Phytophthora infestans is an oomycete or water mold 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”).
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 10 °C (50 °F) and humidity is over 75% for 2 days or more. Rain can wash spores into the soil where they infect young tubers. Spores can also travel long distances on 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.
In 2009 scientists completed the sequencing of the genome of P. infestans. It was found that the genome is considerably larger (240 Mb) compared to other Phytophthora species whose genomes have been sequenced; Phytophthora sojae has a 95 Mb genome and Phytophthora ramorum had a 65 Mb genome. It also contained an diverse variety of transposons and many gene families encoding for effector proteins that are involved in causing pathogenicity. These proteins are split into two main groups depending on whether they are produced by the water mould in the symplast (inside plant cells) or in the apoplast (between plant cells). Proteins produced in the symplast included RXLR proteins, which contain an arginine-X-leucine-arginine (where X can be any amino acid) sequence at the amino terminus of the protein. RXLR proteins are avirulence proteins, meaning that they can be detected by the plant and lead to a hypersensitive response, killing the oomycete. P. infestans was found to contain around 60% more of these proteins than other Phytophthora species and this may allow it to overcome host defences more quickly. Those found in the apoplast include hydrolytic enzymes such as proteases, lipases and glycosylases that act to degrade plant tissue, enzyme inhibitors to protect against host defence enzymes and necrotizing toxins. Overall the genome was found to have an extremely high repeat content (around 74%) and to have an unusual gene distribution in that some areas contain many genes whereas others contain very few.
The Scottish potato industry is now working on a new strain of potato that will be blight resistant by cross breeding varieties from Peru that contain the essential blight resistant DNA.