GREY LEAF SPOT (GLS)

Symptoms

Lesions tan becoming grey as they mature; lesions typically rectangular, limited by and running parallel to the veins; starts on lower leaves progressing upwards.

General Information

Moderate to high temperatures and high humidity (>95%) favour disease development. The increased adoption of reduced tillage has increased the frequency and severity.

Causal Organism

Fungus: Cercospora zeina

NORTHERN CORN LEAF BLIGHT (NCLB)

Symptoms

Lesions elongate becoming elliptical or cigar-shaped; grey-green becoming tan in colour with distinct dark zones of fungal sporulation; lesions often surrounded by pale green, water-soaked border.

General Information

NCLB is favoured by moderate temperatures between 18-27°C and prolonged periods of leaf wetness. Conidia are produced on old residue and are carried by the wind or rain to lower leaves of young maize plants.

Causal Organism

Fungus: Exserohilum turcicum (Setosphaeria turcica)

PHAEOSPHAERIA LEAF SPOT

Symptoms

Bleached, dried, circular to elongate spots with dark brown margins, similar to paraquat herbicide (trade name – Gramoxone) damage; pinpoint, black fruiting bodies develop within lesions.

General Information

Lesions are usually first visible on the edges of maize fields or on the top leaves of plants. Usually occurs towards the end of the season as winter approaches and cold conditions prevail.

Causal Organism

Fungus: Phaeosphaeria maydis and Phoma sorghina

EYESPOT

Symptoms

Small, circular to oval lesions with tan-cream centres surrounded by a brown-purple margin and distinct yellow halo. Easily confused with physiological changes and genetic leaf spots.

General Information

Favoured by long periods of cool (10-12°C), moist weather during the growing season. Conidia are dispersed by wind or light rain to the leaves of nearby young maize plants.

Causal Organism

Fungus: Aureobasidium zeae = Kabatiella zeae

DIPLODIA LEAF STREAK

Symptoms

Brown lesions with characteristic yellow halo; develop from an oval centre extending along the veins; black fruiting bodies develop within the centre of the lesion.

General Information

More prevalent in warm, humid regions. Unbalanced fertility, low K, poor drainage, mechanical and insect damage, cultivar and planting density all influence disease severity.

Causal Organism

Fungus: Stenocarpella macrospora = Diplodia macrospora

BACTERIAL LEAF STREAK

Symptoms

Lesions begin at edges or midrib of leaves; lesions enlarge becoming elongate, irregular spots or streaks which are brown surrounded by a yellow border; lesions run parallel to and are confined by the leaf veins.

General Information

Within fields, it is likely that X. campestris pv. zeae is most effectively disseminated by wind-driven rain or irrigation, but the pathogen may also be disseminated by aphids, wind and plant-to-plant contact.

Causal Organism

Bacteria: Xanthomonas campestris pv. zeae

GOSS'S BACTERIAL WILT AND LEAF BLIGHT

Symptoms

Early leaf symptoms are oblong or elongated lesions of water soaked, greyish-green tissue that progress to long dead streaks. Dark green to black water-soaked spots, called freckles, often develop near lesion edges. A shiny exudate is visible on the lesions where bacteria  oozes out on the leaf surface and may appear shiny after drying to form a glistening residue, or varnish. Plants may also be infected systemically by Goss’s wilt, especially in the seedling stage. These plants have discolored vascular tissue with a slimy bacterial exudate in the stalk.

Plants are commonly stunted and wilt and die as if drought stressed.

General Information

Bacterial invasion requires an injury to the plant often caused by wounding from wind, sandblasting and especially hail which provide openings for bacteria. Infection is spread through the canopy by rain splash, irrigation and wind.  Goss’s wilt overwinters in residue of maize and several grass species. 

Causal Organism

Bacterium: Clavibacter nebraskensis (Clavibacter michiganensis subsp. nebraskensis)

MAIZE STREAK DISEASE (MSV)

Symptoms

Long streaks parallel to the leaf veins developing to alternating dark and light green variegated stripes; stunting; infected plants yellow-green colour; deformed cobs.

General Information

MSV is only vector transmitted, primarily by Cicadulina spp. leafhoppers of which C. mbila, is the most prevalent. MSV is not transmitted mechanically, by pollen or via seed.

Causal Organism

Virus: Maize streak virus

COMMON RUST

Symptoms

Pustules form on both upper and lower sides of leaf; brownish-red oblong pustules; rust band formed close to the base of leaf.

General Information

Moderate temperatures (16-25°C) and high relative humidity (>95%), associated with mist or dew for prolonged periods, favour rust development and spread.

Causal Organism

Fungus: Puccinia sorghi

POLYSORA RUST

Symptoms

Light orange to cinnamon-red, circular to oval raised pustules which are densely scattered over the upper leaf surfaces.

General Information

Favoured by warm and humid (extended dew or high humidity) conditions, with an optimal temperature range between 24-28°C. In temperate regions, Polysora rust occurs towards the end of the season.

Causal Organism

Fungus: Puccinia polysora

SORGHUM DOWNY MILDEW

Symptoms

Infected sometimes stunted; white-striped leaves; leaf chlorosis includes base leaf; transverse margin sharply defining diseased and healthy tissue (‘half-leaf symptom’); leaves narrower and more erect.

