Phytophthora infestans
| Phytophthora infestans | |
|---|---|
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| Symptom of late blight on the underside of a potato leaf | |
| Scientific classification | |
| Domain: | Eukaryota |
| (unranked): | SAR |
| Superphylum: | Heterokonta |
| Class: | Oomycota |
| Order: | Peronosporales |
| Family: | Pythiaceae |
| Genus: | Phytophthora |
| Species: | P. infestans |
| Binomial name | |
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Phytophthora infestans (Mont.) de Bary |
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Phytophthora infestans is an oomycete that causes the serious potato disease known as late blight or potato blight. (Early blight, caused by Alternaria solani, is also often called "potato blight".) Late blight was a major culprit in the 1840s European, the 1845 Irish, and the 1846 Highland potato famines. The organism can also infect tomatoes and some other members of the Solanaceae. At first, the spots are gray-green and water-soaked, but they soon enlarge and turn dark brown and firm, with a rough surface.
The genus name Phytophthora comes from the Greek φυτό-, meaning "plant", plus the Greek φθορά, meaning "destruction, ruin". The species name infestans is the present participle of the Latin verb infestare, meaning "attacking, destroying".
The asexual life cycle of Phytophthora infestans is characterized by alternating phases of hyphal growth, sporulation, sporangia germination (either through zoospore release or direct germination, i.e. germ tube emergence from the sporangium), 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. Hormonal communication triggers the formation of the sexual spores, called oospores. 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, 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.
Under ideal conditions, the life cycle can be completed on potato or tomato foliage in about five days. Sporangia develop on the leaves, spreading through the crop when temperatures are above 10 °C (50 °F) and humidity is over 75%-80% for 2 days or more. Rain can wash spores into the soil where they infect young tubers, and the spores can also travel long distances on the wind. The early stages of blight are easily missed. Symptoms include the appearance of dark blotches on leaf tips and plant stems. White mould will appear under the leaves in humid conditions and the whole plant may quickly collapse. Infected tubers develop grey or dark patches that are reddish brown beneath the skin, and quickly decay to a foul-smelling mush caused by the infestation of secondary soft bacterial rots. Seemingly healthy tubers may rot later when in store.
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Wikipedia