Blight Warnings & Smith Periods
The way to stop blight spreading is to have hot, dry days. Infection is dependent on warm, wet weather, and since the conditions for blight infection are so specific and well understood, periods of high risk can be predicted with some accuracy.
The humidity around plants is increased by watering, and this can accelerate the spread of blight. If watering is necessary it is better to do it from beneath to avoid wetting the leaves.
The conditions under which blight infections can be expected are known as a "Smith Period". This is a 48 hour period in which the minimum temperature is 10°C or more and the relative humidity exceeds 90% for at least 11 hours during the first 24 hours and for at least 11 hours again during the final 24 hours.
One can receive automatic warnings of Smith Periods and blight infection from the UK Potato Council by registering for 'blight alerts'. The service is free and is available to commercial and amateur growers.
The pathogen overwinters in neglected tubers - those not removed from the ground, outgrades, unused seed, and self-setters. It is important to clean up and dispose of this material. The majority of infections in gardens however arise from wind-blown blight spores which probably originated miles away.
Infections are generally first noticed during early June, often in the South-West of the country.
The genetic makeup of blight is always changing and evolving. Before 1976 all blight outside Mexico reproduced asexually and showed little genetic variation.
Now we have 'Blue 13' which is more infectious and aggressive because it can reproduce sexually. It is thought to have arrived in Europe from Mexico (the probable home of blight) on some imported potatoes.
Because it can reproduce sexually it forms new strains (and overcomes resistance) more quickly than before.
Blue 13 is now the predominant blight strain in the UK, and it has overcome the resistance of a number of varieties.
The Sarpo varieties are still resistant at the time of writing. (2010)
ND / Diversity website
