The abundance at our farmers’ markets this time of the year covers up an insidious threat that was lurking in farm fields. The notorious organism that set off the Irish potato famine in the mid-19^th century is threatening potato and tomato crops in the United States this year.

Authorities have no way to stop the culprit in its tracks, but an international team of scientists reported in the journal Nature this week that they have decoded its genome and learned a lot about the reasons it has been so elusive all of these years.

Phytophthora infestans was long considered to be a fungus, but now it is known to be from the water-mold family, closely related to the malaria parasite. It thrives in cool, wet weather and infects plants to cause a “late blight” disease that can decimate entire fields in just a few days.

Among other strengths, this destructive pest has a remarkable ability to change and, therefore, to adapt to new plant hosts. In other words, it races to keep ahead of the farmers who plant resistant potatoes, said scientists at the Broad Institute of MIT and Harvard who helped lead the study. The other lead group was Sainsbury Laboratory in Norwich UK.

With the full genome in hand, scientists were able to compare P. Infestans with its close relatives and learn that its genetic structure is huge - up to four times the size of its cousins. Much of the bulk comes from repetitive or so-called junk DNA. That in itself makes the genome valuable for future research because the role of junk DNA has been something of a mystery.

Wouldn’t it be great, though, if they also could use the genome to safeguard our mashed potatoes?