Phytophthora infestans

Phytophthora infestans is a specialized oomycete, often called a water mold. This fungus-like microorganism is the pathogen responsible for late blight, also known as potato blight. This disease is devastating because it targets essential crops like potatoes and tomatoes. It can also infect other members of the Solanaceae family. Throughout history, this organism has caused massive human suffering. It was a primary cause of the European, Irish, and Highland potato famines during the 1840s.

The life cycle of this pathogen is a complex sequence of stages. It begins with hyphal growth, where the organism spreads through plant tissue. Next, it enters the sporulation phase to create reproductive structures. The pathogen produces sporangia, which are sacs that spread via wind or water to infect new hosts.

Some sporangia release zoospores, which are specialized cells with two flagella. These zoospores are chemotactic, meaning they can move toward chemical signals in water films on leaves or soil. Under ideal conditions, the entire cycle can be completed in just five days.

Scientists categorize the organism into two distinct mating types, labeled A1 and A2. These types are distinguished by the specific mating hormones they produce. The A1 type produces a diterpene called α1, while the A2 type produces α2. When isolates of these opposite mating types meet, hormonal communication triggers the formation of oospores. Unlike the short-lived sporangia and zoospores, oospores are incredibly hardy. They can remain viable in the environment for many years. This ability to persist makes the pathogen very difficult to eradicate.

The history of studying this pathogen involves several key scientific figures. The genus and species name, Phytophthora infestans, was established in 1876 by the German mycologist Heinrich Anton de Bary. The name combines Greek words for "plant decay" and the Latin word for "attacking." Later, in 1956, John Niederhauser discovered the A2 mating type in the Toluca Valley of Mexico. This discovery was made while he worked for the Rockefeller Foundation's Mexican Agriculture Program. Understanding these mating types has been vital for tracking how the disease spreads globally.

Identifying an infection requires looking for specific physical symptoms in the field. On leaves and stems, you may see dark green, brown, or black spots. In humid conditions, a white mold often appears on the underside of the foliage.

If the infection reaches the tubers, they develop grey or dark patches. These patches are often reddish-brown beneath the skin. Eventually, the tubers decay into a foul-smelling mush. This rot is often worsened by secondary soft bacterial infections.

Modern genetics has revealed the incredible complexity of the P. infestans genome. Scientists completed its genome sequencing in 2009 and found it contains about 18,000 genes. The genome is approximately 240 Mbp, which is much larger than related species like P. sojae or P. ramorum. A significant portion of the genome, about 74%, consists of repetitive sequences. The organism uses specialized effector proteins to cause disease. Some of these, called RXLR proteins, are produced inside the plant cells. Others are enzymes like proteases and lipases that degrade plant tissue in the spaces between cells.

There has been significant scientific debate regarding the original home of this pathogen. For a long time, many researchers believed the highlands of central Mexico were the center of origin. This was supported by the high genetic diversity found in Mexican populations. However, more recent research has offered a different perspective. A 2016 study using large-scale whole genome sequencing suggested an Andean origin. This study found that lineages from the Andes were more closely related to historical famine-era lineages. This suggests the pathogen may have originated in the Andes before migrating and diversifying in Mexico.