Late blight of Potato : Symptoms and Disease Management (with Diagram)

In this article we will discuss about (1) Meaning of Of Late blight of Potato (2) Symptoms Of Late blight (3) Disease Management Of Late blight

(1) Meaning of Of Late blight of Potato

Host: Solanum tuberosum L.

Causal Organism: Phytophthora infestans

The disease causes blight symptom which appears towards middle to late in the season, that’s why the name is given. Potato is a native of the North Andes in South America. Initially, late blight of potato was available in that area as an endemic disease. Gradually, by 1840-1847, it spread throughout USA and Europe.

The disease became well established in Ireland, Europe and England by 1842. Later, in 1845, it spread as epiphytic disease throughout the Europe and brought famine in Ireland, causing the death of one million (1,000,000) people and more than one million migrated to other countries.

In India, the Late blight was introduced in the Nilgiri hilts between 1870-1880 from England. Now-a-days, the disease is frequently available in the potato growing areas of India.

The loss from this disease is less in West Bengal and Bihar (5-10%), which gradually increases in Punjab (20-25%), U.P. (15-50%) and Haryana (40-50%) and causes a monetary loss of Rs. 4 crores per year. The loss is gradually reduced by the introduction and popularisation in culti­vation of Kufri Chandramukhi, and some other varieties.

According to A. Basu (2002), some exotic Dutch cultivars like Cardinal and Diament show better resistance along with good yield in West Bengal.

(2) Symptoms Of Late blight

Symptoms of Late Blight of Potato are Available on Foliage and Inside the Tuber:

1. On Foliage:

Symptoms appear as hydrotic areas with indefinite margins at the apex and edges of the leaflets (Fig. 5.18A). The infec­ted regions gradually turn brown to black due to necrosis. In early stage of disease development, chlorotic border develops around the necrotic regions.

During moist weather, the disease progre­sses very rapidly, causes much more decay of the infected regions and produces a characteristic bad odour. The sporangiophore and sporangia are visible as matty growth on the abaxial surface of the leaflet. The entire plant may be damaged completely within a few days.

During dry weather, the disease progresses very slowly and the affected regions curl and shrivel. The diseased areas become hard and easily break with small disturbance.

2. Inside the Tuber:

The pathogen does not go down through the stem to the tuber.

The Tuber Infection takes place in different ways:

(a) During Growing Season:

Hydrotic areas develop on the tuber surface and the regions become necrotic. If condition favours, the entire potato turns brown and becomes damaged completely before harvest.

(b) During Harvest:

During harvesting, the tuber with delicate skin may get rup­tured and the tuber may find contact with the infected leaf and cause infec­tion. Later, the symptom is visible after cutting the tuber as a wheel marked by small brown dots (Fig. 5.18B).

Disease Cycle :

In the dormant phase, the fungus could perennate in the tuber as mycelium and in plant debris as oospore. During favourable condition, the perennating mycelium becomes active and develops active mycelium. The active mycelium develops branched sporan­giophore bearing sporangia on it.

On the other hand, the oospore germinates by producing germ tube bearing sporangia at their tip. In both the cases, the developed sporangia disperse by wind or rain, germinate on host surface by producing zoospores and cause infection to the host tissue.

The infected host again develops sporangia on sporangiophore. The sporangia develop zoospore which cause further infection. This process repeats several times in the growing season — this is the secondary cycle.

During favourable condition, the zoospores may come down into the soil and may cause infection to the tubers. The tuber infection may also take place on contact with infected foliage during harvest.

The perennating mycelium inside the tuber remains active, if the infected tubers are kept in the storage for seed potato. In the next season, after sowing the seed, the perennating mycelium becomes active and causes further infection.

Towards the end of the season, inside the aerial parts, both the sex organs — antheridium (male) and oogonium (female) — develop and undergo sexual reproduction of amphigynous type (Fig. 5.19G).

