The late blight pathogen is a microscopic, fungus-like organism whose sporangia (spore-bearing structures) easily break away from infected foliage and may be wind-blown for long distances. The actual infective spores (zoospores) are released from the sporangia into water and need to swim in a water film before settling on the plant surface and penetrating into leaf tissues; this is why the disease is so serious in wet summers. The pathogen then spreads rapidly, killing the cells. Under humid conditions, stalks bearing sporangia grow from freshly killed tissues and the disease can spread rapidly through the crop.
The pathogen overwinters in rotten potatoes left in the ground or by the sides of fields. However, the great majority of infections in gardens arise from wind-blown sporangia originating in other gardens, allotments and commercial crops. In the UK, outbreaks may occur from June onwards, usually earliest in the South West.
The presence of new strains in the UK means that the pathogen now has the potential to produce resting spores (oospores) in the affected plant tissues. The oospores are released from the rotting tissues to contaminate the soil. These resting spores have yet to be found in the UK, however, and little is currently known about their survival and their potential as a source of the disease. The investigations into oospores are continuing and more information may be available in a few years.
Attacks of blight occurring in late summer defoliate potato crops, but if the disease arrives after the tubers are set, and they are harvested before they become infected, little is lost. However, early attacks can be devastating and blight is the most important disease of potatoes, both for gardeners and commercial growers. Outdoor tomatoes are at high risk of infection if the weather is suitable. The disease is less of a problem under glass as the spores have to find their way into the glasshouse through doors and vents. If, however, blight establishes in a glasshouse the high humidity inside usually leads to very rapid development of symptoms.