Not all Xylellas are born equal V

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.

Fruit trees susceptible to Xylella fastidiosa | BIOVEXO compilation
Pecan

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.

Xylella-infected pecan tree | www.sepga.com
PBLS has been confirmed to occur in the southern United States, including the states of Louisiana, Georgia, Texas, New Mexico, Arizona, and California. Scientific studies on PBLS have shown the pecan cultivar, ‘Cape Fear,’ to be highly susceptible to infection. No significantly resistant cultivars have been identified.

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.

Coffea

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.

Coffee leaf scorch | www.elmundo.cr
In 2012, four coffee plants with leaf scorch symptoms growing in a confined greenhouse were detected and intercepted in France. One strain, isolated from Coffea canephora (Robusta coffee) imported from Mexico, was assigned to Xylella fastidiosa subsp. fastidiosa/subsp. sandyi. The two other strains, isolated from Coffea arabica imported from Ecuador, were allocated to X. fastidiosa subsp. pauca.

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.

Olives

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.

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