Late blight of Potato :
A. OCCURRENCE AND IMPORTANCE
Late blight of potato is the most serious of all potato diseases. This disease is of wide occurrence and is known in all potato growing parts of the world.
Potato is a native of South America (northern Andes). Late blight occurred on potatoes in the Andes in epidemic form.
It was then introduced into Europe and North America. The famous Irish famine in 1845-1846 was caused due to the destruction of the potato crop by late blight disease.
In India, it occurs in epidemic form only on the hills at an elevation of 2000 m and above. Due to cold storage facilities of tubers, the disease is at present of wide occurrence in the plains of India, because the pathogen survives at the cold temperature.
Late blight of potato was first introduced in India in Nilgiri hills between 1870 and 1880. Soon after. it spread to the Darjeeling district in the Himalayas.
Late blight damages plants by killing the leaves and stems of potato plants at any time during the growing season.
it also attacks potato tubers in the field. the tuber may also rot in the field or while in storage, transit and market.
Late blight may cause total destruction of all plant-parts in the field within one or two weeks if favorable conditions prevail and when no control measures are applied.
Depending on the temperature and moisture of the growing season and on the control measures practiced, losses caused by late blight damage, vary from one area to another and from year to year.
The symptom appears first on the leaves as black or purplish-black circular or irregular water-soaked patches, generally at the tips or edges of the lower leaves.
In moist weather, these patches enlarge rapidly and form brown, blighted areas with indefinite borders. often a pale yellowish-green zone surrounds the rapidly enlarging lesions.
On the undersurfaces of the leaves, a zone of white, downy fungal growth appears near the border of the lesions. Very soon, the entire and all the leaves become infected, die, and limp.
Under continuously moist conditions, all tender and above-ground plant parts blight and rot away quickly giving off a characteristic odor.
In dry weather, the fungal activities are checked – existing lesions stop enlarging, turn black, curl and wither away.
The tubers are also attacked. Affected tubers at first show irregular, purplish-black or brownish blotches with a metallic dull, dark color.
When tubers are cut open, the affected tissue appears water-soaked (i.e. soft), dark, reddish-brown and extends a few mm into the flesh of the tuber.
later on, the affected areas become firm, dry and sunken-such lesions may be small or may involve almost the entire surface of the tuber.
It is to be noted that. the rot may continue to develop even after the tubers are harvested, or infected tubers may be subsequently invaded by secondary fungi and bacteria causing soft rots.
C. THE CAUSAL ORGANISM
Late Blight of Potato is caused by Phytophthora infestans (Mont.) De Bary. This pathogen is a phycomycete.
D. DISEASE CYCLE (Development of Disease)
The disease is propagated by sporangia which germinate directly or indirectly i.e. by zoospore formation. 90% relative humidity favors the sporangial germination.
Production of sporangia is accomplished in a saturated atmosphere within a temperature range between 20°C to 26°C.
But the infection declines at a temperature above 20°C, because of a fall in germination percentage and the slow growth of germ tube.
Cool-weather accompanied by abundant moisture is ideal for the bulk production of inoculum.
Various theories have been put forward to explain the disease cycle and availability of inoculum.
The persistence of the fungus in the soil is out of the question because the temperature during summer becomes very high.
The only source of inoculum is through infected seed tubers. The large scale practice of storing the potato seeds (tubers) in cold storage has mainly resulted in the perpetuation of the disease in the plains of India.
Since oospores are formed in nature, it is also presumed that the pathogen possibly perennated in that stage.
Therefore primary inoculum of the disease in the field comes from the planting of infected seed tubers and from oospores in the previous year’s plant debris, the pathogen next invades developing sprouts to form lesions and sporangia or zoospores above ground.
After the formation of sporangia or zoospores, the disease spreads to a large number of plants. The infection by sporangia or zoospores may take place through any part of leaves and stems, either through stomata or through the epidermis.
In tubers, the infection takes place through ‘eyes’, lenticels, or wounds.
E. DISEASE CYCLE WITH DIAGRAM
Late blight of potatoes can be controlled successfully by the following methods :
- Only disease-free potatoes should be used for seeds.
- Potato dumps or cull piles should be burned before planting time or sprayed with strong herbicides to kill all sprouts or green growth.
- All volunteer potato plants in the area (whether in the potato or other fields) should be destroyed, because any volunteer potato plant may be the source of infection.
- Only the resistant varieties available i.e. varieties of potato that are either immune from or resistant to late blight should be planted. Such varieties include Kufri-Sinduri, K. Chandramukhi, K. Kissan, R.K.M., K. Chamatkar, etc. such varieties resist one or more races of the late blight fungus.
- Late blight may be controlled successfully by chemical spraying with fungicides.