Phytophthora infestans
Oomycete pathogen that caused the Irish potato famine.
Phytophthora infestans is a water mold, a type of oomycete that looks like a fungus but is actually more closely related to algae and diatoms. Its cell walls are made of cellulose, not chitin, and it spreads using swimming spores. This organism causes late blight, a serious disease in crops like potatoes and tomatoes. (Early blight, caused by *Alternaria solani*, is also sometimes called potato blight.) Late blight thrives in cool, damp conditions: spore production peaks between 12–18 °C (54–64 °F) in nearly saturated air, and swimming spores form best below 15 °C (59 °F). Lesions grow fastest at 20–24 °C (68–75 °F). Historically, this pathogen was a key cause of the European potato famines of the 1840s, including the Irish famine of 1845–1852 and the Highland famine of 1846.
The name *Phytophthora* comes from Greek words meaning "plant" and "decay," while *infestans* is Latin for "attacking" or "destroying." German mycologist Heinrich Anton de Bary coined the full name in 1876.
The organism’s asexual life cycle alternates between hyphal growth, spore production, and spore germination—either by releasing swimming spores or by growing a germ tube directly. A sexual cycle occurs when two mating types (A1 and A2) meet, triggered by hormones, producing long-lasting oospores. Sporangia spread by wind or water between plants, while swimming spores move through water films on leaves or soil. Sporangia and swimming spores are short-lived, but oospores can survive for years.
On plants, late blight first shows as dark green, then brown, then black spots on leaf tips and edges where water collects. White mold (sporangia and sporangiophores) appears on the undersides of leaves. Infected tubers may show white mycelium on their surface. Under ideal conditions, the life cycle on potato or tomato foliage takes about five days. Spores develop when temperatures exceed 10 °C (50 °F) and humidity stays above 75–80% for two days or more. Rain washes spores into soil, infecting young tubers, and wind carries spores long distances. Early symptoms are easy to miss: dark blotches on leaf tips and stems, white mold under leaves in humid weather, and rapid plant collapse. Infected tubers develop gray or dark patches, reddish brown beneath the skin, and quickly rot into a foul-smelling mush due to secondary bacterial rots. Even healthy-looking tubers may rot later in storage.
Quick Facts
- Genus
- Phytophthora
- Species
- infestans
Facts from the source article.
Lore & Background
The genus name Phytophthora comes from the Greek φυτόν (phyton), meaning 'plant', plus the Greek φθορά (phthora), meaning 'decay, ruin, perish'. The species name infestans is the present participle of the Latin verb infestare, meaning 'attacking, destroying', from which the word 'to infest' is derived. The name Phytophthora infestans was coined in 1876 by the German mycologist Heinrich Anton de Bary.
The asexual life cycle of P. infestans is characterized by alternating phases of hyphal growth, sporulation, sporangia germination (either through zoospore release or direct germination), and the re-establishment of hyphal growth. There is also a sexual cycle, which occurs when isolates of opposite mating type (A1 and A2) meet, triggering the formation of sexual spores called oospores. Sporangia are spread by wind or water, and zoospores released from sporangia are biflagellated and chemotactic, allowing movement on water films. Oospores can persist in a viable form for many years.
People can observe P. infestans produce dark green, then brown then black spots on the surface of potato leaves and stems, often near the tips or edges. The sporangia and sporangiophores appear white on the lower surface of the foliage. Infected tubers develop grey or dark patches that are reddish brown beneath the skin, and quickly decay to a foul-smelling mush caused by secondary soft bacterial rots. Under ideal conditions, P. infestans completes its life cycle on potato or tomato foliage in about five days.
Reader's Guide
Phytophthora infestans is historically significant as the primary cause of the 1845–1852 Irish potato famine, which led to widespread starvation and emigration. Its impact on agriculture continues today, as it remains a devastating pathogen of potato and tomato crops worldwide. The organism favors moist, cool environments: sporulation is optimal at 12–18 °C in water-saturated conditions, and zoospore production is favored below 15 °C. Lesion growth rates are optimal at 20–24 °C. The pathogen survives poorly apart from its plant hosts; hyphae and asexual sporangia survive only briefly in plant debris or soil, but oospores and hyphae within tubers can persist. Volunteer plants sprouting from infected tubers are a major source of inoculum at the start of a growing season. The genome of P. infestans was sequenced in 2009 and found to be considerably larger (240 Mbp) than that of most other Phytophthora species, containing about 18,000 genes and a diverse variety of transposons and effector proteins involved in pathogenicity. Mating types A1 and A2 produce distinct hormones, and populations are grouped into clonal lineages. The pathogen's ability to evolve resistance to fungicides, as seen in the EU_43_A1 lineage, poses ongoing challenges for disease management.
Did You Know?
- Despite its resemblance to true fungi, P. infestans is classified among the Stramenopiles, with cell walls made of cellulose instead of chitin.
- The species name infestans is derived from the Latin verb infestare, meaning 'attacking, destroying'.
- Oospores, the sexual spores of P. infestans, can persist in a viable form for many years.
- The genome of P. infestans, sequenced in 2009, is 240 Mbp, considerably larger than that of most other Phytophthora species.
Frequently Asked Questions
What is Phytophthora infestans?
It is a water mold in the oomycete group, making it phylogenetically closer to algae and diatoms than to true fungi. Despite its fungus-like look, its cell walls are composed of cellulose rather than chitin.
How does Phytophthora infestans spread between plants?
It releases swimming spores that travel through thin water films on leaf surfaces to reach new tissue. This water-dependent dispersal is the reason wet, cool weather triggers rapid outbreaks in the field.
What conditions does Phytophthora infestans need to thrive?
Spore production peaks at 12–18 °C in nearly saturated air, and swimming-spore formation works best below 15 °C. In practical terms, cool and persistently damp weather is its ideal breeding environment.
What is Phytophthora infestans's most famous historical impact?
It is the pathogen behind late blight, the potato disease that destroyed crops across Ireland during the 1845–1852 famine. The resulting harvest failure drove mass starvation and a wave of emigration.
Who named Phytophthora infestans and what does the name mean?
Heinrich Anton de Bary assigned the binomial in 1876. The genus fuses Greek words for 'plant' and 'decay,' while the species epithent draws on Latin for 'attacking' or 'destroying.'
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