''Phytophthora infestans'' is an
oomycete or
water mold
Oomycota forms a distinct phylogenetic lineage of fungus-like eukaryotic microorganisms, called oomycetes (). They are filamentous and heterotrophic, and can reproduce both sexually and asexually. Sexual reproduction of an oospore is the resul ...
, a fungus-like
microorganism
A microorganism, or microbe,, ''mikros'', "small") and ''organism'' from the el, ὀργανισμός, ''organismós'', "organism"). It is usually written as a single word but is sometimes hyphenated (''micro-organism''), especially in old ...
that causes the serious
potato
The potato is a starchy food, a tuber of the plant ''Solanum tuberosum'' and is a root vegetable native to the Americas. The plant is a perennial in the nightshade family Solanaceae.
Wild potato species can be found from the southern Uni ...
and
tomato
The tomato is the edible berry of the plant ''Solanum lycopersicum'', commonly known as the tomato plant. The species originated in western South America, Mexico, and Central America. The Mexican Nahuatl word gave rise to the Spanish word ...
disease known as late blight or potato blight.
Early blight
''Alternaria solani'' is a fungal pathogen that produces a disease in tomato and potato plants called early blight. The pathogen produces distinctive "bullseye" patterned leaf spots and can also cause stem lesions and fruit rot on tomato and ...
, caused by ''
Alternaria solani'', is also often called "potato blight". Late blight was a major culprit in the
1840s European, the
1845–1852 Irish, and the
1846 Highland potato
famine
A famine is a widespread scarcity of food, caused by several factors including war, natural disasters, crop failure, population imbalance, widespread poverty, an economic catastrophe or government policies. This phenomenon is usually accompan ...
s. The organism can also infect some other members of the
Solanaceae
The Solanaceae , or nightshades, are a family of flowering plants that ranges from annual and perennial herbs to vines, lianas, epiphytes, shrubs, and trees, and includes a number of agricultural crops, medicinal plants, spices, weeds, and or ...
.
The pathogen is favored by moist, cool environments: sporulation is optimal at in water-saturated or nearly saturated environments, and
zoospore production is favored at temperatures below . Lesion growth rates are typically optimal at a slightly warmer temperature range of .
Etymology
The genus name ''Phytophthora'' comes from the Greek –(), meaning : "plant" – plus the Greek (), meaning : "decay, ruin, perish". The species name ''infestans'' is the present participle of the Latin verb , meaning : "attacking, destroying", from which we get the word "to infest". The name ''Phytophthora infestans'' was coined in 1876 by the German mycologist
Heinrich Anton de Bary
Heinrich Anton de Bary (26 January 183119 January 1888) was a German surgeon, botanist, microbiologist, and mycologist (fungal systematics and physiology).
He is considered a founding father of plant pathology (phytopathology) as well as the fou ...
(1831–1888).
Life cycle, signs and symptoms
The asexual life cycle of ''Phytophthora infestans'' is characterized by alternating phases of
hypha
A hypha (; ) is a long, branching, filamentous structure of a fungus, oomycete, or actinobacterium. In most fungi, hyphae are the main mode of vegetative growth, and are collectively called a mycelium.
Structure
A hypha consists of one or ...
l growth, sporulation, sporangia germination (either through
zoospore release or direct germination, i.e. germ tube emergence from the
sporangium
A sporangium (; from Late Latin, ) is an enclosure in which spores are formed. It can be composed of a single cell or can be multicellular. Virtually all plants, fungi, and many other lineages form sporangia at some point in their life cy ...
), and the re-establishment of hyphal growth.
There is also a sexual cycle, which occurs when isolates of opposite mating type (A1 and A2, see
Mating types below) meet. Hormonal communication triggers the formation of the sexual
spore
In biology, a spore is a unit of sexual or asexual reproduction that may be adapted for dispersal and for survival, often for extended periods of time, in unfavourable conditions. Spores form part of the life cycles of many plants, algae, ...
s, called
oospore
An oospore is a thick-walled sexual spore that develops from a fertilized oosphere in some algae, fungi, and oomycetes. They are believed to have evolved either through the fusion of two species or the chemically-induced stimulation of mycelia ...
s. The different types of spores play major roles in the dissemination and survival of ''P. infestans.'' Sporangia are spread by wind or water and enable the movement of ''P. infestans'' between different
host plants. The zoospores released from sporangia are
biflagellated and
chemotactic
Chemotaxis (from '' chemo-'' + '' taxis'') is the movement of an organism or entity in response to a chemical stimulus. Somatic cells, bacteria, and other single-cell or multicellular organisms direct their movements according to certain chemica ...
, allowing further movement of ''P. infestans'' on water films found on leaves or soils. Both sporangia and zoospores are short-lived, in contrast to oospores which can persist in a viable form for many years.
People can observe ''Phytophthora infestans'' produce dark green, then brown then black spots on the surface of potato leaves and stems, often near the tips or edges, where water or dew collects.
The sporangia and