Following the blogs describing Xylella fastidiosa subspecies attacking grapevine and citrus species, this time we take a look at other fruit trees, at trees of the Prunus family (plums, peaches, apricots…).
Prunus genus
Cultivated plants belonging to the genus Prunus are globally widespread. For some countries they are economically highly important crops. They can also play a key role in the composition of a landscape.
Some species belonging to the Prunus genus are among the most significant hosts of different Xylella fastidiosa subspecies. In fact, almond leaf scorch disease (ALSD), phony peach disease (PPD), and plum leaf scald (PLS) are of considerable importance in the agricultural history of the United States and South America due to the damage they cause. In most new European and Asian outbreaks, almonds trees are the most affected.; however, peach, plum, apricot, and cherry are also widespread and profitable crops in threatened areas.
Peach
Peach trees (Prunus persica) are plagued by several different diseases throughout the growing season, but the Xylella-caused phony peach disease (PPD) has been causing noticeable losses within orchards in Georgia and the Southeast of the U.S. It results in stunted tree growth and reduced fruit quality and quantity. PPD was first documented in peaches in 1890 near Marshallville, Georgia, and has been a challenge for growers ever since.
A number of large outbreaks have been documented, particularly following warm winters. As a consequence of the disease, millions of PPD-affected trees have been removed from peach orchards over the last century. The disease is transmitted by insects, in particular sharpshooters, which are prevalent in warm south-eastern climates.
The geographic distribution of this disease ranges roughly from northern Florida to North Carolina, and west as far as Missouri and eastern Texas though there are historical accounts of the disease further north.
Although the disease occurs throughout the Southeast, it is an especially significant problem in Georgia. This is for several reasons, such as year-round vector activity and greater bacterial survival in the tree.
This disease is of particular importance because it can be one of the primary limiting factors associated with peach orchard longevity.
PPD causes several symptoms that can be used for identification, though other ailments can mimic these symptoms, making accurate identification difficult to the novice:
- PPD trees have shortened internodes that can result in a nearly flat leaf canopy on top, dark green foliage, and a generally denser and shorter tree.
- Infected trees also tend to bloom earlier and have a delayed leaf senescence in the fall.
- Fruit quality is also substantially diminished, as PPD reduces the tree’s ability to produce the equivalent size and number of peaches as compared to noninfected trees.
- Fruit produced from trees infected with PPD also tends to ripen before healthy peaches.
Xylella fastidiosa infects peach trees either through graft transmission or, more often, through insect-vectors (i.e. transmission during sharpshooter feeding).
Bacteria slowly migrate from the point of infection through the xylem of the tree. Bacteria tend to gather primarily in the plant’s roots, and it can take more than eighteen months before phony peach symptoms develop.
As with almost all Xylella-induced diseases, there is no known cure for PPD. Accordingly, its control is directed towards prevention or minimizing the spread of the disease.
In the fall, after phony peach trees have been flagged, the trees should be rogued (i.e. removed) to prevent further spread of the disease and to extend the orchard’s productive life. In fact, several million PPD-affected trees have been removed from peach orchards in the south-eastern United States over the course of the last century.
As an additional method of control, peach orchards should not be planted within three hundred yards of host plants, such as wild plums. This is to prevent sharpshooters travelling from infected plums and further infecting the peach orchard.
For this reason, farmers often eradicate wild plums near peach orchards. A good weed control program with a weed free strip will help to further reduce habitat for the insect vectors that transmit the bacterium.
Plum
Moving on to South America; in Brazil, plum production is based on “Japanese” cultivars (Prunus salicina) as they require less chilling for transportation than “European plums” (Prunus domestica).
Although the term ‘Japanese plum’ was originally applied to the species Prunus salicina, most commercial cultivars now involve crosses with several plum species of the same genus.
The most important limiting factor for plum production in Brazil is the plum leaf scald disease (PLS), which is caused by the bacterium Xylella fastidiosa. The bacterium was probably introduced from Argentina, where it was first reported in 1935.
Disease symptoms take three or more years to manifest in contaminated plants, which can be identified by apex drought, marginal necrosis, and leaf fall, delaying the growth and fruit ripening, reducing plant development, and finally, causing the plant to die. Affected trees produce fewer and smaller fruits, and after 3-5 years, they become economically worthless. Thus, in many cases, orchards usually have a short life and are abandoned ahead of its lifetime of ca. 14 years.