General Information

Oospores survive in soil and attack susceptible seedlings through underground parts, resulting in systemic infected. Requires a saturated atmosphere or free water and moderate temperatures of 21-25°C.

Causal Organism

Fungus: Peronosclerospora sorghi

CRAZY TOP

Symptoms

The most characteristic symptom is the partial or complete replacement of normal floral parts by small leaves.

General Information

Oospores present in soil are the main inoculum source of S. macrospora. Infection occurs when maize plants are subjected to saturated soil conditions for 24 to 48 hours from planting to about the 5-leaf stage of growth. Pooling of soil and water in the whorl of small plants can also lead to infection. Infection occurs at a wide range of soil temperatures.

Causal Organism

Fungus: Sclerophthora macrospora

HEAD SMUT

Symptoms

Floral structures partially or completely replaced by sac-like structures containing masses of black spores; tassel proliferates to form bizarre leafy structures; ears are smutted or aborted, small, round to teardropshaped, with no evidence of a cob or kernels inside and lack silks.

Causal Organism

Fungus: Sphacelotheca reiliana

BOIL SMUT

Symptoms

Galls replace kernels of ear; galls semi-fleshy, initially firm and silver-grey then turn black; membrane ruptures releasing brown powdery spores.

General Information

The fungus can survive for long periods of time in the soil or crop residues. Spores are carried by air currents or splashed by water to young, actively growing plant parts. Infection normally takes place through the silks or wounds created by hail, insect feeding or other mechanical injury to any part of the plant.

Causal Organism

Fungus: Ustilago zeae (Ustilago maydis)

DIPLODIA EAR ROT

Symptoms

Dense whitish fungal growth will be matted between the kernels and between the ear and the husks; begin at base of ear progressing toward the tip; small, black fungal fruiting bodies embedded in the cob tissue and kernel bases.

General Information

Dry weather early in the season followed by abnormally wet conditions just before and after silking favours ear infection, while a wet early season followed by drier conditions or heat stress later is likely to result in more Diplodia stem rot. Ears are most susceptible from silking to about three weeks later. Hybrids with poor husk cover or thin pericarps are often very susceptible. 

Causal Organism

Fungus: Stenocarpella maydis ( = Diplodia maydis)

DIPLODIA STALK ROT

Symptoms

Brownish pith discolouration at basal internodes; black specks embedded in rind tissue near rotted internodes; pith disintegrates, weakening stalk predisposing plants to lodging.

General Information

Infection usually takes place 2-3 weeks after silking under favourable conditions. Wet, early season followed by drier conditions or heat stress is likely to result in more widespread Diplodia Stem Rot.

Causal Organism

Fungus: Stenocarpella maydis (= Diplodia maydis)

GIBBERELLA EARROT

Symptoms

Pink to reddish mould usually starting at the tip of the ear; mould growth developing between husks and ears; superficial black specks develop on the husks and ear shanks.

General Information

Maize ears are generally susceptible only when they are very young. Cool, wet weather within three weeks of silking favours disease development. The fungus infects silks, grows into the ear progressing downward during grain fill.

Causal Organism

Fungus: Gibberella zeae (Fusarium boothii)

GIBBERELLA STALK ROT

Symptoms

Root and stem lodging; lower internodes turn straw-coloured, internal reddish discolouration, superficial black fruiting bodies on stalk, often at the internodes.

General Information

Favoured by cooler, moist weather. Stalk infection usually occurs shortly after pollination, developing at the origin of the leaf sheaths or around the prop roots.

Causal Organism

Fungus: Gibberella zeae (Fusarium graminearum)

FUSARIUM EAR ROT

Symptoms

Dense whitish fungal growth will be matted between the kernels and between the ear and the husks; begin at base of ear progressing toward the tip; small, black fungal fruiting bodies embedded in the cob tissue and kernel bases.

Causal Organism

Fungus: Fusarium verticilliioides = F. moniliforme (Gibberella fujikuroi)

FUSARIUM STALK ROT

Symptoms

Premature senescence and lodging; whitish-pink pith discolouration often confused with the red colour of Gibberella stalk rot; shredding of internal stalk; black fruiting bodies absent.

General Information

F. verticilioides is most common in warm, dry areas and typically occurs in complexes with other root and stalk rots. Infection of plants facilitated by wounds caused by hail or insects.

Causal Organism

Fungus: Fusarium verticilioides = F. moniliforme (Gibberella fujikuroi)

CHARCOAL ROT

Symptoms

Black discolouration of lower stem internodes; shredding of interior; numerous minute black specks (microsclerotia) visible on vascular strands.

General Information

High soil temperatures (30-42°C) and low moisture or when unfavourable environmental conditions stress the plant, especially during postflowering period.

Causal Organism

Fungus: Macrophomina phaseolina

ERWINIA

Symptoms

Sudden wilt and lodging; water-soaked, dark areas at base of stalk; most distinctive symptom of Erwinia infection is the foul smell of diseased, slimy plant parts.

General Information

Xanthomonas vasicola pv. vasculorum
 

Causal Organism

Bacteria: Erwinia pectobacterium pv. zeae. (before Erwinia chrysanthemi pv. zeae.)

