Diseases Flashcards
When soybean is susceptible to Anthracnose?
All stages of development, but more commonly during bloom and podfill (spores spread from infected plant residue)
Implications of seed infected with Anhtracnose
- Early disease development
- Dark brown lesions develop on cotyledons
- Stem may collapse
- Damping off
Regular symptoms of Anthracnose
Irregularly-shaped brown blotches on stems, pods, and leaf petioles
Severe symptoms of Anthracnose
Leaves: leaf rolling, premature defoliation, and stunted plants
Pods: shriveled pods with less seeds, moldy seed, or no seed
Symptoms of Anthracnose-infected soybean stem
Randomly distributed lesions are covered by small black dots (acervuli)
Fungus that causes Antracnose in Soybeans
Colletotrichum truncatum
How does Colletotrichum truncatum overwinter/survive?
Overwinter as mycelia on crop residue or infected seed
Weather that favors Anthracnose
- Wet, warm, humid weather
- Infection may occur when leaf wetness, rain, or dew periods exceed 12 hours/day.
Agents that spread the spores of Colletotricun truncatum
Wind and rain
Management of Anthracnose
- Crop rotation (reduces inoculum on soil)
- Tillage (bury crop residues)
- Foliar fungicides applied between the R3 and R5
Fungus that causes Brown Stem Rot (BSR) in Soybeans
Phialophora gregata (strain A is highly aggressive, and strain B is mild)
Time for BSR infection and for symptoms
- Infection: early in the season
- Symptoms of vascular system damage: during reproductive development
Favorable conditions to BSR
- Between 60 and 80 F -
- Little or no disease develops above 90 F
- Soil moisture near field capacity (also optimal for crop development)
- Drought stress during pod fill
- soil pH near to 6.0
Where does Phialophora gregata survives?
- Infected soybean residue left on soil surface
- Does not produce survival structures
What influences inoculum levels of BSR in the spring?
Fungus can continue to reproduce throughout the winter
When and how occur infections of new soybeans?
Occurs through roots (by growth stage V3) and progresses to stem
Impact of BSR on soybeans
- Infection causes a gradual disruption of the vascular system
- Premature plant death may occur, especially if heat and drought stress impacts badly diseased plants
Management of BSR
- Resistant Varieties
- Crop Rotation: Effective in reducing disease inoculum - 2 years away from soybeans
- Tillage
- Manage SCN
- Sanitation of equipment is not needed
Fungus that causes Cercospora Leaf Blight in Soybeans
Cercospora kikuchii
Favorable conditions to CLB
Humid conditions and temperatures of 75 to 80 F or higher
Which stage CLB occurs?
Generally occurs during pod-filling stages
Impact of CLB in soybean
- Early infection: plants may lose their cotyledons, become stunted, or die.
- During cycle: loss of leaf tissue or entire leaves may occur.
- Defoliation may reduce yield if disease occurs early relative to pod fill.
Where does Cercospora k survives?
Survives as mycelia on soybean residue and on the seed coat of infected seed.
Conditions to sporulation of Cercospora k
High humidity and temperatures of 75 F or higher. Sporulation increases as temperatures rise above 80 F.
Agents that spread the spores of Cercospora k
Wind and water
Sporulation from lesions results in secondary infections.
How does Cercospora k infect seeds?
Seeds become infected when the fungus invades the pod and grows through the upper vein. The hilum and eventually the seed coat become infected.
Symptoms of Cercospora k em Soybeans
- Sun-exposed leaves on the upper part of the plant develop a bronze to reddish-purple discoloration.
- Discoloration results from numerous irregular-shaped lesions
- Lesions form large necrotic blotches as the disease progresses and lesions merge.
- As plants mature, infected leaves develop a leathery appearance.
- Severely affected upper leaves may drop but the petioles remain on the plant; lower leaves of the plant remain green and attached
Symptoms of Cercospora k in seeds (Purple Seed Stain)
- Lesions and a purplish discoloration are symptoms of infected pods.