The product of sexual reproduction is oospore, which has hard protective covering. The oospore remains dormant inside the host tissue during unfavourable season. During favourable condition in the next season they germinate and produce next crop of zoospores.

Oospore formation is very rare in USA and Canada and may not play a significant role in the survival of the fungus. But it is frequently avai­lable in Mexico and indicates the possibility of playing significant role in the annual recurrence.

But in India, the potato-growing season is followed by hot summer season in the plains, ruling out the possibility of survival of inoculum in the soil and also in the plant debries. The perennating mycelium in the tuber is killed by exposure at 40°C for 4 hours or at 30°C for 65 hours. Thus, in plains, the tubers stored in the house at room temperature becomes free from disease.

In the northern hills and also in its surrounding plains, the mycelium may remain active due to high humidity and low temperature and causes disease in more severe form. The disease also appears with more intensity, possi­bly due to survival of inoculum inside the seed tubers kept in cold storage.

(3) Disease Management Of Late blight

The disease can be controlled or reduced by the following procedures:

A. Cultural Methods:

The methods useful in controlling the late blight of potato are:

1. Seed tubers should not be collected from the disease field.

2. Tubers should be harvested after the maturity of the tubers (when skin of the tubers comes tough).

3. In infected field, the aerial part of the plants should be dried completely before harvest of tubers.

4. Infected plant should be cut out and collectively buried deep in the soil.

5. The previous year’s plant debris should be cleaned from the field and it must be burnt outside the field.

6. The volunteer potato plants that may harbour the infection mainly in hills should be eradicated during appea­rance.

7. Planting of alternate row of susceptible and resistant variety will reduce the intensity of the disease.

8. Cultivation of tuber on high ridges and covering the tuber with soil will prevent the fungus to grow out from the infected mother tuber, thus reduce the spread of the disease.

9. The nitrogen fertiliser should be applied at a reduced rate.

10. During harvesting, the potato plants should be placed on one side and tubers on other side, thereby contact between tuber and infected plant can be avoided.

B. Physical Methods:

Following methods are used to control the disease:

1. The perenating mycelium in the tuber can be killed by exposure to 30°C for 65 hours or to 40°C for 4 hours.

2. The perenating mycelium can be killed by dipping the tuber in water at 45°C for 4-5 hours or at 40°C for 24 hours.

C. Chemical Methods:

1. Dutt (1962), while working with 16 different fungicides in hilly region, found that Bordeaux mixture (5:5: 50) is the most effective giving better result than. Burgundy mixture.

2. Fungicides like Brestan (1 kg/ha), Dithane M-45 (2 kg/ha) and Difolitan 80WP (2.5 kg/ha) are used as foliar spray and found highly effective in con­trolling late blight disease of potato in plains of India.

3. According to Khanna and Sharma (1981), Metalaxyl (1 kg/ha), a systematic fungi­cide, is found highly effective in con­trolling the disease, thus increasing the yield about 3 times than the control.

D. Biological Methods (Biocontrol):

1. Jindal et al., (1988) reported the role of Epicoccum purpurascens, Trichoderma koningii and Stachybotrys atra in con­trolling the disease.

2. Roy et al. (1991) reported the role of Penicillium aurantiogriseum and Myrothecium verrucaria in late blight disease management.

3. Arora (1999) reported the role of Penicillium viridecatum, Chaetium brasilense and Trichoderma viride in controlling the late blight disease. E. Disease tolerant varieties. It is better to use tolerant varieties.

In India, Central Potato Breeding Station (Simla) has developed some tolerant varieties like Kufri Jyoti, Kufri Naveen, Kufri Badshah, Kufri Jeevan, Kufri Neela etc. for commercial cultivation. Cardinal and Diament (Dutch cultivrs) and Kufri Chandramukhi and Kufri Chipsona-1 (Indian cultivars) are the varieties showing good performance in alluvial zone of West Bengal.