RED SPIDER MITE

Symptoms

Leaves show yellowing, stippling (tiny white or yellow spots), and bronzing on leaves.  Fine silk webs is visible on the underside of leaves, where mites cluster. Severe infestations cause stunted growth, leaf drying and reduced grain filling.

General Information

Red spider mites attacks maize, especially under hot and dry conditions. These mites feed on plant sap, causing yellowing, wilting, and reduced photosynthesis, which leads to lower yields. Mites thrive in drought-stressed maize fields. An insecticide application often kills natural predators causing red spide mite infestations. 

Causal Organism

Red spider mite is a tiny arachnid pest. Tetranychus urticae (Two Spotted Spider Mite) is the most common on maize in South Africa.

STRIGA

Symptoms

Striga plants are visible around the base of the maize plants and produces reddish flowers (hence the name Rooiblom in Afrikaans). Striga attaches to the roots of maize, siphoning off water, nutrients, and carbohydrates, leading to stunted growth and reduced yields.

General Information

Its seeds remain dormant in the soil and only germinate when stimulated by compounds released from maize roots. A single Striga plant can produce thousands of tiny seeds, which can remain viable in the soil for years. Infestations can cause yield losses ranging from 20% to total crop failure, especially in nutrient-poor soils.

Causal Organism

Striga, also known as witchweed, is a parasitic plant that severely affects maize. It belongs to the Striga genus, with Striga asiatica being the most destructive species for maize in Africa.

GENETIC STRIPE, LEAF SPOTS AND FLECKS

Symptoms

Some genes alter or limit the ability of the maize plant to produce chlorophyll and result in albino plants or plants with white to yellow stripes or other variegations. Some leaf spots and mosaic patterns mimic symptoms caused by infectious diseases.

Causal Organism

Genetic disorder – genes cause abnormal morphology and colour pattern in plants.

SILK JAMMING

Symptoms

Stresses slow silk elongation causing a delay or failure of silks to emerge from the ear shoot. If the delay is long enough, pollen shed may be almost or completely finished before receptive silks are available resulting in nearly blank or totally blank ears.
 

General Information

Result from heat and/or moisture stress or other stresses before or during pollination. Excessive plant populations and nutrient deficiency may also encourage this disorder. This phenomenon is also common in hybrids with tight husks when cold night temperatures are followed by warm nights during silking.

Causal Organism

SILK-CUT

Symptoms

Silk-cut is characterised by a horizontal crack over the face of the kernel. The name silk-cut is probably derived from silks, which often become embedded in ruptured areas on the tip of the kernel or occasionally on sides of kernels between the kernel rows.

General Information

As in the case of popping, the exposed starch also gets colonised by various fungi. This secondary colonisation enhances the kernel damage. This phenomenon is rarely seen on commercial hybrids.

Causal Organism

A dry period followed by wet weather, before full maturity, results in resurgence of deposition of dry matter in the kernel – this stresses the seed coat which cracks and breaks. Another theory is that viable silks of unpollinated ovules push against the developing pericarp of kernels and may also cause this rupturing.

POPPING

Symptoms

Popped kernel is an irregular break in the seed coat over the kernel crown. The kernel resembles a partially expanded popcorn kernel. The crack in the seed exposes starch providing the ideal substrate for secondary, saprophytic fungi e.g. Fusarium, Aspergillus and Penicillium spp.

General Information

Popping seems more prevalent in genotypes with hard, flinty kernels. It is common in breeders’ nurseries, but rarely seen on commercial hybrids.

Causal Organism

Popping is believed to be the result of irregular growth, especially since it is common in years with irregular rainfall and particularly when conditions are very hot and dry.

PRE-GERMINATION

Symptoms

Pre-germination of kernels occurs during the latter part of grain filling and is normally only noticed at harvesting.

General Information

Pre-germination has been observed in certain seasons in KwaZulu-Natal and Mpumalanga.

Causal Organism

Predisposition to premature germination has been shown to be genetically controlled and is also affected by various environmental factors. This phenomenon normally occurs during seasons with low sunlight hours, lots of overcast weather and low heat units. In addition, high temperatures during the later stages of grain growth may break embryo dormancy allowing kernels to germinate on the ear if rain occurs around harvest time. Some ear diseases, such as Diplodia Ear Rot, may induce pre-germination of kernels. Furthermore, molybdenum deficiencies in the grain may also be responsible, the severity of which is enhanced by heavy, late dressings of nitrogenous fertiliser.

PINKING

Symptoms

The most characteristic symptom is a general light-pink superficial discolouration of the cob as a result of the production of a red pigment, anthocynin. The discolouration is more noticeable when the cobs are exposed to sunlight. The discolouration is limited to the pericarp, and does not affect meal colour after milling. The pink discolouration may be confused with Fusarium spp.

General Information

Certain white hybrids are more prone to pinking under specific climatic conditions. Grain may be downgraded at silos and milling plants, having direct financial implications.

Causal Organism

Pinking is a physiological disorder resulting from environmental conditions, occurring sporadically. Limited information is available on the specific temperature and humidity that predispose cobs to pinking.

