What is Xylella and which nut trees are affected?


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A spreading disease among European olive trees could cost the economy over 20 billion euros, according to a recent study. The disease is caused by the bacterium Xylella fastidiosa, believed to be the most deadly plant pathogen in the world, and there is no cure. It showed up in an Italian olive orchard in 2013, but it also infects cherry, almond, and plum trees. Xylella makes it difficult for a tree to move water and nutrients, killing the tree. Sap-sucking insects spread the pathogen, making it very hard to control. It recently showed up in olive trees in Spain and Greece and has spread quickly up and down Italy. Farmers are fighting the disease by burning infected trees, but the researchers say the only effective way of stopping it will be producing a resistant tree.The Xylella fastidiosa bacteria, spread by insects, has been destroying almond trees around Spain, and can also be found in olive trees, apricot trees and grape vines.

The bacteria was first detected in El Castell de Guadalest, near Altea, in July 2017, and has since spread across 134,000 hectares, invading 76 municipalities in Marina Baixa, Marina Alta and El Comtat. The current strategy of the Ministry of Agriculture, in Valencia, is to quarantine and rip up thousands of trees where the bacteria is present, with 30,000 already uprooted.

The containment plan eradicates trees within 100 metres of an infected specimen, even if it is healthy, with many specialists believing this will destroy 20% of all trees in Alicante. Now an association of those affected, the AXFA, has hit back against the plans, insisting it will cause ‘irreparable damage to the environment, landscape and local economy’. “Tourism will be badly damaged and this could further hasten depopulation, which is already a problem in rural areas,” said a spokesman. Farmers in the association, who have not even been able to use the wood as fuel, have demanded that only infected trees be cut down.

Xylella fastidiosa is a xylem-limited, nutritionally fastidious bacterium that causes several plant diseases including Pierce’s disease (PD) in grape and leaf scorch in almond (ALS) and oleander (OLS). OLS strains belong to X. fastidiosa subsp. sandyi, PD strains belong to X. fastidiosa subsp. fastidiosa, and strains from almond designated as ALS strains are of two general types belonging either to X. fastidiosa subsp. multiplex or X. fastidiosa subsp. fastidiosa. The ALS strains assigned to X. fastidiosa subsp. multiplex belong to two different genotypes (ALSI and ALSII) below the subspecies level. The OLS strains do not infect grape or almond. PD strains produce diseases in grape, alfalfa, almond, and some weeds, but they do not infect oleander, oak, peach, or citrus. ALS strains that belong to X. fastidiosa subsp. multiplex do not produce disease on grape. In this study, a relatively simple polymerase chain reaction (PCR) based method was developed to distinguish among PD, OLS, and ALS strains. PCR performed with primers XF1968-L and XF1968-R amplified a 638-bp fragment from OLS strains but not from PD strains or ALS strains that belong to X. fastidiosa subsp. fastidiosa. PCR with primers XF2542-L and XF2542-R amplified a 412-bp fragment from PD strains, but not from OLS strains. PCR with primers ALM1 and ALM2 produced a fragment of 521 bp from strains isolated from almond that belong to X. fastidiosa subsp. multiplex. The combination of the three primer sets allowed the distinction of the two ALS genotypes of X. fastidiosa subsp. multiplex. These results are in agreement with those obtained from analysis of sequences of 16S-23S rDNA intergenic spacer regions sequence analysis and with previous results based on randomly amplified polymorphic DNA analysis.

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