUM WILT AND ROOT ROT Causal organism: Fusarium spp. Favorable conditions: Root rot typically favored by cool, wet soils; wilt more likely with hot, dry conditions; plant stress (herbicide injury, high soil pH, nematodes, nutrient deficiency) predisposes the crop to infection. When to scout: Planting through pod development. Impact on yield: Variable. Symptoms and signs: • General – pre- and post-emergence damping off; stunting and reduced vigor (Fig. 15.1); wilting and death (Fig. 15.2). • Roots – discolored (brown to black) roots, poor nodulation. • Distribution – patches in the field. May be confused with: Phytophthora root and stem rot, other seedling diseases, herbicide injury. Management: • Plant less susceptible varieties (resistant varieties not available). • Minimize plant stress. • Fungicide seed treatments.

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Mid-Atlantic Soybean Disease Scouting Guide

Figure 15.1 Stunting associated with Fusarium infection. Photo credit: M. Hansen, Virginia Tech, Bugwood.org

Figure 15.2

Wilting symptoms associated with Fusarium infection. Photo credit: D. Mueller, Iowa State University, Bugwood.org

43

STEM AND ROOT DISEASES

PHYTOPHTHORA ROOT AND STEM ROT Causal organism: Phytophthora sojae Favorable conditions: Poorly drained soil with a soil temperature greater than 60°F. When to scout: Emergence to pod filling. Impact on yield: Variable, potentially high with susceptible varieties and poorly drained soils. Symptoms and signs: • General – seed rot and pre- and post-emergence damping off, stunting, wilting, death (Fig. 16.1). • Leaves – chlorosis between veins, leaves wilt and die but remain attached. • Stems – water-soaked lesions, dark-brown lesion on lower stem extending up from the tap root that girdles the plant (Fig. 16.2). • Distribution – single plants or patches in the field. May be confused with: Anthracnose stem blight, brown stem rot, charcoal rot, Fusarium root rot and wilt, seedling diseases, stem canker, sudden death syndrome, herbicide injury. Management: • Resistant varieties (partial and complete to certain pathogen populations). • Improve soil drainage with tillage. • Fungicide seed treatments.

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Mid-Atlantic Soybean Disease Scouting Guide

Figure 16.1 Field view of wilting due to Phytophthora infection. Photo credit: D. Mueller, Iowa State University, Bugwood.org

Figure 16.2 Dark brown lesion on the lower stem, characteristic of Phytophthora root and stem rot. Photo credit: D. Mueller, Iowa State University, Bugwood.org

45

STEM AND ROOT DISEASES

POD AND STEM BLIGHT Causal organism: Diaporthe sojae Favorable conditions: Extended wet and warm (above 68°F) periods during the pod-development and maturation stages. When to scout: Full pod through maturity. Impact on yield: Seed decay can reduce yield and seed quality. Symptoms and signs: • Stems – small black dots (pycnidia) in linear rows on the stems of mature plants; pycnidia can also be observed on petioles and pods (Fig. 17.1). • Seeds – see Phomopsis seed decay. • Distribution – widespread in field. May be confused with: Anthracnose stem blight, charcoal rot. Management: • High-quality, disease-free seeds. • Non-host crop rotation (e.g. corn or small grains) and tillage to bury infested crop residue. • Foliar fungicides (R5 application, protects seed quality but does not impact yield).

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Mid-Atlantic Soybean Disease Scouting Guide

Figure 17.1 Pod and stems with pycnidia. Photo credit: Clemson University - USDA Cooperative Extension Slide Series, Bugwood.org

STEM AND ROOT DISEASES

SCLEROTINIA WHITE MOLD Causal organism: Sclerotinia sclerotiorum Favorable conditions: Shaded, moist and cool to moderate temperatures When to scout: Beginning pod development (R3) through maturity. Impact on yield: Not common in the Mid-Atlantic, but potential for significant yield loss. Symptoms and signs: • Leaves – gray-green discoloration between veins, leaves die and turn brown but remain attached to stems. • Stems – water-soaked lesions, infected stems may become bleached as lesions spread; fluffy white fungal growth (Fig. 18.1); formation of black survival structures (sclerotia) on and inside the stem (Fig. 18.2). • Pods and seeds – white fungal growth and black sclerotia can be observed inside pods. • Distribution – small to large patches. May be confused with: Downy mildew (seed), Phomopsis seed decay, brown stem rot, charcoal rot, Phytophthora root and stem rot, stem canker, sudden death syndrome. Management: • Resistant varieties (moderate resistance). • Non-host crop rotation (e.g. corn or small grains) and no-till reduce number of viable sclerotia in soil. • Foliar fungicides (R1 through R3 growth stages, applied prior to infection). • Control of weed hosts. 48

