Late Blight in Potato

Introduction image

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

In this article, we will discuss the effects, favourable factors and the mode of action of  the pathogen responsible for potato blight. In addition, the symptoms, disease cycle and the control measures of  the late blight in potato are explained, important for the exam point of view.

Content: Late Blight in Potato

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

Causal Organism

Phytophthora infestans is a causal organism that causes late blight or potato blight. A term phytophthora infestans itself denotes as a plant destroyer, which was first coined by a scientist named Heinrich Anton de Bary.

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 by its slender and unbranched haustoria, and spreads intracellularly.

Primary sources of inoculum include the infected seeds or potatoes kept in storage, culled or volunteer potatoes etc. The sterilisation 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


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

  • Leaf or foliage develops spots of different colours during the crop development.
  • The 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 that are necessary for the pathogen 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 firstly attack plant foliage. During this stage, there we can see the appearance of blisters or lesions on the leaf as small, dead spots with varying colours (brown to purple-black).

Later, the plant foliage, including veinlets and margins, the infection also spread to the areas like rachis, petiole and stem. When the pathogen gets its favourable conditions, the size of lesions proliferate, and the infection spreads upward from the lower leaves and petioles.

Then, the plant foliage acquires rapid blistering. In dry climatic conditions, the foliage becomes shrivelled, while in the moist condition the leaf becomes dead and gives a foul smell.

Symptoms of Late Blight in Potato

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

  1. The appearance of watery, light green or yellow blisters on the leaf foliage.
  2. Lower leaf develops brown, 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, the leaf starts falling and cause rotting in tubers as well. Rotting in tubers are of two types:
    • Dry rot: The potato tuber develops bluish-black growth outside, and becomes reddish brown inside. P. infestans causes 5-15mm deep rotting in the tuber.
    • Wet rot: The potato tuber develops the growth of white colonies outside and secretes water. P. infestans causes 24-45mm deep rotting in the tuber, or we can say it can affect the whole potato.

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

Disease Cycle

The asexual mode of disease cycle occurs via sporangia either indirectly by the zoospores or without an intermediary through the germ-tube formation. The sexual mode of disease cycle occurs through the oospore, resulted after the fusion of oogonium and anthredium. Generally, the asexual phase is more common. Lesions or blisters develop in the seedlings or tubers by the attack of fungus (P. infestans).

The fungus may disperse into the new infection sites, and then a fungal mycelium develops sporangia. In wet conditions, the sporangia form biflagellate zoospores within three to five hours and requires 12 hours of free moisture. On unfavourable condition, 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 can indirectly infect the host cell wherever they fall via wind or rain transmission.

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

disease cycle of late blight in potato

The sexual phase of P. infestans involves diploid oospores, resulted after the nuclear fusion or karyogamy between the male antheridium and female oogonium. Later, the oospores germinate to develop sporangia, which then follows the same pathway as we can 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 can start a new infection with the visible symptoms. Lesions in the form of small or wide patches appear on the leaves and stem after an incubation period of a few days.

Therefore, the high growth rate of the 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 that occurs every five to seven days, resulting in dissemination of the late blight infection.


The disease management of late blight in potato includes the following:

  1. We must use healthy or resistant varieties of seeds like Kuphri, Alankar etc.
  2. The plant debris must be cleaned relative to field sanitation.
  3. To control the infection of late blight, one should employ a sprinkler irrigation technique that allows foliage to dry at regular intervals.
  4. The application of phosphorous acid to potatoes after harvest and before piling can minimise the occurrence of late blight.
  5. Harvesting of potatoes must be done in the dry season.
  6. The upper portion of the plant should be excised out before 15 days of harvesting.
  7. The earthing must be 10-15 cm deepened.
  8. There should be balanced used 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 the 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 disease.


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

The disease cycle may continue either by the biflagellate zoospores or oospores, produced as a product of asexual or sexual reproduction of Phytophthora infestans. Late blight is a community disease, which might infect other fields or crops if persists longer.

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