Late Blight Of Potato

Potato is a native of Andes (South America), where this disease occurred in endemic form. The disease was recorded simultaneously in Europe and North America in 1830. It became very well established in Europe by 1842 and was one of the important causes of the great Irish famine in 1845 and 1846. It invaded the sub-continent between 1870 and 1880. This is extremely destructive to tomato and eggplant and many other Solanaceous hosts except pepper. It may bring about as much …

Potato is a native of Andes (South America), where this disease occurred in endemic form. The disease was recorded simultaneously in Europe and North America in 1830. It became very well established in Europe by 1842 and was one of the important causes of the great Irish famine in 1845 and 1846. It invaded the sub-continent between 1870 and 1880. This is extremely destructive to tomato and eggplant and many other Solanaceous hosts except pepper. It may bring about as much as 10-15 percent losses to world production of potatoes during certain years.

Symptoms :The disease manifests itself only after the blossoming period. The first symptoms appear in the form of brown dead spots or extended necrotic areas more frequently until the leaves are killed. These blighted areas first appear as faded green patches, which soon turn to brownish black lesions not delimited in size. These lesions enlarge rapidly under favourable weather. They appear on tips of margins of the leaves and spread downwards or inwards. In moist weather, entire leaf may be killed in 1-4 days, while in dry weather, infection advances slowly and affected areas curl and shrivel and the spots are restricted in size and look hard, easily breaking away. On lower surface of leaves, a whitish or grayish mildew growth appears on the surface of lesions where pale and purplish tissues merge. This contains sporangiophores and sporangia, which grow out through stomata.

Tubers are also affected in the field, having dry or wet rot according to the moisture and temperature prevailing at that time. There is brown to purple discolouration of skin followed by brownish dry rot, which extends about ½″ below the skin.

Casual Organism : Phytophthora infestans

Disease Cycle: Fungus survives mostly as persistent mycelium in the infected tubers, but it may over winter in soil as well. This mycelium grows up in the stem and produces sporangia and zoospores on small dwarf shoots. The secondary infection starts from these spores. Infection takes place epidermis of leaves and stem either through stomata or directly. Tuber infection occurs through eyes, lenticels or wounds. Susceptibility of eyes and resistance of lenticels increases with storage and maturity of tubers. Spores from blighted leaves are washed down to soil and cause tuber infection.

Epidemiology : Chances of epidemic development of disease are the maximum when unusually cool weather combined with abundant moisture prevails at the time of sporangial formation. Conidia are formed at a minimum relative humidity of 91 % (optimum = 100 percent) and a temperature range of 3-26°C (optimum = 18-22°C). The conidia formed at 15°C and optimum temperature for zoospores formation is 12°C. Cool moist nights are thus required to provide most rapid build up of inoculum and are the most favourable for the formation and germination of zoospores. It is possible to make an accurate forecast of the disease if accurate meteorological data are available and current weather trends are known. For example, in Holland they have worked out the following four criteria necessary for the occurrence of late blight epidemics:

1. Night temperature below the dew point for about four hours or more.
2. Night temperature not more than 10°C.
3. Mean cloudiness not below 0.8 on the following day.
4. Rainfall of at least 0.1 mm on the following day

Very fortunately, in plains of Pakistan, the weather conditions are unfavourable for the development of epidemics. But in the hills, sometimes, temperature and moisture conditions are favorable and therefore, late blight epidemics may occur in certain years.

Control :

1. Seed tubers should be obtained from areas where disease does not occur.
2. Use of resistant varieties: All the commercial varieties within the species Solanum tuberosum are susceptible. However, Solanum demissum, a species from Mexico is highly resistant. Hybridization of this variety with commercial types has been tried with good results. e.g. Cord
3. Foliage spraying is the most satisfactory control method. Bordeaux Mixture and some new fungicides such as Acrobat MZ, Ridomil Gold MZ and Banko (0.2 – 0.25%) give good results.
4. General Sanitary Measures, killing and removing of foliage a few days before actual digging of potatoes is beneficial in reducing chances of tuber infection.