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

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)

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

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)

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

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

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

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

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

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

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

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.

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

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

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

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

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

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)

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)

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