Diseases of sunflowers
Note Number: AG0121
Published: November 1999
Updated:
About 30 diseases of sunflowers have been recorded throughout the world; not all occur in Australia, and only six are of importance in Victoria.
Sclerotinia stem rot (Sclerotinia minor) is the most serious disease of sunflowers in Victoria.
Sclerotinia head rot Occurrence and outcome
This disease is caused by the fungus Sclerotinia sclerotiorum, which attacks a range of vegetable and field crops including celery, cauliflower, cabbage, beans, lupins, canola, soybean and linseed. It was first recorded in Victoria in 1929. In sunflowers the head rot phase usually occurs in late sown crops (December-January) that ripen in autumn in cool, often wet conditions, ideal for disease.
Economic importance
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Fig 1. Head rot |
To date there have been only occasional outbreaks of this disease. Infections that occur in the late-flowering and seed development stages result in minimal losses in seed yield and oil content, unless plants collapse.
The seed is unsuitable for sowing as it will be mixed with the fungus fruiting structures (sclerotia).
Symptoms
Head rot
A soft, light-brown rot of the spongy tissue at the back of the head extends some distance down the stalk. As the disease develops only the fibrous strands on the back of the head remain, and the seed-bearing face generally falls away under the weight of the seed. Large, black irregular sclerotia develop below the seed layer and around the seeds. The sclerotia are about the size and density of the seed and therefore difficult to grade out.
Stem rot
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Fig 2. Stem rot - light |
Sudden wilting of the leaves is the first symptom. Encircling the stem base is a soft, light-brown to bleached water-soaked area extending up the stem 80-250 mm. The pith is filled with a white cottony fungal growth, in which large, black, irregularly shaped sclerotia form. Affected areas sometimes occur from 500 mm to a metre above the ground and stems usually break at this point.
Carry-over and spread
This disease is not seed-borne, but the sclerotia are of similar size and density to the seed and can mix with and contaminate the seed. The fungus may also be carried over as sclerotia in crop debris or in the soil. Sclerotia are capable of surviving for several years.
During cool moist weather sclerotia near the soil surface germinate to produce small, creamy, mushroom-like structures, which contain large numbers of spores. These spores are dispersed by wind and rain, and lodge on leaves, stems or heads to infect the next crop.
Control
As all sunflower cultivars are susceptible to this disease, it is best to avoid planting sunflowers and other susceptible crops in sclerotia-infested soil for from four to six years. Sow seed that is free from sclerotia..
Red rust
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Fig 3. Red rust - brown |
Occurrence
Rust caused by the fungus Puccinia helianthi was first recorded on sunflowers in Victoria in 1896. It is now a common disease of sunflower crops.
Economic importance
Only light outbreaks causing minimal losses have occurred in Victoria so far, but severe outbreaks with yield losses of 70% have been reported in open pollinated cultivars in other states.
High amounts of rust can be tolerated by early-sown crops. When spore density is high early in the growth of late-sown crops, yields may be significantly reduced.
Symptoms
Small reddish-brown pustules appear on the underside of lower leaves. If weather conditions are favourable infection will spread to all leaves, and eventually to the stem and floral bracts. Leaves may yellow, but rarely fall from the plant.
Carry-over and spread
Rust spores survive from season to season on crop debris and volunteer plants. Wind-borne spores spread the disease from out-of-season sunflowers to the crop. Infection occurs during wet weather or heavy dews.
Control
• Volunteer sunflowers should be destroyed before a new crop is sown.
• Use resistant cultivars.
• Sow susceptible open-pollinated cultivars early, so that they mature before wind-borne spore numbers become too high.
White blister rust
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Fig 4. White blister rust - yellowish-green |
Occurrence and importance
The fungus Albugo tragopogonis was first recorded at Katunga in 1979. It is not as widespread as red rust. Field observations indicate that sunflower yields are not significantly reduced, even when most leaves are infected.
Symptoms
Raised (blistered) yellowish-green areas, up to 20 mm across, occur on the leaves and often cover the entire leaf. On the underside of the blisters is the white spore growth of the fungus. Occasionally the centres of blisters dry and fall out.
Control
Because this disease does not cause significant yield losses, no control measures are warranted.
Rotation of crops reduces spore numbers carried over on crop debris.
Rhizopus head rot
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Fig 5. Rhizopus head rot - back of head softens |
Occurrence and importance
The fungus Rhizopus oryzae was first recorded at Echuca West in 1979.
If it is to develop, the rot occurs after flowering in warm, wet conditions on heads that have been damaged by the abrasions of insects, birds, hail or head-to-head contact.
No economic losses have been reported in Victoria.
Symptoms
The spongy tissue at the back of the head softens and turns brown. This usually starts around either abrasions or the perimeter of the head. A grey fur like fungal growth containing small round black bodies develops inside affected heads. There are no large black sclerotia present as is the case with sclerotinia head rot.
Control
The control of insects such as Rutherglen bug and Heliothis caterpillars, and tactics that scare away birds, will reduce the incidence of this disease.
Charcoal stalk rot
Occurrence and importance
The fungus Macrophomina phaseoli was first recorded in Victoria in 1981, although it almost certainly was present before then. The fungus has a wide host range including soybean, linseed, onion, lucerne and sub clover, and is widespread in irrigation districts.
The extent of yield losses in Victoria has not been determined. Charcoal rot is reported to be most destructive when excessive heat and dry conditions follow periods of rapid growth.
Symptoms
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Fig 6. Charcoal rot |
Symptoms are not apparent until after flowering when poorly filled heads and premature ripening and drying of the stalks occur. Affected stalks are discoloured at the base and the pith rots away to leave shredded water conducting tissues. After a period of hot, dry weather the conducting tissues become covered with small black fruiting bodies (sclerotia). These are smaller than those of Sclerotinia minor (Sclerotinia stem rot).
Control
The best method of reducing the effects of this disease is to minimise moisture stress by sound irrigation management.
Acknowledgements
This Note was developed by Rod Clarke, November 1999.