FALSE CHINCH BUG

Symptoms

Crops are damaged from March onwards when headlands or adjacent weedy crop fields, hosting the bug, are cultivated. Insects move from weeds during senescence and die-off prior to winter, to late planted sunflower during seed fill. Developing sunflower seeds are pierced and the contents are sucked from it. After seed-set, feeding reduces grain yield and seed germination, while oil content of seed can also be reduced.

Notes on disease:

• Outbreaks in South Africa sporadic and often serious.
• Well adapted and widely distributed. Wide range of wild hosts.
• Able to survive, develop and reproduce in areas with a wide range of temperatures and rainfall.
• Control: Integrated management - use of cultural practices and chemical control.

Causal Organism

Insect: Nysius natalensis

CABBAGE SEMI-LOOPER

Symptoms

The larvae feed mainly on the leaves of plants. If large numbers are present, defoliation may occur and the stems of the flower buds may be damaged. Damage does not occur on the edges of fields and is therefore not visible from outside the field.

Notes on disease:

• It is a very sporadic pest and damage is of economic importance only when large numbers occur and plants are defoliated.
• Control: No insecticides are registered in RSA.
  

Causal Organism

Insect: Trichoplusia orichalcea (Noctuidae)

AFRICAN BOLLWORM

Symptoms

Larvae of the African bollworm (Noctuidae) are regularly present during the reproductive stage of cultivated sunflower. They feed on leaves, bracts, florets, pollen and achenes and burrow into the receptacle. Sunflower plants can tolerate large numbers of larvae, as the preferred feeding sites of larvae are not the florets and achenes.

Notes on disease:

• Plants have the ability to compensate for losses of florets and achenes of up to 20%, damage therefore rarely of economic importance.
• Control: Various insecticides are registered.
  

Causal Organism

Insect: Helicoverpa armigera (Noctuidae)

ASTER YELLOWS

Symptoms

Most distinct symptom is a wedge-shaped sector in the sunflower head where abnormal growth took place, no seed is set and leaves form in these sectors.

Notes on disease:

• Rarely observed and presence in South Africa not confirmed.
• Mycoplasmas are transmitted by leafhoppers.
• Control: Insect control and use of resistant hybrids.
  

Causal Organism

Mycoplasmas

SUNFLOWER CRINKLE VIRUS

Symptoms

Virus causes distortion (crinkle) of the leaves, irregular yellow vein banding, chlorotic blotches and reduced leaf size. The affected plants normally become stunted.

Notes on disease:

• Not very common has been observed in Kenya, Uganda, Malawi and other tropical areas.
• Virus transmitted by insect vectors.
• Control: Insect control.
  

Causal Organism

Virus: Possibly Umbravirus

PHYTOPTHORA STEM ROT

Symptoms

Black, sunken lesions girdling the base of the stem. The pith eventually rots causing lodging.

Notes on disease:

• Not very common.
  
• In heavy soils following over-irrigation.
• Control: Cultural practises include irrigation management.

Causal Organism

Fungus: Phythopthora cryptogea

CHARCOAL ROT

Symptoms

Symptoms only observed at flowering when leaves turn yellow and plant wilts. Silvery-grey stem lesions develop at the soil surface. Mircosclerotia (visible with a hand lens) form on the outside of the taproot and within the lower stem. Split open lower stalk to reveal unique compression of pith into layers. Pith tissue dries up resulting in premature senescence and poor head fill.

Notes on disease:

• Associated with hot, dry stress conditions e.g. soil temps 30-35⁰C.
• Major disease of many other crops including soybeans and maize.
• Control: Cultural practises - minimising stress, esistant hybrids and rotation.
  

Causal Organism

Fungus: Macrophomina phaseolina

RHIZOPUS HEAD ROT

Symptoms

Brown sunken water-soaked lesions develop on the back of the flower. The interior of the head becomes soft and filled with grey or dirty-white mycelium. Characteristic black pepper grain-like spore-bearing structures are visible in mycelium. Head eventually rots completely and dries up (mummifies).

Notes on disease:

• Rhizophus common in soils and rotting plant material.
• Spores spread by water and wind. Any physical damage (by insects, hail, birds) provides a wound for this weak fungus to enter the head.
• Control: Controlling insects and birds will minimise if not control infection.

Causal Organism

Fungus: Rhizophus microsporus

BOTRYTIS HEAD ROT

Symptoms

Often associated with water pooling in the back of the head. Sunken brown spots develop on the back of the head and no sporulation inside the head distinguishes it from Rhizopus. Heads are distinctly covered by a grey mouldy growth. These hulls of seeds from infected heads are highly flammable. At maturity, the heads may shred giving them a broom-like appearance. Check for sclerotes to help avoid confusion with sclerotinia head rot.

Notes on disease:

• Common in all sunflower growing areas.
• Occurs during wet cool conditions.
• Control: Resistant hybrids. Chemical control ineffective. Desiccating the crop at 30% moisture could prevent late season infection.

