department of plant pathology and weed research , volcani center (aro)
plants get sick. just like humans and animals plants are infected by bacteria, fungi, viruses and other microorganisms which may lead to a severe decline in plant productivity and even death.
plant diseases may have huge impact on our society and our well-being. perhaps the most known example to the effect plant diseases had on humanity was the devastating outbreak of the late blight disease, which hit the potato cultivation in ireland, and led to the great famine during the years 1845-1850.
since in ireland (and other european countries at the time), a large proportion of the population depended on potatoes as the main dietary food, the rapid and disastrous loss of potatoes due to the disease, resulted in the starvation and death of about one million people and spurred a massive immigration wave from ireland to america.
xylella fastidiosa (x. fastidiosa), is a bacterial pathogen, which has been known in the americas for over a century. there, it infects mostly grapevines (north america) and citrus (south america). for decades, the disease was restricted to the americas and was not found elsewhere in the world. this changed in 2013, when italian researchers had first detected x. fastidiosa on european soil. unlike in the americas, in italy, x. fastidiosa was found in olive trees, causing significant damage to the olive culture of south italy (puglia). the damage was most evident in ancient olive trees, some of which are centuries-old, that had fell victim to this pathogen, leading to a very dramatic and sad change of puglia’s landscape.
typical leaf scorch symptoms caused by xylella in almond trees, photo: ofir bahar
differently from many other disease agents, x. fastidiosa required a suitable insect vector to be transmitted from one tree to another. unfortunately for the italians, even though x. fastidiosa is not endemic to europe, a suitable and wide-spread insect vector (a spittlebug called philaenus spumarius) was already present in italy, ready to carry and transmit the new pathogen from infected to healthy trees. the combination of a devastating pathogen, an abundant and fairly efficient insect vector, and a very sensitive host plant, was deadly. these factors, in combination with a rather late discovery of the pathogen, led to a rapid and almost unstoppable spread of the disease from the southern part of italy northwards. thus far, more than one million olive trees had died or were uprooted by the authorities to limit the spread of the disease, yet, the disease had since been detected in northern italy, france, spain and portugal.
the x. fastidiosa species includes four main sub-species, and many more genetic variants, which altogether can infect a wide range of host plants (over 350 plant species). each sub-species has a set of host plants it can or cannot infect. thus far, in europe, at least three different sub-species of x. fastidiosa, infecting highly important crop plants such as olive, almond and grapevines, were detected.
philaenus spumarius. the main insect vector of xylella fastidiosa in italy, spain and france.
p. spumarius nymph emerging from spittle (a) and adult on a leaf (b)
following insect feeding from trees, x. fastidiosa cells are introduced into the plant xylem vessels, these are the water conducting elements of plants. in these vessels x. fastidiosa cells multiply, aggregate and spread throughout the plant leading to a disruption of water flow in the tree which eventually is manifested as desiccation symptoms of the leaves. while in italy it is reported that olive trees die as a result of x. fastidiosa infection, in almond and grapevine in the usa, x. fastidiosa infection is described as leading to a slow decay of the tree, and not necessarily to its death.
during the second half of 2016, first indications for the presence of x. fastidiosa in almond plantations in israel had surfaced. a nation-wide survey in the following year confirmed that indeed certain almond plantations in the hula valley are infected with the deadly bacterium and that the southern part of the country appears to be free of the disease. in infected orchards in the hula valley ~5% of the trees may show clear disease symptoms. in almond orchards further south, only sporadically distributed, single almond trees were found infected with x. fastidiosa. thus far, x. fastidiosa was not found in other potential hosts such as grapevines and olives. genetic analysis of the x. fastidiosa sub-species and the genetic type found in israel revealed that it belongs to the sub-species fastidiosa, of the genetic type called sequence type 1 (st1). this genetic type is known to infect mainly almond and grapevine trees and is different than the sub-species pauca (st53), which is associated with the epidemic in olive trees in italy.
as mentioned earlier, x. fastidiosa can be transmitted from one tree to another by insect vectors. some of these insect vectors are polyphagous in nature (can feed on multiple plant species). for example, in spain, the same insect vector can transmit x. fastidiosa from almond to grapevines and vice versa. nevertheless, surveys conducted in the past few years in israel, in an attempt to identify possible x. fastidiosa vectors, have yet to identify such a vector. while the immediate and direct damage caused by x. fastidiosa to the almond industry in israel at present is not overwhelming, the potential direct and indirect damages are enormous. for example, in the presence, or appearance of an efficient vector for x. fastidiosa, additional host plants such as grapevine may be infected with the disease. the indirect damages are related to the ability to export plant material outside of the infected area specifically and from israel in general. although x. fastidiosa was not found in other host plants, countries which do not have x. fastidiosa in their territory will most likely be reluctant to take the risk of importing x. fastidiosa host plants from an infected region or country. therefore, nurseries in the hula valley and the surrounding regions may be requested to perform additional assays in order to test whether the plant material they are exporting is indeed free of x. fastidiosa, and these additional tests may be very costly for the farmers.
to date, there is no robust method to cure infected trees and therefore infected trees that have not been uprooted, will continue to serve as a source of the disease (assuming the presence of appropriate insect vectors). disease management practices include chemical application to reduce the insect vector population, and tilling of the soil to minimize the development of annual grasses within and around the orchards, as those serve as important host for the insect vectors at certain stages of its life cycle.
in march 2019 a multidisciplinary research team was put together in order to study the disease in israel and to devise practices for its management. this team includes researchers from the agricultural research organization (volcani center), the northern research & development station (mop zafon) and the plant protection and inspection services of israel (ppis), gathering experts in plant pathology, epidemiology, entomology and more. the funding for the work of this research team comes from the chief scientist of the ministry of agriculture and from the israeli fruit council. efforts to uproot infected trees and to detect early appearances of newly infected trees are undergoing, aiming to limit as much as possible the spread of the disease and to avoid leakage to other hosts.
in some places in the world, x. fastidiosa was referred to as the cancer of trees, highly persistent and very difficult to cure. only time will tell whether x. fastidiosa was detected early enough in israel and whether the efforts that are being taken now were efficient enough to halt the disease and to minimize its damage to israel’s agriculture.