The first report of leaf scald in Brazil was made in the 1950s. At that time, ‘Santa Rosa’ was the main cultivated variety. This variety, unfortunately, was extremely susceptible to leaf scald. Between 1975 and 1982, this disease destroyed more than 90% of plum orchards in Santa Catarina State, in Southern Brazil. Nowadays, plum production has been made possible by growers turning to Xylella-resistant cultivars, amongst other efforts. In colder areas, moderately resistant cultivars, such as ‘Laetitia’ and ‘Fortune’, are being used. In warmer areas, the attack of Xylella fastidiosa is more severe, and ‘Gulf Blaze’ is now the most planted cultivar because of its higher level of resistance.
Planting Xylella-free material and spraying insecticides during all seasons to kill vectors are also commonly used methods to combat PLS, in spite of the environmental concerns the wide-spread use of insecticides causes.
The bacteria often resides in alternative weeds and hosts present in orchards, which act as reservoirs for the microorganism and make inoculum dissemination practically unavoidable. Thus, the primary challenge for plum growers in Brazil is to produce plum cultivars resistant or tolerant to leaf scald whilst still being commercially viable as fruit crops. Although some cultivars as ‘Carazinho’, ‘Sanguinea’, ‘Chatard’, and ‘Piamontesa’ have been reported as resistant, they don’t have high enough fruit quality for commercial purposes.
Resistant cultivars originate from genetic material from the Paraná Delta region, Argentina, where this disease was initially identified in South America. These cultivars, as well as some cultivars created in Florida, have been used as sources of resistance in the plum breeding program of Santa Catarina State.
Recently, a specific plum genotype was found that seems to be immune to Xylella fastidiosa, leading to the launch of ‘Zafira’ cultivar, the first plum cultivar for commercial purposes that is not naturally infected with Xylella fastidiosa. ‘Zafira’ plants have been kept free from Xylella fastidiosa for over ten years, without infection symptoms, in addition to showing negative results in PCR tests.
Almond
Almond leaf scorch disease (ALSD) was first reported in almonds (Prunus dulcis) in California in the mid-1930’s, and more recently there have been confirmed outbreaks, in Alicante in eastern Spain and in the Hula Valley, Israel.
The recent introductions of the bacterium Xylella fastidiosa into Europe are linked to the international plant trade. However, both how and when these entries occurred remains poorly understood.
Almond trees, an important icon of the agricultural landscape of Majorca, Balearic Islands (Spain), have experienced severe decline and mortality over the last 15 years. The almond disease was preliminarily studied from 2008 to 2010 in the main initial focus in Son Carrió, east of Majorca. Although no etiological agent was ascribed, this disease was associated with a complex of fungal trunk pathogens and their interactions with known disease-predisposing factors such as prolonged drought and tree aging. This early disease diagnosis, however, was recently challenged after the detection of Xylella fastidiosa in Majorca in October 2016.
More than 120 almond samples tested positive for Xylella fastidiosa in a 2017 analysis conducted by the Balearic Islands Official Plant Health Laboratory (LOSVIB), which raised suspicion that Xylella could underlie the sudden emergence of this unprecedented disease.
Over 7287 plant samples comprising 274 plant species were analysed for Xylella fastidiosa at the Balearic Islands Official Plant Health Laboratory (LOSVIB) between 2016 and 2019. As a result, three Xylella fastidiosa subspecies have been identified, incl. subsp. fastidiosa strain ST1, which causes Pierce’s disease on grapevines and ALSD in California.
In addition to ST1, ALSD is also caused by strains of the subsp. multiplex ST6/7.
ST1 strains infect grapevines and can cross-infect almonds as well, whereas almond strains of ST6/7 do not cause disease on grapevines. Both subspecies, however, coexist sympatrically in almond orchards in the San Joaquin Valley of California.
First-year symptoms may be confined to a few inches from the infection site and not be noticed for 2 or 3 years. Movement within a tree may be slow and require several years to infect the entire tree. In other cases, spread throughout the tree appears to have occurred within a year. If discovered early and only in one branch, the infection may be removed by pruning off a primary scaffold.
Presently, there are no chemical or nutritional treatments that can control or cure ALSD. In Iran, it’s been reported that Xylella survived overwinter in root tissues in almond orchards with winter temperatures even below -15°C.
If the orchard is young (5–10 years old), the best course of action may be to remove infected trees. In older orchards (16 –20 years old), it may be more cost-effective to keep infected trees, because the entire orchard is normally removed between 22 and 25 years of age and infections will probably not significantly impact yields before then.
In Europe, infected trees are typically uprooted and a sanitary cordon around the infected plot established.
Cherries and Apricots
Cherries (Prunus avium and Prunus cerasus) and apricots (Prunus armeniaca) seem not to be highly susceptible to attacks by Xylella fastidiosa.
In Europe, Xylella fastidiosa has been detected in three cherry trees at a garden centre on the island of Majorca in 2016, and in an apricot plant once in 2018 in mainland Spain with samples coming from an isolated apricot tree located close to plots with infected almond trees.