Causal Organism

Fungus: Botrytis cinerea

DIAPORTHE STEM CANKER

Symptoms

First symptoms seen prior to flowering on lower/middle leaves. Infection starts at leaf margin, a necrotic area with chlorotic border moves up the leaf to the mid-vein, resulting in a typical V-shape. Infection progresses up the vein to the petiole and stem. Stem lesions are brown, oblong, centred on the leaf axil and may be dotted at each node. Lesions range from caramel to dark brown, may have water soaked appearance and sometimes a black edge. Pith damage can lead to lodging and/or early senescence. At maturity, Phomopsis lesions can develop silvery-grey centres.
Disease often confused with Phoma which has very black and more oval lesions.

Notes on disease:

• Sunflower debris is main source of infection. Infection favoured by frequent rain during the growing season. Lesions only seen from flowering onwards.
• Seed infection can occur in infected crops. The role of seed transmission low.
• Control: Fungicides, resistant hybrids, crop rotation and tillage to bury residues.

Causal Organism

Fungus: Diaporthe helianthi, Diaporthe gulyae (Previously Phomopsis). Other species may be involved.

BLACK STEM

Symptoms

Infection starts at the leaf margin and progresses down leaf petiole forming a distinct dark or black lesion on the stem. Lesions only superficial and affects only epidermal tissue, in contrast to that of Phomopsis.

Notes on disease:

• Frequently observed in sunflower and yield losses slight.
• Fungus is specific only to sunflower.
• Spores from crop residue are rain splashed or spread by wind or transmitted by leaf insects. Frequent rain and high temperatures favour disease development.
• Control: Resistant hybrids and crop rotation. Tillage to bury infected plant residue will decrease fungal survival.

Causal Organism

Fungus: Phoma macdonaldii

APICAL CHLOROSIS

Symptoms

Top leaves of the sunflower plant develop chlorosis and these symptoms are usually localised to one side of the plant.

Notes on disease:

• Disease follows hail or other mechanical damage.
  

Causal Organism

Bacteria: Pseudomonas syringae pv. tagetis

BACTERIAL HEAD ROT

Symptoms

Close inspection of sunflower heads show small dark spots on the face. Cutting open the head reveals a slimy mass associated with a foul smell. Foaming occurs and slime eventually runs down the stem and leaves, and secondary colonisation leads to the plants and field appearing black.

Notes on disease:

• Not common.
• Disease not well understood, but is very hybrid specific. Initial damage on the flower is caused by an unknown insect.
• Control: Hybrid selection. No chemical control possible.

Causal Organism

Complex of bacteria (Erwinia spp) and yeasts

VERTICILLIUM WILT

Symptoms

Leaves, starting at the bottom of the plant, show interveinal chlorosis near flowering giving it a mottled appearance. Chlorotic tissue between veins eventually turns brown and become necrotic. Chlorotic areas may appear quite yellow. Vascular system is discoloured (cut across stem). Diseased plants occur in patches or individuals throughout field and can display symptoms of sudden wilting of the entire plant especially in hotter temperatures.

Notes on disease:

• Soil-borne microsclerotia survive in plant residue.
• Yield losses can be high in heavily infested soils.
• Control: Resistant hybrids and crop rotation.

Causal Organism

Fungus: Verticillium dahliae

RUST

Symptoms

Lowest leaves usually become infected first. Small cinnamon/brown rust pustules develop on both top and lower leaf surfaces. Later in the season these pustules may turn black as the resting spores (telia) develop. Infected leaves eventually wilt, dry up and die. Severe infections will cause defoliation.

Notes on disease:

• Loss of photosynthetic leaf area causes serious yield losses on susceptible hybrids.
• Rust spores infect sunflower leaves between 10-25⁰C and when there is dew or free moisture.
• Pustules can produce new spores after 10-14 days, which are spread by wind, and thus several cycles of rust infection can occur in the one cropping cycle.
• Control: Registered fungicides and resistant hybrids.

Causal Organism

Fungus: Puccinia helianthi

POWDERY MILDEW

Symptoms

Airborne spores infect the lowest leaves first. Greyish white spots develop on the top of the leaf surface. White spores produced above the lesions give a powdery white appearance which eventually covers the whole leaf surface. The disease will spread rapidly up the plant under cool conditions with high humidity. Dense plant stands may lead to increased disease.

Notes on disease:

• Severe in some seasons and could cause yield losses particularly if infection develops quickly prior to budding. Oil quality may be affected.
• Spores are dispersed by wind, germinate on leaves at 20-25⁰C under high humidity. Life cycle as short as 5-7 days under ideal conditions so crop monitoring is essential to minimise losses.
  
• Control: Registered fungicides. Good air movement within canopy minimises humidity build up.

Causal Organism

Fungus: Golovinomyces cichoracearum

DOWNY MILDEW

Symptoms

Main symptoms are chlorotic (yellow) areas between veins or affecting entire leaves of seedlings. Underside of leaves covered with white spores. Affected plants often die as seedlings, and if they survive, the plants are stunted and produce horizontal heads with blank seeds. Local lesions, which are angular yellow spots, may occur also. These could be mistaken for white blister rust, but downy mildew lesions are not raised

Notes on disease:

• Present in all sunflower areas, can cause severe yield loss.
• Spores survive in soil and infect roots when soil is saturated after planting.
• Control: Fungicide seed treatments and resistant hybrids
  

Causal Organism

Fungus: Plasmopara halstedii

WHITE BLISTER RUST

Symptoms

Yellow raised, isolated spots develop on top of the leaf with white spores produced on the lower leaf surface, and even on bracts of head. Lesions can merge under heavy infection covering most of the leaf. Superficial grey stem lesions (bruises) could develop and cause lodging.

