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Mycologia, MASTIGOMYCOTINA, PERONOSPORALES, PYTHIACEAE .

Crops attacked: potato .

Late blight of potato,

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

[R]Description
The disease occurs in isolated foci in the field but can extend rapidly and even cause complete loss of the crop in a few days.
Browning of the stems accompanied by lesions cause the destruction of the plants at the emergence or consequent necrosis at different points on the stem and then lodging.
Brown spots circled by a pale greenish halo occur on the leaves.
In conditions of high humidity a characteristic white downy mildew develops on the margin of the spots on the lower surface of the leaves followed by rapid necrosis.
When the tubers are pulled out the disease shows slight symptoms : the blue greyish sunken spots are ill-defined.
In section a rust marble-like zone can be detected which extends from the skin to the center of the tuber.
If the conditions are favourable stored potatos soften up and rotten.

[R]Biology
The fungus survives as mycelium in the contaminated tubers.
Mycelial intercellular hyphae colonize the leaf tissues and cause cell death.
Lesions occur 5 - 7 days after contamination.
In conditions of high RH white thick sporangiophores form around the lesions.
Tubers are contaminated by the zoospores of infected leaves carried by rain.

[R]Epidemiology
Alternate humid and relatively hot periods favour the development and spread of the disease.
Sporangia form and germinate only if free water persists at least 15 hours on the leaves.
Over 18 °C. temperature and 100 % RH are optimum conditions for the development of the fungus in storage places.

[R]Treatement
Destroy debris and shoots which generate the primary foci.
Early preventive treatment, before occurence of any symptom according to climatic conditions and choice of cultivars.
Wilt at the end of the vegetative cycle under fungicide protection up to the complete destruction of vegetative parts.
Remove the infected tubers.

[R]Possible misleading
.

[R] Images

1. Phytophthora infestans , Downy mildew (VEGH I., INRA)
   Downy mildew on potato Symptoms of downy mildew on the aerial part of potato.
   
2. Phytophthora infestans , Downy mildew (INRA)
   Downy mildew on potato External view of potato downy mildew.
   
3. Phytophthora infestans , Downy mildew (VEGH I., INRA)
   Downy mildew on potato Internal symptoms of potato downy mildew.
   
4. Phytophthora infestans , Downy mildew (BASF)
   Downy mildew on potato Symptoms of downy mildew on a potato leaf.
   
5. Phytophthora infestans , Downy mildew (BASF)
   Downy mildew on potato Section of a potato tuber infected by downy mildew showing an internal reddish-brown dry rot which develops superficially under the skin of the tuber.
   
6. Phytophthora infestans , Downy mildew (BASF)
   Downy mildew on potato Section of a potato tuber infected by downy mildew showing an internal reddish-brown dry rot which extends from the skin to the center of the tuber.
   

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