Early blight of potato cause, symptoms, and control
It is one of common and the most important anamorphic fungal disease in the potato growing regions. A potato grower has to follow effective control measures against the early blight of potato to avoid severe loss in total crop production. Early blight of potato is a very common disease in India, the other important disease of potato is late blight of India. Identify the symptoms to control or manage it. Although, the intensity of latter is comparatively less.
It can cause up to 40% loss in yield of tubers. Damage is more severe when attack occurs during the time of tuber formation. The disease appears at the early or young stage of plant. It is a disease of cool as well as warm regions. Hence, it is common in hills as well as in the plains.
Also read: What is Seed Plot Technique in Potato?
|(1). Hosts of the disease|
|(2). Symptoms of ealry blight of potato|
|(3). Causal organism|
|(4). Important points about disease cycle|
|(5). Management of the disease|
(1). Hosts of the disease
Potato, tomato, and chilli are the three major host of the causal organism of this disease.
(2). Symptoms of early blight of potato
2.1: Presence of small spots
At first, we see small, pale brown, and scattered spots on the leaflets. There is a deep greenish-blue growth of fungus on these spots. First attack appears on the leaves near the soil surface, now it progresses upward.
2.2: Formation of target spot
In the necrotic area of the spots, some concentric ridges develop which form a target board. This is the most characteristic symptom of ealry blight of potato.
What is the cause of target board symptom?
It is due to interruption of fugal growth. Unfavorable weather conditions are the cause of interruption of fungal growth. Presence of a narrow chlorotic zone around the spots is also a symptom of this disease. Further, this chlorotic zone turns into the normal green. Size of this chlorotic zone depends on the size of target board.
Number of spots may be less or more. The symptom of chlorotic is more when spots are present in veins.
2.3: Yellow zone around the lesions
What is the cause of yellow zone around the lesions?
This is due to the diffusion of fungal metabolites like the toxin alternaric acid. It is translocated through the veins.
2.4: Dry spots
Curling of leaves and formation of rotting patches
- Spots turn hard in dry season and the leaves curl.
- In humid conditions, the spots coalesce and big rotting may appear.
- Leaves shrivel and fall off in severe attack.
2.5: Symptoms on stem
- Potato stem show brown to black necrotic lesions on the skin.
- These may cause collapse of of the entire plant above the ground.
2.6: Effect on tuber
Infection and rotting may appear.
(3). Causal organism
|Scientific name||Alternaria solania|
|Major features||Production of melanin especially in the spores, and the production of host species toxin.|
- Inter cellular mycelium: It consists of septate and branched, light brown hyphae.
- Intra cellular mycelium: It also consists of septate and branched, light brown hyphae.
- The hyphae in the host are at first intercellular which penetrate the cell later.
It emerges from the dead centres of the spots. Melanins are dark, brown to black, and high molecular weight pigment. Melanins play role in the development of conidia. Conidia born singly on the conidiophores. They develop from a apical cell bud of the conidiophores. Moist weather favours germination of conidia. Optimum temperature is 28 to 30°C.
3.3: Disease cycle
3.3.1: Primary cause of infection
The fungus is resistant to dry and hot weather. Mycelium remains viable in dry leaves for a year. Conidia may remain viable for up to 17 months at room temperature. Mycelium and conidia are the primary cause of infection.
184.108.40.206: Primary source of inoculum
Contamination of tubers with conidia or mycelium is another source of primary inoculum. Hence we can say, host seed is the primary source of inoculum.
220.127.116.11: Site of first infection
- Infection: Lower leaves are the site of first infection. Conidia forms during periods of warm rainy, and humid weather conditions through conidia.
- Penetration: Mode of penetrations are, stomata, direct through cuticle or through wounds.
3.3.2: Site of secondary infection
Infection: Primary spots are the site of secondary spread of the disease. It occurs through conidia. Wind, rains, and insects are the major spreading agents. Incubation period is 48 to 72 hours.
- Upper leaves are less susceptible.
- Young leaves are less susceptible.
(4). Important points about disease cycle
- Penetration occurs most frequently through the junction of epidermal cells.
- Abundant moisture and frequent rains favourable for this disease.
- Increase of inoculum concentration from 6.2 to 11.5 conidia per ml increases the percentage of leaf area infection.
(5). Management of the disease
- Crop rotation and field sanitation are essential for effective control of the disease.
- Destroy dead haulm by burning it.
- Maintain good plant vigour.
5.1: Agronomical measures
The intercropping induces significant reduction in early blight bin following ways:
(1). Allelopathic effect on conidia.
(2). Alteration of the microclimatic conditions around the canopy.
(3). Providing physical barrier against spread of conidia.
5.2: Biological control
Aureobasidium pullulans, Epicoccum nigrum, Penicillium aurantiogriseum, Rhizopuz stolonifer, Trichoderma koningii, and Trichothecium roseum are the natural enemies of Alternaria solania.
5.3: Chemical control
Benzothiadiazole treatment of the potato foliage at 50 mg a.i. per liter induces agains this disease.
5.4: Important points about management practices
Timely spray of fungicide is very important. Start applying fungicide just after one month of planting. Spray fungicide regularly at the intervals of 10 to 21 days, and continue it throughout the growth of plant.