Mid-Atlantic Soybean Disease Scouting Guide

Figure 18.1 Fluffy white fungal growth of Sclerotinia on a soybean stem. Photo credit: N. Kleczewski, University of Delaware

Figure 18.2 Black sclerotia produced by Sclerotinia inside a soybean stem. Photo credit: N. Kleczewski, University of Delaware

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STEM AND ROOT DISEASES

SEEDLING DISEASES

(RHIZOCTONIA ROOT AND STEM ROT AND PYTHIUM SEEDLING BLIGHT)

Causal organism: Rhizoctonia solani, Pythium spp. Favorable conditions: Sandy, dry soil (Rhizoctonia); wet, poorly drained soil (Pythium). When to scout: Variable. Symptoms: Rhizoctonia root rot is characterized by reddishbrown lesions on the hypocotyl near the soil line. It causes post-emergence damping off where the seedlings are killed after the infection girdles the hypocotyl. Damping off caused by Rhizoctonia usually occurs in small patches or in rows. Symptoms and signs: • General – pre- and post-emergence seedling blight and damping off (Fig. 19.1), reduced vigor and stunting. • Hypocotyl – reddish-brown lesions near soil line that may girdle plant (Fig. 19.2) (Rhizoctonia); water-soaked lesions that develop into a brown soft rot (Pythium). • Roots – discolored, rotten roots. • Distribution – single plants or patches in field. May be confused with: Fusarium wilt and root rot, Phytophthora root and stem rot, other seedling diseases, herbicide injury. Management: • Minimize stress to plant. • Avoid planting into cold, wet soil. • Fungicide seed treatments. 50

Mid-Atlantic Soybean Disease Scouting Guide

Figure 19.1 Seedling damping off. Photo Credit: D. Mueller, Iowa State University, Bugwood.org

Figure 19.2 Reddish-brown lesions on roots and hypocotyls caused by Rhizoctonia infection. Photo credit: M. Hansen, Virginia Tech, Bugwood.org

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STEM AND ROOT DISEASES

STEM CANKER

(NORTHERN AND SOUTHERN)

Causal organism: Northern Stem Canker – Diaporthe caulivora, Southern Stem Canker – Diaporthe aspalathi Favorable conditions: Extended periods of wet weather early in the growing season and moderate temperatures (70-85°F). When to scout: Early reproductive stages through maturity. Impact on yield: Significant yield loss if the disease is widespread and results in premature plant death. Symptoms and signs: • General – wilting, death (Fig. 20.1). • Leaves – interveinal chlorosis and necrosis. • Stems – small, reddish-brown lesions near the nodes on the lower portion of the stem; lesions expand to form sunken cankers (Fig. 20.2); northern stem canker can girdle the stem, southern stem canker develops on one side of the plant; some plants develop black “zone lines” within the lower portions of the stem. • Distribution – patches within field, sometimes entire fields are affected. May be confused with: Anthracnose stem blight, brown stem rot, charcoal rot, Phytophthora root and stem rot, sudden death syndrome, herbicide injury, fungicide injury. Management: • Resistant varieties (southern stem canker only); avoid highly susceptible varieties. • Delay planting until temperatures are warmer (southern stem canker). • Tillage to bury infested crop residue. 52

Mid-Atlantic Soybean Disease Scouting Guide

Figure 20.1 Field view of plants wilting and dying due to stem canker infection. Photo credit: H. Mehl, Virginia Tech Tidewater AREC

Figure 20.2 Stem canker lesion. Photo credit: D. Mueller, Iowa State University, Bugwood.org

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STEM AND ROOT DISEASES

SUDDEN DEATH SYNDROME

(SDS)