- Seeds are infected through their attachment to the pod, the hilum.
Management of Cercospora k
- Crop rotation: 1- to 2-year rotation to corn or small grains will reduce inoculum levels (not legumes).
- Tillage
- Seed treatment
- Strobilurins, triazoles and mixtures of both.
Fungus that causes Charcoal Rot in Soybeans
Soil fungus Macrophomina phaseolina
What are other names to Charcoal Rot
- Summer wilt
- Dry-weather wilt
Optimum soil temperature for Macrophomina phaseolina
80 to 95 F
How does Macrophomina overwinter/survive?
- Dry soil or residue of host plants
- The fungus can survive on available nutrients in plant debris or in the soil, or by infecting alternate hosts
- Microsclerotia (hardened fungal survival bodies) are formed in infected tissue
When does infection by Macrophomina occur?
Typically occurs early in the spring when soil moisture is high
When does Charcoal Rot disease manifest?
Disease can manifest early if weather is hot and dry, but symptoms usually occur later in the summer during reproductive growth stages.
Symptoms of Charcoal Root
- Smaller than normal leaves, reduced vigor, premature yellowing of top leaves and plants wilting during the midday heat.
- A light gray discoloration develops on the surface tissues of the roots and lower stem
- Plants reach the permanent wilt point and die prematurely
Impact of Charcoal Root in Soybean
- Root mass is reduced, decreasing drought tolerance
- Plants have smaller leaflets and reduced vigor
- Wilting of plants is common; premature death may occur
- Pods abort and seeds are fewer and lighter, reducing yield
- Seed may crack and shrivel and microsclerotia may form in fissures
Management of Charcoal Root
- Crop rotation: up to 2 years
- Irrigation
- Seeding and Fertility: Avoid excessive seeding rates and maintain adequate soil fertility levels
- Reduced Tillages: may help conserve moisture
- Weed Management: alternate hosts and competition for available soil moisture
Fungus that causes Frogeye Leaf Spot in Soybeans
Cercospora sojina
Favorable conditions for development of Frogeye
- Warm, humid conditions and frequent rains
- Dry weather severely limits disease development
Where does Cercospora s survives?
Soybean residue and seeds
Source of spores of Cercospora
- Infected residues or cotyledons (provides inoculum to continue disease cycle in next soybean crop)
- Lesions on the soybean (secondary infections)
Leaf Symptoms of Frogeye Leaf Spot
- Dark, water-soaked spots develop into lesions with dark brown centers surrounded by red or dark reddish-brown margins
- As lesions age, the center becomes light brown to light gray, and the border remains dark
Stem, Pod and Leaf Symptoms of Frogeye Leaf Spot
- Lesion development on pods is similar to that of the leaves
- Seeds may show cracking of the seed coat and discoloration
Management of Frogeye Leaf Spot
- Resistant varieties
- Tillage
- Crop rotation
- Foliar fungicides
Order of most important diseases
Soybean cyst nematode, sudden death syndrome, and Phytophthora root
When does Fusarium occur?
After warm, humid weather
Symptoms of Fusarium
- Browning or blackening of the vascular system in roots and stems
- Flaccid leaves and wilting of stem tips are most common on young plants
- Pods of infected plants are often poorly developed, but root rot is minor
Management of Fusarium
- Grow resistant varieties
- Plant high-quality seed in warm well-drained soil
- Delay cultivation until soil moisture is adequate
Fungus that causes Phytophthora Root Rot and Stem Rot in Soybeans
Soilborne fungus Phytophthora sojae (also known as Phytophthora megasperma f. sp. glycinea)
When Phytophthora s attacks soybean
Any time during the growing season
Conditions favoring Phytophthora s
- Wet soil conditions: commonly occurs on heavy, poorly drained or compacted soils
- Ideal temperature for infection is 60 to 80 F
- Successive years of soybeans
- Application of high levels of potash, manure or municipal sludge immediately before planting may increase disease severity
