Early blight of potato

Host: Potato (Solanum tuberosum L.)
Pathogen: Alternaria solani Sorauer

Distribution

Early blight of potato is the most common disease of potato found in all the potato growing regions of the world. As the name 'early' blight suggests, it appears before the 'late' blight. It attacks more severely in early maturing cultivars in contrast to late blight, which attacks more severely in the mid and late maturing cultivars. This disease is reported from wide range of environmental condition in across the globe including countries like, Australia, Brazil, Canada, Germany, India, Poland, United States. In Indian subcontinent, loss yield of more than 79 % is reported in severely infected field. Foliar wetness caused by irrigation, rain or dew, the susceptibility of cultivar and nutritional status of plants are the factors which determine the severity of the early blight.

Symptoms

On foliage

Early symptoms appear as dark brown dot-like small blotches on the older (lower) leaves. Soon they become small and circular lesions. As the disease progresses, the circular lesions enlarge and cover the whole leaf areas restricted only by the leaf veins. The lesions can grow up to the diameter of half an inch. This time the lesions become angular and give the appearance of blight. In severe infection elongate lesions appear on the petioles and stems. If untreated, leaves withers and entire foliage die.
The infection of early blight can be easily identified by the presence of alternating dark-brown and light colored concentric rings in the lesions, which give characteristics 'target board' appearance.

On tubers

Tubers of early blight infected plants bear circular to irregular, slightly sunken lesions that are surrounded by a raised dark brown border. These reduce the marketability of potato tubers.

Pathogen

Early blight of potato is caused by several species of Alternaria across the world. Alternaria solani however, is the most reported and studied culprit. Conidia are produced singly or in short chains, which are pale to olive-brown, straight to flexuous, mostly obclavate and double-walled. Conidia are muriform containing 0–8 longitudinal or oblique and 6–19 transverse septa.

Alternaria solani
Various stages of conidial development in Alternari solani. bar =  20 μm

Disease cycle

Primary infection occurs by means for conidia coming from other infected hosts. Overwintering conidia on the debris in the infected field also comes in contact with the potato leaves through the rain splashes, air currents, windblown soil particles and irrigation water.
After successful infection pathogen produces numerous conidia on the leaves under favorable environmental conditions. These conidia provide secondary inoculum and increases the disease spread exponentially. Alternating wet and dry periods are the most favorable condition for the sporulation and dispersal. Temperature ranging between 5 - 30 °C is favorable for sporulation and infection, while 20 °C is optimum.

Control measure

  • Late-season variety should be sown. Early maturing varieties are are less resistant and more prone to be infected.
  • Disease appears only a week or two after infection. Hence, fungicides should be sprayed in advance. A combination of contact plus systemic fungicides often gives good result. Mancozeb and chlorothalonil well fulfill the purpose.
  • According to Sharma et al. (2020) three spray of combination of fungicides fenamidone 10% + mancozeb 50% WDG @ 0.2% treatment (2.5 gm/l) or metiram 55% + pyraclostrobin 5% WG @ 0.2% (2 gm/l) or cymoxanil 8% + mancozeb 64% WP @ 0.25% (2.5 gm/l) at 15 days interval are very effective in controlling the disease.
  • Rotation in types and modes of fungicides should be practiced in order to avoid the fungicides resistance development in the pathogen.
  • In severely infected field crop rotation with non-host crops for up to 5 years should be practiced.
  • Resistant varieties available in the region should be sown.
  • Scout the field on regular basis and monitor the physiological days (P-days). Follow the instructions of local advisories based on the forecasting systems set for early blight of potato.

References

  • Sharma, R.K., Patel, J.K., Patel, D.R. and Patel, R.N., 2020. Management of Early Blight of Potato (Solanum tuberosum L.) caused by Alternaria solani [(Ellis & Martin) Jones & Grout] through Fungicides and its Impact on Yield. Int. J. Curr. Microbiol. App. Sci, 9(3), pp.1683-1693.

Late blight of potato

Host: Potato Solanum tuberosum
Pathogen: Phytophthora infestans (Mont.) de Bary

History and distribution

Late blight of potato is historically very significant disease. The causal organism of the disease originated in the Central Mexico, where both of its mating types, A1 and A2 as well as other relative species of Phytophthora are prevalent. Late blight of potato was brought to the North America and Europe with infected potato by the migrants.
The disease is the culprit of European, Irish and Highland famine during 1840s. Great Irish Potato Famine that seen during 1845 to 1849 caused mass starvation because of unavailability of potato (staple food). About one million people died and up to two million became refugee in the neighboring countries. The total population of Ireland dropped by 20 to 25 %. 1847 was the worst year of this famine, which is recognized as "Black 47" and this decade is known as the "Hungry Forties".
In Indian sub-continent late blight of potato was first recorded between 1870 to 1880 in Nilgiri Mountains. Until mid of the 20th century, the disease has been reported from almost all potato growing states of India. In Uttar Pradesh, late blight of potato was reported for the first time in 1943 in Dehradoon and Meerut.

