Late Blight is a serious fungal disease of plants belonging to the family Solanaceae, especially potatoes and tomatoes. It is distributed worldwide and destructive to crops grown in the rainy season. The Irish famine, which resulted from an outbreak of this disease during 1845-1850, killed over one million people.
Interesting Science Videos
Causal organism of Late Blight Disease
It is caused by the pathogen Phytophthora infestans which belongs to the family Peronosporaceae under the class Oomycetes under phylum Oomycota belonging to the kingdom Chromista.
Symptoms of Late Blight Disease
The disease first appears on the tops of the plants generally after the blossoming period, mostly in January. Low temperature and high humidity are the most favorable conditions for the growth of the pathogen.
It may appear as well at any time during the growth period of the plant. The disease makes its appearance as small, dead, brownish to purplish black areas or lesions. These appear on the tips and margins of the leaflets, rachis, petiole, and stem. Under favorable conditions, the lesions rapidly increase in size involving the whole surface of the leaf.
The disease generally first attacks the leaves and petioles near the ground and the lower surface of the leaflets on individual plants and then spreads upwards. Finally, a rapid and general blighting of foliage occurs. The blighted leaves curl and shrivel in dry weather. Under moist conditions, they decay and emit a characteristic offensive odor.
The lesions on the lower leaf reveal a delicate growth of the fungus parasite in the form of a white powdery bloom. It consists of sporangiophores and sporangia of the pathogen pushing out through the stomata. The sporangia serve to spread the disease in the growing season.
The potato tubers are often infected in the field and there is the presence of brownish discoloration of the tubers which lie nearest to the soil.
Morphology and reproduction- The fungal mycelium is aseptate, coenocytic, hyaline, and branched with the absence of cross walls (septa). The mycelium develops between cells intercellularly and only extensions of it i.e. haustoria enter the cells.
Phytophthora Reproduction
Phytophthora reproduces asexually and sexually.
Asexual reproduction
Between three and ten days after infection, depending upon environmental conditions, sporangiophores emerge through openings (stomata) on the leaf surface. Zoosporangia or conidia develop at the end of these sporangiophores. After reaching maturity, the zoosporangia easily break off and are dispersed by the wind. Zoosporangium contains motile zoospores that possess two flagella. It germinates directly at temperatures between 20°C-24°C and indirectly at temperatures between 12°C-16°C.
On leaves and stems, the germ tubes may directly penetrate the epidermis whereas on tubers, it germinates through lenticels or wounds.
Sexual reproduction
When mycelia of different mating types of fungus generally known as A1 and A2 grow together, the oogonium fertilizes with the antheridium giving rise to diploid thick-walled oospore which germinates in a similar way to that of asexual reproduction.
Late Blight Disease Cycle
The fungus overwinters as a dormant mycelium in the infected tubers. It becomes activated at the time of germination of the diseased seed tubers or infected tubers remaining in the ground after the previous crop. The activated mycelium invades the healthy sprouts.
Primary Infection
The thick-walled resting oospores are the source of primary infection. At the time of planting, the oospore germinates and the germ tube ends in terminal sporangium. The contents of the sporangium divide to form zoospores. The infected sprouts emerge above ground to produce shoots containing the mycelium. It grows and ramifies in the intercellular spaces absorbing nutrition through haustoria into the host cells.
Under optimum temperature and humidity, the mycelium pushes out branched sporangiophores through the stomata of host leaves.
The thin-walled ovoid lemon-shaped sporangia, each with an apiculate tip, are borne singly at the tips of sporangiophores or their branches. As the sporangium reaches maturity, the supporting hyphal branch immediately below it swells slightly and continues to grow turning the attached sporangium to the side.
The elongation of the branch proceeds and a new sporangium is borne. A fertile sporangiophore is thus characterized by 9 or 10 such swellings occurring at intervals.
Each nodular swelling marks the point where the sporangia are borne. The mature sporangia are readily detached and spread by splashing rain or air currents to new plants.