Notes on disease:

• Frequently observed, but only occasionally damaging.
• Spores survive in soil.
• Infection when cool (10-15⁰C) and there is free water.
• Control: Registered fungicides and resistant hybrids. Early planting dates could escape disease.

Causal Organism

Fungus: Pustula tragopogonis (Previously Albugo tragopogonis)

SEPTORIA LEAF SPOT

Symptoms

Angular to diamond-shaped brown necrotic lesions develop on the upper leaf surface and lighter brown lesions on the lower leaf surface. Could be confused with Alternaria Leaf Spot - microscopic confirmation may be required. Small black fruiting bodies in the older lesions can be seen with a 5x handlens - a diagnostic feature of Septoria.

Notes on disease:

• Common in sunflower production areas. Survives in residues.
• Usually infection starts on lowest leaves.
• Minor impact, but may be severe under warm humid conditions
• Control: Registered fungicides, crop rotation and resistant hybrids.

Causal Organism

Fungus: Septoria helianthi

ALTERNARIA LEAF SPOT

Symptoms

Dark brown round/square lesions develop on leaves, stems, petioles and heads, merge and form large dead areas. Individual lesions may display a halo. Defoliation could follow. Alternaria may also cause seedling blights. Leaf symptoms are often confused with Septoria Leaf Spot and bacterial leaf infection; microscopic confirmation may be required.

Notes on disease:

• Severe infection could cause defoliation and consequent yield loss under warm and humid conditions.
• Spores survive in infected debris. Spores are spread by wind and splashing water. Dew is required for infection.
• Control: Registered fungicides and resistant hybrids.

Causal Organism

Fungus: Various Alternaria species (e.g. A. helianthi & A. zinneae, and others)

SCLEROTIUM STEM ROT (Southern Blight)

Symptoms

Roots and the base of infected stem show caramel to orange necrotic lesions with brown margins. Lesions may exhibit diural banding. White mycelium can develop on lesions and grow across the soil at the base of the plant if infection is advanced. Small, round brown sclerotia form. Plants may wilt and eventually die.

Notes on disease:

• Not very common. Prefers warmer temperatures than Sclerotinia minor.
• Control: Crop rotation. No chemicals are registered.

Causal Organism

Fungus: Sclerotium rolfsii

SCLEROTINIA HEAD ROT

Symptoms

Water-soaked soft lesions develop on the back and white mycelium on the face of the head. Whitish mycelium turns into large flat, net-like black sclerotia. The head disintegrates forming a broom-like structure after shedding seed and sclerotia. Heads light tan in colour when viewed from a distance.

Notes on disease:

• Widespread disease with vast pathogen host range, sclerotia can survive in soil for several years.
• Continuous wet periods of 30-40 h are necessary for infection.
• Control: Difficult to control. Crop rotation will reduce inoculum.
• Control weeds, as many are hosts.

Causal Organism

Fungus: Sclerotinia sclerotiorum

SCLEROTINIA BASAL ROT AND WILT

Symptoms

Individual plants suddenly wilt and disease then spreads to adjacent plants. Pale brown basal lesions (S. sclerotiorum, S. minor) or stem lesions (S. sclerotiorum) can eventually encircle the stem. Whitish mycelium may be seen as the disease develops and later hard, black, irregularly shaped sclerotia will form outside and/or in the pith of the stem. Stems may show characteristic shredding. Lodging can occur.

Notes on disease:

• Widespread disease with vast pathogen host range. Sclerotia survive in soil for years, stimulated by root exudates to germinate and infect roots and stem.
• Control: Very difficult to control. Some hybrids more tolerant. A 3-4 year crop rotation will reduce inoculum. All broadleaves are susceptible so be careful not to rotate to another susceptible crop such as soybeans, canola, etc. Rotate to a cereal crop, e.g. maize, sorghum, wheat.
• Control weeds as many are hosts.

Causal Organism

Fungus: Sclerotinia sclerotiorum, S. minor

Soybean Rust (Asian soybean rust)

Symptoms

Early symptoms appear as small, pale green to yellow spots on the upper leaf surface and could initially be mistaken for Downy Mildew.  As the disease progresses the leaf appearance turns a bronze yellow colour and tan to reddish-brown pustules (uredinia) form predominantly on the underside of leaves. Severe infections cause premature leaf drop, reduced photosynthetic area, poor pod fill, decreased seed quality, and can result in significant yield losses if not controlled.

General Information

P. pachyrhizi spreads rapidly via airborne urediniospores. It cannot overwinter in regions with hard frost but persists year-round in warmer, frost-free areas. Soybean rust prefers warm, humid environments with extended leaf wetness periods caused by frequent rain or dew. Ideal temperatures for infection range between 15–28°C, with relative humidity above 75% promoting rapid disease development. Wind can carry spores over hundreds of kilometers, allowing fast regional spread under favorable weather conditions.  Preventative fungicide applications is effective in controlling the disease.

