Phytophthora infestans

This is the fungus that causes potato blight, which is historically the first and most important plant disease, and was responsible for the 'Hungry Forties' of the nineteenth century, and the great Irish famine. It was this disease that initiated the science of plant pathology.

Phytophthora infestans has two mating types, known as A1 and A2. Each mating type is hermaphrodite but self-sterile. This means that oospores can be formed only if both mating types are present, as happens in the centre of origin in Mexico. When blight was accidentally taken to New York, and then to Europe, in the mid-nineteenth century, it was taken as A1 only and, for 150 years, it could reproduce asexually only. This meant that the fungus could over-winter only in potato tubers. The initial inoculum was small, and the epidemic developed slowly. It was known as 'late blight' for this reason. In the late twentieth century, A2 was taken to Europe, and it was spread all over the northern hemisphere in certified seed potatoes. This means that functional oospores are now being formed in most of the potato-growing areas of the world. The initial inoculum will now be much greater, and late blight is likely to become early blight.

The variability of the fungus is likely to increase considerably, and unstable resistances (i.e., vertical resistances) and unstable fungicides will break down much more quickly. And higher levels of horizontal resistance will now be required to provide a full control of the disease.

Potatoes are an excellent crop for amateur breeders , and the need for horizontal resistance to blight and other pests and diseases is acute. Phytoplasma

A mycoplasma that is parasitic in plants. A mycoplasma is a micro-organism smaller than a bacterium and without a cell wall.

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