Symptoms

Symptoms on leaves, petioles and stems:

Early symptoms of late blight of potato includes blackish/brown circular to irregular water-soaked lesions on the lower leaves. In cool and moist weather conditions, these lesions expand rapidly and appear greasy. The infected area is sometimes surrounded by chlorotic border (yellow halo). As the lesions expand, they coalesce and cover the entire leaf. During active growth, fungus produces white mildew-appearing area at the lower (abaxial) surface of the leaves. These are sporangia and sporangiophores of the pathogen. If the weather changes to warm and dry, these lesions become tanned so-called 'blight'.
If not properly treated with fungicides, the lesions appear on the petiole and stem and with in few days of infection whole plant is destroyed.
The late blight infected field of potato give characteristic odor.

Symptoms on potato tuber:

The sporangiospores produced on the leaves washed on the soil and enter to the potato tuber, where they infect the tubers. Symptoms on the tuber appear first in the region of eyes, lenticels and cracks. Infected tubers show irregularly shaped, slightly depressed brown to purplish area on the skin. Infection of late blight often host the secondary pathogens including soft rot bacteria, which grow more rapidly than late blight pathogen and convert the whole potato tuber into rotten mass.

Blight of potato
Photographs showing the potato crops. a: healthy plants of potato, b: late blight affected potato in the field, and c: abaxial surface of infected leaves.

Pathogen and disease cycle

Late blight of potato is caused by Phytophthora infestans. It was first named as Botrytis infestans by M. J. Berkeley in the 1840’s, which later was renamed as Phytophthora infestans by Anton de Bary in 1876. It is an obligate parasite (biotroph) that can not survive without its host. P infestans is a heterothallic fungus like organism, that requires two mating types for sexual reproduction. Two mating types are A1 and A2. In most of the places, A1 mating type is found, restricting the sexual reproduction in the region. Both A1 and A2 mating types are reported only is several countries of the world including Mexico, Switzerland, England, Wales, Netherland, Poland, Finland, Norway, Hungary, France and Czech Republic (Mazáková et al., 2010). Hence, in most of the times and places, P. infestans reproduce only by means of asexual reproduction.
Infected potato tubers missed during harvest, from storage and seed tubers are source of primary inoculum. After primary infection in the healthy tuber and plants, P. infestans produces zoospores, which act as secondary inoculum and spread the disease in the field.

Epidemiology

Cool temperature (3 - 26 °C) and high moisture (>90 %) favor the disease development and spread. Pathogen produces numerous sporangia on the infected stems and lower side of the leaves at 90 % relative humidity and temperature between 18 - 22 °C. At temperature between 21-26 °C, sporangia directly germinate by producing germ tube (behave as conidia), while below 18 °C, produce 6 to 8 zoospores. Zoospores are flagellated, swims in film of water (moisture), encysted and germinate to spread the infection.
Spores of the pathogen develop in 3 to 5 days and within 12 hours cause a new infection. This is why, disease become severe in cool and wet conditions. Cool nights and warm days with extended wet conditions (caused by rain, dew, fog and sprinkler irrigation) favor the onset of late blight epidemics.
In norther states of India, late blight epidemics is known as "Pala of potato".

Control measures

  • As, humidity in cool temperature increase the diseases severity, fields should be with proper air passage and good drainage.
  • Plantation should be done with proper spacing.
  • Infected tubers are the source of primary inoculum. Hence infected tubers should be buried in landfill or below 1m deep.
  • Volunteer plants are also good source of inoculum. Hence should be destroyed.
  • Only certified tubers should be used for plantation.
  • Freshly cut seed tubers should be treated in Mancozeb fungicides.
  • Over-watering and night irrigation should be avoided to keep the humidity at its minimum.
  • Excessive nitrogenous fertilizer makes the canopy dense allowing more humidity to retain, which should be avoided.
  • Contact fungicides should be sprayed on foliage as a preventive measure. On appearance of symptoms, systemic fungicides (e.g, dimethomorph, cymoxanil) should be used.
  • In infected field, crop rotation of two to three years with non-host plant should be practiced.
  • If disease forecasting system is available, advise from the center should be followed. Some of the forecasting systems for late blight of potato is listed here.
  • Varieties resistant to late blight should be used. Some of these varieties are Kufri Sona, Kufri Khyati, Kufri Megha and Kufri Arjun (Pradel et al., 2019).
  • Covering of foliage with ash reduces the humidity and diseases incidence (Farmer's perception from norther states of India).
  • Vines should be killed two to three before harvesting.
  • Burning of hay and straw near small field of potato also reduces the humidity and disease incidence (Farmer's perception from norther states of India).


References




Content first created on 15-01-2024
last updated on 15-01-2024