Causal Organism

Fungus: Phakopsora pachyrhizi

Sclerotinia Stem Rot of Soybeans (White Mold)

Symptoms

Infections typically begin during flowering, when fallen petals serve as infection sites on stems and leaves. Water-soaked lesions form initially and soon become covered with dense, white, cottony fungal mycelium. As the disease progresses, stems dry out, become brittle, and may collapse, leading to plant wilting and lodging. Black, hard sclerotia may develop inside or on the surface of infected tissue, aiding the pathogen’s long-term survival. Severe infections during flowering and pod fill stages can lead to major yield losses.

General Information

S. sclerotiorum produces sclerotia that can persist in soil for over 5–8 years, acting as a survival source. Cool (15–25°C), wet conditions during flowering favor disease outbreaks. Under these conditions sclerotia in the soil germinate and produce small, flesh-coloured structures called apothecia.  These produce and release ascospores which infect the plant.  High humidity, dense crop canopies, and extended periods of leaf and soil moisture increase infection risk. The pathogen’s broad host range (over 400 plant species) and persistence in the soil makes crop rotation as management tool very difficult. Fields previously planted to other susceptible crops such as sunflowers or beans is a high risk.

Causal Organism

Fungus: Sclerotinia sclerotiorum

Sudden Death Syndrome (SDS)

Symptoms

Typical foliar symptoms include interveinal chlorosis and necrosis, where veins remain green while surrounding tissue turns yellow and eventually brown. Leaves often drop prematurely, with petioles remaining attached to stems. Below ground, infected plants exhibit stunted root systems, root rot, and discolored vascular tissues.

General Information

SDS infections begin early in the season, when the fungus colonizes roots, but symptoms only manifest later during the reproductive growth stages. Cool, wet soil conditions at or shortly after planting favor infection, while symptom expression is enhanced by warm, wet weather mid-season. Soybean fields with soybean cyst nematode (SCN) pressure tend to experience more severe SDS outbreaks, however, SCN have not been found in South Africa.

Causal Organism

Fungus: Fusarium virguliforme

Ascochyta Leaf Spot

Symptoms

Small, circular to irregular brown lesions with concentric rings appear on leaves and stems, often with dark margins. With severe infections, lesions coalesce into larger necrotic areas, leading to premature leaf drop and, occasionally, stem girdling. In rare cases, pods are also affected, resulting in poor seed development and quality loss.

General Information

The fungus survives on soybean residue and infected seeds. It is primarily spread via rain splash and wind during moist conditions. Cool to moderate temperatures (20–25°C) combined with prolonged leaf wetness (from dew or rain) favor infection. Fields with poor drainage, high planting densities, and heavy canopy closure are especially prone to outbreaks.

Causal Organism

Fungus: Ascochyta glycines

Charcoal Rot

Symptoms

Symptoms typically appear after flowering during periods of drought stress. Plants show premature yellowing, wilting, and eventual death. Lower stems and taproots exhibit a silvery-grey appearance and when stems are cut open fine black microsclerotia are visible, giving a characteristic "charcoal" appearance.

General Information

Macrophomina phaseolina is a soilborne pathogen capable of infecting over 500 plant species. It survives in soil and crop residues as microsclerotia for several years. Hot (>30°C), dry conditions greatly favor disease development. Charcoal rot is especially damaging during late reproductive stages when plants are under drought or heat stress, leading to substantial yield reductions.

Causal Organism

Fungus: Macrophomina phaseolina

Downy Mildew

Symptoms

Early symptoms appear as small, yellowish-green spots on the upper leaf surfaces. A fine, grey to purple downy growth is visible on the underside of affected leaves. Severe infections can cause leaf curling, distortion, and reduced plant vigor. The initial pale leaf spots are sometimes mistakenly identified as Soybean Rust.

General Information

Although considered a minor disease in most commercial soybean production systems, Downy Mildew can affect seed quality and seedling vigor. The pathogen survives in infected seed and crop debris. Cool (18–24°C), wet conditions with high humidity favor infection and sporulation. It is common early in the growing season and disperses via wind and rain splash.

Causal Organism

Oomycete: Peronospora manshurica

Purple Seed Stain

Symptoms

Seeds exhibit a purple to dark reddish discoloration on the seed coat. Infected leaves may show foliar symptoms resembling sunscald or leaf blight, and lesions may occur on pods and stems, though seed staining is the most common and visible symptom.

General Information

Purple Seed Stain is widespread and affects seed quality by reducing germination rates and marketability. The pathogen persists on infected seed and crop residues. Disease development is favored by warm, humid conditions during late pod fill and seed maturation stages. Delayed harvests, particularly in wet seasons, increase infection severity.

Causal Organism

Fungus: Cercospora kikuchii

Red Leaf Blotch

Symptoms

Symptoms begin as small, reddish to purplish-brown blotches on leaves, often surrounded by a faint yellow halo. Lesions typically appear on lower leaves first and may merge, causing large areas of blight. Infected plants often suffer from premature defoliation and reduced vigor, leading to yield loss.

General Information

Although not yet reported in South Africa, Red Leaf Blotch is prevalent in neighbouring countries such as Zambia and Zimbabwe. It is most problematic in tropical regions with warm (25–30°C), humid conditions and frequent rainfall. Continuous soybean cropping, minimal residue management, and lack of crop rotation contribute to disease buildup and severity.

