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Mycologia, MASTIGOMYCOTINA, PERONOSPORALES, PERONOSPORACEAE .

Crops attacked: sunflower .

Downy mildew

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

[R]Description
Persistent dwarfism is observed in the most affected young plants ; chlorotic "angular spots " on the adaxial surface of the leaves ; white "downy" cover on the abaxial surface of the leaves.
At maturity the capitulum of the less affected plants faces upwards (horizontal habit ) and has some areas without seed : morphological heterogeneity of the floral parts (up to complete sterility ) affecting a number of florets which will not produce achenes.

[R]Biology
Oospores -the resting bodies of the fungus - are preserved in the soil as plant debris ; less frequently the mycelium is preserved in the seeds ; oospores are spread in the soil by running water and germinate producing zoosporangia.
The mobile zoospores coming from the zoosporangia will form the primary inoculum which will infect the seedlings ; they germinate in the vicinity of the roots in the root hair zone.
The hypocotyl, the stem are colonized by systemic intercellular mycelium.
Formation of zoosporangia takes place on the roots and the abaxial epidermis of the leaves.
The zoosporangia on the roots release new zoospores which infect the plants close to the primary focus of infection while the airborne zoosporangia are spread by the wind on apical buds of new plants making the disease an epidemic.
At the end of the growing period of the plants, oospores are produced in the infected leaves.

[R]Epidemiology
Wet weather (free water in the soil or on plants) and an optimum temperature of about 15 °C. favour the germination of zoosporangia.
Wind allows the airborne zoosporangia to spread.

[R]Treatement
Useresistant varieties.
Registration of a new sunflower variety on the French catalogue requires resistance to the French downy mildew races.
Compulsory treatment of all imported seeds and the seeds of any still susceptible female lines used in breeding.

[R]Possible misleading
Dwarfism due to various types of phytotoxicity in the early growing stage.

[R] Images

1. Plasmopara helianthi , Sunflower downy mildew (TOURVIEILLE D., INRA)
   Sunflower downy mildew Foci of downy mildew in a sunflower field.
   
2. Plasmopara helianthi , Sunflower downy mildew (ALLARD C., INRA)
   Sunflower downy mildew Focus of downy mildew in a parcel of sunflower showing dwarf plants (primary infection).
   
3. Plasmopara helianthi , Sunflower downy mildew (LAMARQUE C.,INRA)
   Sunflower downy mildew Dwarf plant of sunflower (primary infection by Plasmopara helianthi ).
   
4. Plasmopara helianthi , Sunflower downy mildew (LAMARQUE C., INRA)
   Sunflower downy mildew Dwarf plant of sunflower (secondary contamination by Plasmopara helianthi ).
   
5. Plasmopara helianthi , Sunflower downy mildew (TOURVIEILLE D., INRA)
   Sunflower downy mildew Dwarf plant of sunflower (secondary infection by Plasmopara helianthi ).
   
6. Plasmopara helianthi , Sunflower downy mildew (LAMARQUE C.,INRA)
   Sunflower downy mildew Down induced by downy mildew on sunflower leaf.
   
7. Plasmopara helianthi , Sunflower downy mildew (LAMARQUE C., INRA)
   Sunflower downy mildew Very strong secondary contamination of downy mildew at the apex of a sunflower.
   
8. Plasmopara helianthi , Sunflower downy mildew (LAMARQUE C., INRA)
   Sunflower downy mildew Downy covering of mildew on the underside of a sunflower apex resulting from a strong secondary contamination.
   

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