Causal organism: Fusarium virguliforme Favorable conditions: High soil moisture and cool temperatures at planting; rainfall during early reproductive growth stages favors development of foliar symptoms; soil compaction and soybean cyst nematode increase SDS severity. When to scout: During pod development. Impact on yield: Little to almost complete yield loss depending on growth stage at disease onset and disease severity. Symptoms and signs: • Leaves – small, mottled interveinal chlorotic spots on plants in an area; as the disease develops, the whole leaf will exhibit interveinal chlorosis and necrosis (Fig. 21.1); severe infection results in rapid premature defoliation and death of the plant; petioles remain attached to the stem (Fig. 21.2). • Roots – rotted taproots and lateral roots; blue fungal growth on surfaces of roots (Fig. 21.3). • Stems – when split open, may show brown discoloration in the woody tissue extending up to 8 inches above the soil line. • Distribution – small to large patches in the field. May be confused with: Soybean vein necrosis virus, brown stem rot, charcoal rot, Phytophthora root and stem rot, root-knot nematode, soybean cyst nematode, stem canker, nutrient deficiency, herbicide injury, fungicide injury. Management: • Resistant varieties (partial resistance to SDS and resistance to SCN). • Delay planting until soils are warmer. • Non-host crop rotation (small grains; rotation with corn is not effective). • Seed-treatment fungicides and/or nematicides. • Manage SCN populations. 54

Mid-Atlantic Soybean Disease Scouting Guide

Figure 21.1 Foliar symptoms of SDS. Photo credit: N. Kleczewski, University of Delaware

Figure 21.2 Petioles attached to the stem after leaves have dropped due to SDS infection. Photo credit: N. Kleczewski, University of Delaware

Figure 21.3 Blue fungal growth on the surface of roots. Photo credit: D. Mueller, Iowa State University, Bugwood.org Missouri Soybean Disease Field Guide

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NEMATODES

ROOT-KNOT NEMATODE Causal organism: Meloidogyne spp. Favorable conditions: Sandy soils, drought conditions. When to scout: Late vegetative growth stages through pod development. Impact on yield: High yield-loss potential if not managed Symptoms and signs: • General – patches of decreased vigor, wilting, yellowing, stunting and death. • Roots – large, irregular-shaped swellings (galls) (Fig. 22.1). • Distribution – small to large patches in the field. May be confused with: Soybean cyst nematode, nutrient deficiency, drought stress. Management: • Resistant or tolerant varieties (RKN species-specific). • Non-host crop rotation (need to know RKN species in field). • Sanitize field equipment to avoid spread of inoculum. • Control weed hosts. • Seed treatment nematicides.

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Mid-Atlantic Soybean Disease Scouting Guide

Figure 22.1 Root galling due to infection with RKN. Photo credit: N. Kleczewski, University of Delaware Missouri Soybean Disease Field Guide

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NEMATODES

SOYBEAN CYST NEMATODE (SCN) Causal organism: Heterodera glycines Favorable conditions: Light, sandy soils, high-pH soils. When to scout: Late vegetative growth stages through pod development. Impact on yield: High yield loss potential if not managed. Symptoms and signs: • General – patches of stunting, yellowing, decreased vigor. • Roots – discolored and stunted, reduced nodulation, presence of cysts (small, lemon-shaped white to brown SCN females) (Fig. 23.1). • Distribution – small to large patches in the field. May be confused with: Root-knot nematode, charcoal rot, sudden death syndrome, nutrient deficiency, drought stress. Management: • Resistant varieties (need to know race or HG type of SCN population). • Non-host crop rotation (e.g. corn, cotton, small grains). • Sanitize field equipment to avoid spread of inoculum. • Control weed hosts. • Seed treatment nematicides

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Mid-Atlantic Soybean Disease Scouting Guide

Figure 23.1 Cysts on roots infected with SCN. Photo credit: E. Bush, Virginia Polytechnic Institute and State University, Bugwood.org Missouri Soybean Disease Field Guide

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Technical editing for this guide was led by researchers from University of Delaware, University of Maryland and Virginia Tech. The United Soybean Board neither recommends nor discourages the implementation of any advice contained herein, and is not liable for the use or misuse of the information provided.

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Mid-Atlantic Soybean Disease Scouting Guide