Phytophthora infestans

Phytophthora infestans is an oomycete that causes the serious potato disease known as late blight or potato blight. (Early blight, caused by Alternaria solani, is also often called 'potato blight'). It was a major culprit in the 1845 Irish and 1846 Highland potato famines. The organism can also infect tomatoes and some other members of the Solanaceae.

Biology
The spores of this water mold overwinter on infected tubers, particularly those that are left in the ground after the previous year's harvest, in cull piles, soil or infected volunteer plants and are spread rapidly in warm and wet conditions. This can have devastating effects by destroying entire crops.

Spores develop on the leaves, spreading through the crop when temperatures are above 10C (50°F) and humidity is over 75% for 2 days or more. Rain can wash spores into the soil where they infect young tubers, or else spores can be blown in from miles away by the wind.

The early stages of blight are easily missed, and not all plants are affected at once. Symptoms include the appearance of dark blotches on leaf tips and plant stems. White mould will appear under the leaves in humid conditions and the whole plant may quickly collapse. Infected tubers develop grey or dark patches that are reddish brown beneath the skin, and quickly decay to a foul-smelling mush caused by the infestation of secondary soft bacterial rots. Seemingly healthy tubers may rot later when in store.

Strains


Up until the 1970s, there was only one type of blight (A1) in the UK, and this was unable to produce resistant spores that could survive the winter. There are now two types (A1 and A2) which can mate and after that produce resistant spores, although the indications so far are that this rarely, if ever, happens in the UK. Mating can occur only between moulds of different mating-types and is required for the production of resistant spores.

Management
P. infestans is still a difficult disease to control today. There are many options in agriculture for the control of both damage to the foliage and infections of the tuber. Potatoes fill throughout the season, but it is estimated the assimilates stop going to the tubers (they stop growing) when 75% of the canopy is destroyed. ­ This must also be taken into account when growing pototoes, as it means that plants grown do not have to be 100% resistant to blight.

Sources of Inoculum
Blight can be controlled by limiting the source of inoculum. Only good quality seeds obtained from certified suppliers should be planted. Often discarded potatoes from the previous season and self-sown tubers can act as sources of inoculum.

Environmental Conditions
There are several environmental conditions that are conducive to P. infestans. By using weather forecasting systems, such as BLITECAST, if the following conditions occur as the canopy of the crop closes, then the use of fungicides is recommended to prevent an epidemic.


 * A Beaumont Period is a period of 48 consecutive hours, in at least 46 of which the hourly readings of temperature and relative humidity at a given place have not been less than 20C (68ºF) and 75%, respectively.


 * A Smith Period is at least two consecutive days where min temperature is 10C (50ºF) or above and on each day at least 11 hours when the relative humidity is greater than 90%.

Potato Varieties
Potato varieties vary in their susceptibility to blight. Most early varieties are very vulnerable; so that the crop matures before blight starts (usually in July) plant them early. Many old crop varieties, such as King Edward potato are also very suceptible but are grown because they are wanted commercially. Maincrop varieties which are very slow to develop blight include Cara, Stirling, Teena, Torridon, Remarka and Romano. Some so-called resistant varieties can resist some strains of the blight and not others, so their performance may vary depending on which are around. These crops tend to have had polygenic resistance bred into them, and are known as field resistant.

Use of Fungicides
Fungicides for the control of potato blight are normally only used in a preventative manner, perhaps in conjunction with disease forecasting. In susceptible varieties, sometimes fungicide applications may be needed weekly. An early spray is most effective. Metalaxyl was a fungicide that was marketed for use against P. infestans, but suffered serious resistance issues when used on its own.

Control of Tuber Blight
Ridging is often used to reduce tuber contamination by blight. This normally involves piling soil or mulch around the stems of the potato blight meaning the pathogen has farther to travel to get to the tuber.

The canopy can also be destroyed around 2 weeks before harvest. This can be done via a contact herbicide or using sulphuric acid to burn off the foliage.

Historical Impact
The effects of Phytophthora infestans in Ireland in 1845-57 were one of the factors which caused over one million to starve to death and forced another two million to emigrate from affected countries. Most commonly referenced is the Great Irish famine, during the late 1840s.