Late Blight Disease of Potato

 Late blight caused by the fungus Phytophthora infestans is the most important disease of potato that can result into crop failures in a short period if appropriate control measures are not adopted. Losses in potato yield can go as high as 80% in epidemic years. During  past years, epidemics of the disease have appeared in 1985-86, 1989-90, 1992-93, 1997-98, 2006-07  and recently during 2007-08 season.

 Distribution in State

            Late blight occurs frequently in the districts of Hoshiarpur, Jalandhar, Nawan Shehar, Kapurthala, Ropar and Amritsar. Hoshiarpur is generally considered to be the hot spot for this disease. The disease is generally observed first in certain pockets of district Hoshiarpur like Sham Chaurasi, Mehtiana, Phuglana and areas around Tanda and spreads further to adjoining regions under favourable weather conditions. In the districts like Ludhiana, Patiala, Fatehgarh Sahib, Moga and Sangrur, the disease is noticed in moderate incidence and does not appear every year or develops late in the season. South-western districts such as Bathinda, Faridkot and Ferozepur mainly remain free from the disease or its attack is observed very rarely during prolonged epidemic years only. Based on prevalence of the disease in different regions recorded over the past years, a late blight map for Punjab has been prepared. Normally, early sown crop, planted in early September and harvested in mid-November for table purpose, escapes attack of the disease because of high temperature prevailing during this period or may be affected slightly at the end without causing any significant damage.

Potato Cropping Period and Late Blight favourable Period

Symptoms of Late Blight

On leaves:

• Small water-soaked spots develop at the tips, margins or any other part of the leaf which enlarge to form irregular dark brown lesions surrounded by a light green halo.

• During morning hours, a whitish cottony growth of the fungus is visible around the dark brown lesions on the under surface of the leaves especially when weather remains sufficiently humid or when there is dew fall in the morning hours.

• If the weather turns dry, the lesions become necrotic and dry up.

• Under favourable weather conditions (low temperature, high humidity due to intermittent winter rains) the disease spreads rapidly and whole of the crop may be killed within 10-14 days giving blighted appearance.

Potato Leaf Showing late blight Symptoms	Potato foliage affected by late blight
Sporulation of P.infestans on lower leaf surface	Potato field affected by late blight

On Stem:

• In case of infection originating from infected tubers, elongated brown stripes develop on the stem from below the ground level and encircle it.

• The lesions also appear on the stem near the growing point when the inoculum of the fungus comes from the infected plants.

• The infection can also start at nodes and extend both up and down the stem and the plant topples down under congenial weather conditions.

• The infected portions of the stem bear white fungal growth especially visible in early morning hours but are not as prominent as on leaves.

Late blight symptoms on potato stem

On Tubers

• Infected tubers show irregular, shallow or sunken reddish brown patches. Inside infected tissue is spongy and rusty brown to varying depths. In sub-mountainous areas, the lesions on tubers are locally called "Pathar Dag". Later on, these lesions are often invaded by secondary pathogens especially in wet soils or in storage resulting into soft rot.

• Smaller immature tubers are more prone to infection as compared to the larger ones and rotting is more in heavy wet soils.

• Late blight does not spread from tuber to tuber in cold stores.

• Varieties with short stolons bearing tubers near to stem are more liable to tuber infection as in case of variety Kufri Chandramukhi.

Late blight symptoms on potato tubers

 Late blight on spring crop

          In spring crop, which is planted in January, sprouts are attacked at very young stage as the inoculum of the fungus is readily available from the already infected main season crop. Under such conditions, young leaves and stems near the growing points are infected by the fungal spores called sporangia. Brown necrosis is evident around growing points. Ultimately whole of the young sprouts are killed causing almost complete loss in the yield.

Young potato plant attacked by late blight

Disease source, development and spread

• The primary source of the disease is infected seed tubers planted after cold storage.

• Not all, but a few of the infected tubers give rise to primary sporangial inoculum which further infects the nearby plants creating a primary focus of infection in the field.

• Some of the infected tubers do not sprout or are soon killed after sprouting giving patchy appearance in the field.

Diseased tubers piled up near potato field

• The heaps of culled infected potato seed piled near the cold stores also serve as source of primary inoculum for the adjoining fields.

• In the district of Hoshiarpur and adjoining areas of Nawan Shahar, the disease always appears early due to presence of primary inoculum in the form of infected tubers and favourable weather conditions such as high relative humidity and low to moderate temperature.

            For late sown or spring crop or where successive crops are grown, the  previous infected crop often serve as the major source of huge primary inoculum.

Disease cycle

• The fungus over-summers as mycelium in the infected seed potato kept in cold stores.

• These tubers when planted in the next crop season (main crop and subsequent ones) serve as the source of primary inoculum.

• When the plants emerge from such tubers, the fungus invades a few of the growing sprouts and sporulates (produce sporangia) under humid conditions. Further spread of the disease takes place by these sporangia through air or rain splashes.

• Initiation of the disease generally takes about 3-7 days before clearly visible symptoms develop. The fungus produces white sporulation on the underside of the leaves which is clearly visible in the early morning hours.

• These sporangia further infect new leaves and stems of the nearby plants and this cycle continues after every 4-10 days depending upon the prevailing temperature and humidity levels.

• If the temperature is lower than 10⁰ C, disease development slows down and takes more time up to 12 days while at temperature of 16-18⁰ C, it takes only 4-5 days to complete one cycle.

• Sporangia washed by rain or carried by irrigation water cause infection on tubers in the soil.

• Partially exposed tubers can easily become infected.

• These infected tubers serve as the primary source of inoculum for the next year’s crop.

 Disease Cycle of Late Blight of Potato in Punjab

Races of Phytophthora infestans

Presently, there are multigene complex races of P.infestans on Potato in Punjab. Commonly occurring races in the State are:

• 1.2.3.4.5.7.10.11

• 1.2.3.4.7.10.11

• 1.2.3.4.5.7.11

Sporangia of P. infestans