Plant pathogen focus: Pierce's disease and the vineyards of California

Grapevine dying of Pierce's disease. Photo courtesy of PD-GWSS/CDFA.

Xylella fastidiosa  is not your ordinary kind of bug. It made it to the list of the most wanted plant pathogenic bacteria in 2012! (Mansfield, 2012.)

This is well deserved: X. fastidiosa can infect over a hundred species (grapevine, oleander, citrus, almonds,…), and it causes severe symptoms that can kill the infected plant. The Xylella bacteria colonize the xylem vessels, and by doing so they block the transport of water in the plant. The water-deprived leaves dry and scorch, until finally they drop to the ground. 

In 1892, Newton Pierce, California’s first professional plant pathologist, described the disease that now bears his name, although he failed to identify the causative agent of the disease (X. fastidiosa). Around the end of the 19^th century, an epidemic of Pierce’s disease devastated thousands of hectares of vineyards in the Los Angeles Basin. Since then, southern California has considerably reduced its viticulture; this also explains why present-day Californian vineyards are mostly restricted to the north of the state. There is currently no other cure than getting rid of the infected plants!

Plant pathogen focus: black Sigatoka as a worldwide threat to banana

Banana trees affected by Sigatoka in Malawi. Photo courtesy of APS.

Since I work in the department of plant pathology at UCD—and even though I am not a plant pathologist myself!—I decided to start a series of posts on microbes that cause plant disease, focusing on stories that are of economic and societal importance.

A couple of weeks ago, one of my colleague—him a true plant pathologist—, managed to scare me by claiming during a talk that banana could disappear in the not-so-distant future! The culprit? The fungus Mycosphaerella fijiensis: This ascomycete causes a disease (black Sigatoka) that damages the leaves of banana trees and reduces photosynthesis. Moreover, the fungus triggers a premature ripening that spoils the fruit. Together these effects of black Sigatoka provoke the loss of 50% or more of the fruit production. 

[I learnt a lot about black Sigatoka in an online article by Randy Ploetz on the website of the American Phytopathological Society (APS). When no other source is explicit, the information in this post comes from the Ploetz article. In general, the APS website is a great starting point for everything related to plant pathology!]