Causal Organism

Fungus: Phoma glycinicola

Frogeye Leaf Spot

Symptoms

Characteristic circular lesions with light grey to tan centers and dark reddish-purple borders develop on leaves. Over time, older lesions may coalesce, causing extensive leaf blighting. In advanced cases, lesions may also appear on stems and pods, resulting in reduced seed quality and yield.

General Information

Frogeye Leaf Spot has not been officially reported in South Africa but is present in nearby countries such as Zambia and Zimbabwe. It is one of the most widespread foliar diseases of soybean globally. The disease thrives in warm (24–30°C), humid conditions, especially during reproductive stages. Frequent rainfall, prolonged dew, dense canopies, and poor crop rotation contribute significantly to disease spread. The fungus overwinters in crop residues, allowing carryover from season to season.

Causal Organism

Fungus: Cercospora sojina

Ergot of Sorghum

Symptoms

Ergot infects flowers, replacing normal grain development with a sticky, honeydew-like mass that later hardens into dark, compact fungal structures called sclerotia. Honeydew exudate often drips from infected florets and may attract insects. Severely infected panicles appear distorted and have poor seed set, leading to reduced grain yield.

General Information

Cool, humid weather during flowering greatly favors infection. Extended panicle emergence due to asynchronous flowering increases vulnerability. Fields with dense canopies or prolonged moisture retention are at higher risk. Insect activity can help spread the sticky spores between plants.

Causal Organism

Fungus: Claviceps africana

Rust of Sorghum

Symptoms

Small, round to oval, reddish-brown pustules (uredinia) form primarily on the upper and lower leaf surfaces. These pustules can merge, causing large necrotic areas. In severe infections, leaves dry out, turn brown, and die prematurely, reducing the plant’s photosynthetic capacity. In some cases, pustules may also appear on leaf sheaths and stalks. 

General Information

Sorghum rust is widespread and occurs wherever sorghum is grown.  Warm (20–30°C), humid conditions favor spore germination and infection. High rainfall, heavy dew, and dense plant populations promote disease spread. Wind aids long-distance dispersal of rust spores. Yield losses are usually 5–10% but can exceed 20% if severe rust develops early during the plant's growth stages and affects large portions of the canopy.

Causal Organism

Fungus: Puccinia purpurea

Leaf Blight of Sorghum

Symptoms

Large, elliptical to oblong lesions develop on the leaves, typically light tan in colour with reddish-brown margins. The colour of the margin can vary and normally corresponds with the grain colour of the particular hybrid. Lesions can coalesce, leading to extensive blighting and death of large leaf areas. Severe infections can cause rapid defoliation, especially during grain filling, reducing yield potential. 

General Information

Warm (25–30°C), moist environments with extended periods of leaf wetness favor disease development. Dense planting and overhead irrigation can enhance conditions conducive to spread. Spores are dispersed by wind and rain splash. The fungus survives on infected crop residues left in the field.  Yield losses typically range from 5–20%, but in severe outbreaks with early leaf blighting, losses can exceed 30%, mainly due to reduced photosynthesis. Leaf blight occurs in most sorghum-growing regions worldwide during grain filling. 

Causal Organism

Fungus: Exserohilum turcicum

Smut of Sorghum

Symptoms

Instead of developing normal seed heads (panicles), infected sorghum plants form large, swollen smut galls filled with black spore masses. These galls eventually rupture, releasing spores that contaminate the soil. Early plant growth may appear normal and symptoms only become evident at flowering.

General Information

Infection occurs at the seedling stage via the roots. Warm soil temperatures (25–30°C) and dry conditions after planting promote infection. The pathogen survives for years in soil and infected seed. Continuous sorghum cultivation and deep planting can increase disease incidence. Crop rotation and planting resistant varieties are effective management strategies. Infected plants often fail to produce any grain, leading to 5–20% yield losses in affected fields. In severe cases, losses can exceed 50% if susceptible varieties are planted under favorable conditions for the pathogen.  

Causal Organism

Fungus: Sporisorium reilianum (Sphacelotheca reiliana)

Sooty Stripe

Symptoms

Elongated leaf blight like, narrow, greyish to dark brown lesions form on leaves and leaf sheaths, bordered by a distinct yellow margin. Lesions are often mistaken for Leaf Blight, however, the yellow margin is very distinctive.  As lesions mature, a black, sooty fungal growth appears on the surface, giving the disease its name. Severe infections can cause premature leaf death, reducing photosynthesis The lesions may merge, leading to leaf death and reduced photosynthesis. Sooty spores may give leaves a dirty appearance and rub off on hands or clothes.

General Information

Warm (25–30°C), humid weather with frequent rains and heavy dew promotes the disease. Dense canopies and poor air circulation favor pathogen development. Spores are spread by wind and rain splash. The fungus survives on infected crop residues left in the field from previously infected sorghum crops. Moderate infections typically cause 5–15% yield losses by reducing grain fill. Severe outbreaks, particularly during grain filling, can lead to up to 30% yield loss if large portions of the canopy are destroyed. 

Causal Organism

Fungus: Ramulispora sorghi