Stopping the Rot

In July 2000, the fi nger of blame for a mysterious mass killer of Californian oak trees came to rest on a previously undescribed plant pathogen. From the initial identifi cation of Phytophthora ramorum, it took less than four years to produce a draft sequence of its genome, one of the fastest-ever discovery-to-sequence stories for a complex pathogen. This achievement was a United States initiative, facilitated by the injection of federal and state funding into Phytophthora research. But the US is not alone in the battle against this genus. Phytophthora species cause thousands of millions of dollars of damage to the world’s commercial crops every year: they blight potatoes and tomatoes, devastate the lucrative soybean, and rot cacao, threatening the world’s supply of chocolate (Figure 1). But for Sophien Kamoun, Associate Professor of Plant Pathology at Ohio State University (Wooster, Ohio, United States), these destructive organisms present an exciting opportunity. There are some 60 species of Phytophthora, but so little is known about the genus, he says, that there are things about its species we didn’t even know we didn’t know. ‘What I fi nd really exciting,’ he says, ‘is discovering these unknown unknowns.’ The most infamous of the Phytophthora pathogens is the potato late blight, P. infestans. It was this species that led to the Irish potato famine in the mid-1840s, which resulted in the death or displacement of millions. Today, P. infestans is estimated to cost potato and tomato farmers US$5,000,000,000 a year in lost revenue. The story for the soybean pathogen P. sojae is similar, causing loss of more than US$1,000,000,000 a year to soybean growers. In addition to the direct economic impact of these pathogens, introduced Phytophthora can cause severe damage to native fl ora. The most recent Phytophthora