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Mycologia, ASCOMYCOTINA, HELOTIALES, SCLEROTINIACEAE .

Crops attacked: sunflower .

White rot

Identification, Description, Biology, Epidemiology, Treatement, Possible misleading, Images

[R]Description
From the formation of the sunflower flowering bud to the end of the flowering stage a number of plants wilt in the crop.
After flowering starts, chlorosis of the leaves, petiole and stem will occur followed by a softening of the affected organs ; a thick white mycelium colonizes them and can often be observed along the stem where it clusters in different points before forming sclerotia. When this point of least resistance of the tissues is reached the stem can break and lodge.
At maturity, translucid spots appear on the adaxial side of the capitulum on which the thick white mycelium forms a grid of sclerotia. All the tissues of the capitulum are disintegrated by the pathogen. Only the fibres at the top of the stem are left uninjured which makes it look like a whip.

[R]Biology
This fungus is embedded in the field. It survives as sclerotia and free mycelium in the soil.
It also travels contained in the seed lots usually as sclerotia but occasionally as mycelium on the surface layers of the achenes.
When the sclerotia are in direct contact with the roots they can germinate directly in mycelial form and colonize the plant on the tap root, collar, at the base of the stem. Most frequently sclerotia situated in the upper layers of the soil (3 to 5 cm deep) germinate as apothecia.
The latter produce asci which release ascospores.
Ascospores set on all the plant organs in the vicinity of the foci of apothecia.

[R]Epidemiology
42 hours of free water on the flowering side of the capitulum are necessary for an infection of these inflorescences to take place.

[R]Treatement
Avoid the too frequent rotations of the oleaginous plants and legumes.
Walking on the field before sowing allows detection of infested fields (when in large number, sclerotia can be seen with a naked eye).
Avoid irrigation during the flowering period.
Clean away the grids of sclerotia after an epidemic.
Sow healthy seeds of a tolerant variety.

[R]Possible misleading
Botrytis cinerea on the stem and the capitulum.
The sclerotia of S. sclerotiorum may be mistaken for those of Sclerotinia trifoliorum and even those of Botrytis cinerea but the latter don't havemedulla, which can be seen on a transversal section of the sclerotium.
With a bacterial attack but the colour of the S. sclerotiorum attack is lighter.
Often masked by B. cinerea whose secondary settling and sporulation stop the colonization of S. sclerotiorum .

[R] Images

1. Sclerotinia sclerotiorum , White rot (COURTILLOT M., INRA)
   White rot Apothecia (carpophore) of Sclerotinia sclerotiorum at maturity on a sclerotia.
   
2. Sclerotinia sclerotiorum , White rot (LAMARQUE C., INRA)
   White rot on sunflower Secondary contamination on a sunflower leaf infected by a neighbouring plant previously infected by white rot.
   
3. Sclerotinia sclerotiorum , White rot (LAMARQUE C., INRA)
   White rot on sunflower A sunflower capitulum showing corky fibers after being broken up by white rot.
   
4. Sclerotinia sclerotiorum , White rot (SLAGMULDER C., INRA)
   White rot on sunflower Sclerotia of Sclerotinia sclerotiorum formed by the aggregation of mycelium at the level of a node on a sunflower stem.
   

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