On reaching a suitable host, the sporangia germinate on the leaves in two ways-
- Indirect germination- In cool moist weather the sporangia functions as zoosporangia and the protoplasm of sporangium is divided into eight biflagellate zoospores. The optimum temperature for the germination is 12°C-16°C. The zoospores are liberated in a group through terminal pores formed by the rupture of the apical papilla. The released zoospores, after a brief period of activity in rainwater or dew, come to rest. Each retracts its flagella and secretes a wall around it.
- Direct germination- Under dry and warmer conditions, no zoospores are formed. The sporangium functions as a conidium. It directly puts out a germ tube or infection thread. The optimum temperature for this direct germination is 20°C-24°C.
Secondary Infection
The infection thread produced on the surface of the host leaf enters the host tissue. It makes its entry occasionally through the stoma but more often it penetrates directly through the cuticle by a penetration hypha arising from the appressorium. The lower surface of the leaf is more susceptible to the pathogen. The infected leaves produce another sporangia which are carried out by winds to the healthy plants and are thus infected. This constitutes a secondary infection.
As a result, the disease spreads during the growing season and spreads quickly when cool and wet nights alternate with warm moist days. Low temperature and high humidity favor the spread of the disease.
Factors Affecting Late Blight Disease
Temperature and humidity are the main environmental factors affecting the development of disease.
Temperature– The optimum temperature for late blight is 18°-22°C. The pathogen prefers cool and damp conditions, if the temperature is above 30°C the pathogen is unable to infect the host tissue.
Humidity- Late blight thrives well in humid conditions with a relative humidity of more than 90%.
Control Measures of Late Blight Disease
Selection of seed- The seed or seed tubers must be free from disease. It requires strict seed or tuber inspection.
Storage at 40°F or below- Storage of potato tubers or tomatoes in cold storage reduces the progress of the disease.
Growing disease-resistant varieties- Growing resistant varieties will increase in opportunity to combat a disease.
Use of fungicides- The control of the blight is accomplished by applying fungicides before infection which are applied in various ways-
- Spraying- The best method of control is the timely and repeated foliage spray schedule with copper fungicides such as Fytolan, Perenox, Dithane Z-78, etc. The spraying should start when the plants are 7-8 inches tall.
- Dusting- Dusting the foliage with copper-lime dust is more effective in controlling the pathogen. Dusting is done in the morning when the plants are wet with dew.
Sanitation- Destruction or proper disposal of potato tuber from pits and storehouses is one of the practical methods to reduce the incidence of the disease.
Tuber treatment before storage- Tubers should be placed in 1:1000 mercuric chloride solution for 90 minutes before storage.
References
- Vasistha, B. R., Sinha, A. K., & Kumar, A. (2016).Fungi. S. Chand & Company Ltd.
- Gupta, V. K., & Sharma, S. K. (2011). Plant Pathology. Kalyani Publishers.
- Mehrotra, R. S., & Aggarwal, A. (2003). Plant Pathology. Tata McGraw-Hill Education.
- Late blight of tomato and potato. (n.d.). UMN Extension. https://extension.umn.edu/disease-management/late-blight
- The Editors of Encyclopaedia Britannica. (2024, August 5). Late blight | Description, Symptoms, & Control. Encyclopedia Britannica. https://www.britannica.com/science/late-blight
- Late blight of potato and tomato. (n.d.). Late Blight of Potato and Tomato. https://www.apsnet.org/edcenter/disandpath/oomycete/pdlessons/Pages/LateBlight.aspx
- Admin. (2022, November 3). Late blight of potato. BYJUS. https://byjus.com/biology/late-blight-of-potato/
- Ivanov, A. A., Ukladov, E. O., & Golubeva, T. S. (2021). Phytophthora infestans: An Overview of Methods and Attempts to Combat Late Blight. Journal of Fungi, 7(12), 1071. https://doi.org/10.3390/jof7121071
- Whisson, S. C., Boevink, P. C., Wang, S., & Birch, P. R. (2016). The cell biology of late blight disease. Current Opinion in Microbiology, 34, 127–135. https://doi.org/10.1016/j.mib.2016.09.002
- Late Blight. (n.d.). Bayer. https://cropscience.bayer.co.uk/agronomy-id/diseases/potato-diseases/late-blight