Late blight

Late blight (Phytophthora infestans) is an oomycete or water mold that causes a serious disease of potato. (Early blight, caused by Alternaria solani, is also often called "potato blight"). Late blight was a major culprit in the 1840s European, the 1845 Irish and 1846 Highland potato famines. The organism can also infect tomatoes and some other members of the Solanaceae family.

Potato
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.

Control
P. infestans is still a difficult disease to control today by ordinary methods. There are many options in agriculture for the control of both damage to the foliage and infections of the tuber. Potatoes grow throughout the season, but it is estimated the tubers stop growing when 75% of the canopy has been destroyed.

Genetic engineering
In recent years, a resistance gene effective against all known strains of blight has been identified and successfully copied from a wild relative of the potato, Solanum bulbocastanum, and introduced into the genome of cultivated varieties of the potato.

Sources of inoculum
Blight can be controlled by limiting the source of inoculum. Only good quality seed potatoes 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. During the 2009 growing season, which was colder than average, and with greater than average rainfall, there was a major infestation of tomato plants in the eastern United States. 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.

Potato varieties
Potato varieties vary in their susceptibility to blight. Most early varieties are very vulnerable; they should be planted early so that the crop matures before blight starts (usually in July). Many old crop varieties, such as King Edward are also very susceptible 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 blight and not others, so their performance may vary depending on which are around. These crops have had polygenic resistance bred into them, and are known as "field resistant". New varieties such as Sarpo Mira and Sarpo Axona show great resistance to blight even in areas of heavy infestation. Defender is an American cultivar whose parentage includes Ranger Russet and Polish potatoes resistant to late blight. It is a long white-skinned cultivar with both foliar and tuber resistance to late blight. Defender was released in 2004. These varieties are likely to gain great popularity as consumers increasingly embrace organically produced crops and reject food items that have been grown using fungicides and other chemicals. Researchers are currently working to develop a variety of potatoes that will be resistant to both late and early blight.

Researchers have found the wild potato species Solanum verrucosum resist to the late blight disease. They aim to make cultivated potatoes resistant to late blight by crossing the wild, resistant strain with the vulnerable, cultivated strain. In addition to this, researchers are crossing Solanum verrucosum with another wild potato species that is resistant to early blight, making a hybrid that is resistant to both late and early blight. They plan to cross the hybrid with cultivated potatoes to pass both resistant genes onto the cultivated species.

Earthing up
The practice of earthing up is often used to reduce tuber contamination by blight. This normally involves piling soil or mulch around the stems of the infected plant meaning the pathogen has farther to travel to get to the tuber.

The canopy can also be destroyed around 5 weeks before harvest. This can be done via a contact herbicide or using sulfuric acid to burn off the foliage.

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 is a fungicide that was marketed for use against P. infestans, but suffered serious resistance issues when used on its own. It is strongly advised to use metalaxyl along with carbamate compounds, or the especially effective synergistic Cymoxanil and Mancozeb combination as it is effective at managing metalaxyl resistant strains.

In the past, copper sulfate solution (called 'bluestone') was used to combat potato blight. Copper pesticides remain in use in rare instances on organic crops.