The last blog of this series focuses on other plants susceptible to Xylella fastidiosa, i.e. on pecan trees and on coffea species. A small update on olive trees concludes the blog.
Pecan bacterial leaf scorch (PBLS), caused by Xylella fastidiosa subsp. multiplex, can cause severe disease in some pecan cultivars, resulting in substantial yield loss. It can also infect many other tree species, as we have seen, including oaks, American sycamore, peaches, plums, almonds, and maples.
Symptoms of PBSL include tan to light brown necrotic lesions on leaflet tips that progress in a uniform pattern toward the leaflet’s base and will eventually result in premature defoliation. Xylella fastidiosa causes chronic infections from year-to-year and can live dormant, or in low concentrations, in the host without presenting symptoms. Symptoms of pecan bacterial leaf scorch can be seen on individual branches or systemic across the entire canopy of the tree.
There is presently no cure for PBLS. Care should be taken not to inoculate other trees during grafting. It has been suggested recently that pecan scions shall be completely submerged in water heated to 46°C for 30 minutes. This treatment is claimed to be over 90 percent effective.
Coffee leaf scorch (CLS), due to X. fastidiosa, was first identified in 1995 in Brazil and later in Latin American coffee-producing countries such as Costa Rica. Symptoms of CLS include drying of infected branches, shortening of internode regions, decreased fruit size, chlorosis, and early senescence of leaves. Plants also show irregular growth with an atypical curling appearance that gave rise to the Spanish name “crespera.” CLS affects plant productivity but rarely leads to plant death. Sharpshooter is the insect vectors.
Xylella fastidiosa was presumptively diagnosed during a survey performed on ornamental Coffea arabica plants imported from Costa Rica and Honduras into the Netherlands in the autumn of 2014. These plants showed either mild leaf scorch symptoms, or a range of “crespera” – like symptoms, or no symptoms.
Apart from the looming crises in the Mediterranean area, presence of Xylella fastidiosa has been reported in olive trees showing leaf scorch symptoms in Argentina and Brazil. The two South American strains resulted differently from each other and from the Italian strain but were both classified as belonging to the subsp. pauca.
Xylella fastidiosa subsp. multiplex was also detected in olive trees grown in California. Strains of Xylella fastidiosa subsp. multiplex isolated from California olive trees are not known to cause disease on olive, although some can induce leaf-scorch symptoms on almond.