Late Blight in Potato

Late blight in the potato is a plant disease caused by the pathogenic fungus Phytophthora infestans. It is a potentially devastating disease that primarily affects potato crops but can also be seen in the members of the Solanaceae family, including tomatoes, petunias and hairy nightshades.

Phytophthora infestans is a plant pathogen, which can severely affect the yield and quality of potatoes by infecting the foliage, stems and potatoes at any stage of crop production.

This post describes the effects, favourable factors, and mode of action of the pathogen responsible for potato blight. In addition, you will get to know the symptoms, disease cycle and the control measures of the late blight in potatoes.

Content: Late Blight in Potato

  1. Phytophthora infestans Pathogen
  2. Symptoms
  3. Disease Cycle
  4. Control
  5. Conclusion

Phytophthora infestans Pathogen

Phytophthora infestans is a causal organism causing late blight or potato blight, and the term itself denotes a plant destroyer. Heinrich Anton de Bary coined the term.

P. infestans is a coenocytic oomycete or water mould. It acts like an obligate parasite that requires a living host and invades the host cell. P. infestans spreads intracellularly through its slender and unbranched haustoria.

Primary sources of inoculum include the infected seeds or potatoes kept in storage and culled or volunteer potatoes. The sterilization temperature for P. infestans is 120 °F for a time limit of half an hour. P. infestans has a worldwide distribution.

Systemic Position

  • Kingdom: Fungi
  • Class: Oomycetes
  • Order: Pernosporales
  • Family: Pernosporaceae
  • Genus: Phytophthora
  • Species: P. infestans


Phytophthora infestans can cause severe damage to potato crops by affecting the following:

  • Leaf or foliage develops spots of different colours during crop development.
  • Tubers will develop of small size.
  • It may also cause decay in tubers.
  • It reduces productivity as well as the quality of the crop.
  • In severe cases, there is a complete loss of crop.

Factors Responsible

The infection by the Phytophthora infestans is favoured by the following factors necessary for the pathogen to cause infection and disease development:

  • Free moisture from rain, dew and irrigation methods.
  • Night temperatures of 50 to 60 F.
  • Day temperatures of 60 to 70 F.
  • Cloudiness

Mode of Action

Phytophthora infestans predominantly attack plant foliage. We can see blisters or lesions on leaves as small dead spots with varying colours (brown to purple-black) during this stage.

Besides the plant foliage (including veinlets and margins), the infection also spreads to the areas like rachis, petiole and stem. When the pathogen gets favourable conditions, lesions proliferate, and the infection spreads upward from the lower leaves and petioles.

Then, the plant foliage acquires rapid blistering. The foliage becomes shrivelled in dry climatic conditions, whereas leaves become dead and give a foul smell in moist conditions.

Symptoms of Late Blight of Potato

The infection or late blight in potatoes may occur at any stage of plant growth. The signs and symptoms of late blight in potatoes include the following:

  1. The appearance of watery, light green or yellow blisters on the leaf foliage.
  2. The lower leaf develops brown and black lesions.
  3. In certain areas of the plant, leaf blistering may occur.
  4. Within several days (generally 1-4), leaf rotting occurs by the development of tan coloured blisters (in dry weather) and growth of blue-grey mycelium (in moist weather).
  5. Later, leaves start falling and rotting in tubers also occur. Rotting in tubers are of two types:
    • Dry rot: Bluish-black and reddish-brown fungal growth develop outside and inside of the potato tuber. P. infestans causes 5-15 mm deep rotting in tubers.
    • Wet rot: The potato tuber develops the growth of white colonies outside and secretes water. P. infestans causes 24-45 mm deep rotting in tubers, or we can say it affects the whole potato.

If no one follows control measures for the disease management, then the whole field may be affected that may emit an offensive odour.

Disease Cycle of Late Blight of Potato

The asexual mode of the disease cycle occurs via sporangia either indirectly by zoospores or without an intermediary through the germ-tube formation. The sexual mode of the disease cycle occurs through oospores, resulted after the fusion of oogonium and antheridium.

Asexual Phase

Generally, the asexual phase is more common. Lesions or blisters develop in seedlings or tubers after an attack of the fungus (P. infestans). The fungus may disperse into new infection sites, and later a fungal mycelium develops sporangia.

In wet conditions, the sporangia form biflagellate zoospores within three to five hours and require 12 hours of free moisture. During unfavourable conditions, the sporangia wall bursts out to release free zoospores.

The biflagellate zoospores of P. infestans can survive in dry conditions for an hour and even more in cloudy weather. Zoospores encyst and indirectly infect the host cell wherever they fall through wind or rain transmission.

In dry weather, sporangia act as an individual spore that can form a germ tube to directly invade plant cells. The sporangia germinate to get into the different cellular components.

disease cycle of late blight in potato

Sexual Phase

P. infestans involves diploid oospores, resulted after the nuclear fusion or karyogamy between the male antheridium and female oogonium. Later, oospores germinate to develop sporangia and follow the same pathway as we could see in the above diagram.

Phytophthora infestans has a high reproductive potential that can produce 1 to 3 lakhs sporangia per day from an individual lesion. Then, every sporangium infects new sites with visible symptoms. Lesions in the form of small or wide patches appear on leaves and stem after an incubation period of a few days.

Therefore, the high growth rate of P. infestans can cause rapid defoliation and destruction of foliage and tubers. If the management of the disease is not established correctly, then the pathogen might destroy the whole field in a short time.

If the tuber gets infected with such fungus, then the whole seedling will become diseased. In this way, the disease cycle revolves around every five to seven days, resulting in the dissemination of the late blight infection.

Control Measures

The disease management of late blight in potatoes includes the following control measures:

  1. We must use healthy or resistant varieties of seeds like Kuphri, Alankar etc.
  2. The plant debris must be cleaned to maintain field sanitation.
  3. To control the late blight infection, one should employ a sprinkler irrigation technique that allows foliage to dry at regular intervals.
  4. The application of phosphorous acid to potatoes after harvesting and before piling can minimise the chances of occurrence.
  5. Harvesting of potatoes must be done in the dry season.
  6. Excise the upper portion of the plant before 15 days of harvesting.
  7. The earthing must be 10-15 cm deep.
  8. There should be a balanced use of nitrogen-fixing fertilisers.
  9. Seeds must be treated with 5% of ridomil and dust of 1kg/100kg of seeds.
  10. Seed storage is a primary factor to minimise the consequences of infection by keeping seeds at 3.5-4.4 degrees Celcius.
  11. The sources on inoculum like weeds and infected tubers (culls) must be eliminated.
  12. One can also use foliar fungicides to manage the late blight disease.


Therefore, we can conclude that the late blight disease can cause outbreaks if not appropriately managed. The reproductive power of P. infestans is very high that can form a huge number of sporangia from a single lesion, which may reach the other infection sites.

The disease cycle of late blight of potato continues by biflagellate zoospores or oospores, produced as products of asexual or sexual reproduction of Phytophthora infestans. Late blight is a community disease, which might infect other fields or crops if it persists longer.

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