Phytophthora infestans (Mont.) de Bary   
(cause of) Late Potato Blight

Kingdom: Chromista
Phylum: Oomycota
Order: Pythiales

British distribution: Introduced, widespread and frequent.
World distribution: Widespread, probably wherever the hosts are grown.

potato blight
Potato plants showing symptoms of Phyophthora infestans infection, Kindrogan, Perthshire, September 2000.

A destructive parasite of potatoes (Solanum tuberosum), tomatoes (Lycopersicon esculentum) and other members of the potato family (Solanaceae). In potatoes, rapid destruction of foliage and tubers can occur, infamously causing the Irish potato famines of 1845-7, with social and political repercussions that persist to the present day. Presumably of South American origin, it is said to have first been known in the British Isles in 1845.

The species is heterothallic, requiring presence of two different strains for sexual reproduction to occur. Whether a given strain will be functionally male (producing antheridia) or female (producing oogonia) is, however, under nutritional control.

Asexual reproduction is, in contrast, highly efficient and can result in rapid infestation of crops. Branched sporangia emerge from the leaves, commonly through the stomata, and hygroscopic twisting of the sporangiophore occurs with changes in humidity, flicking off the sporangia into the wind. When wind-borne sporangia land on a moist leaf surface, they germinate by production of germ-tubes or, at cooler temperatures (9 to 15 °C.), by production of motile zoospores that may be dispersed further by swimming and splashing. Infection of the new host plant may take place inside 2 hours and production of new sporangia within 3 to 5 days. Periods of dew and high humidity are important - the sporangia are sensitive to drying. Spells of cool, damp weather produce the rapid spread and crop destruction so devastating in the Irish famines.

Accounts of this "fungus" are to be found in mycology textbooks, e.g. Webster (1980), and in plant pathology textbooks such as Agrios (1998).

Sporangiophores and detached sporangia.
The sporangial contents are not yet differentiated into zoospores and the sporangium may directly produce a multinucleate germ tube that re-infects the host plant.
Source, the potato plants shown above, Perthshire.

•   Agrios, G.N., (1998), Plant pathology (4th ed.), Academic Press, San Diego.
•   Webster, J., (1980). Introduction to fungi (2nd ed.), Cambridge University Press, Cambridge.

© A.J. Silverside
Page first hosted at, April 2001; transferred to with minor edits, January 2011.
For text layout and clarity it is best viewed with Internet Explorer
Return to main Index
Conditions of Use home page