Abstract
Apple replant disease (ARD) is a disorder that affects apple trees when they are replanted in soil where
the same species was previously grown. ARD has been known for a long time, but the precise cause is not
yet identified. Although ARD is most probably due to a combination of abiotic and biotic factors, the fact
that soil fumigation commonly prevents the symptoms, at least temporarily, supports the hypothesis that
microorganisms play an important role in it. In order to
find possible relations between taxa composition
of soil microbial communities and plant growth in ARD-affected orchards, we compared fumigated
(dazomet 99%) and untreated soils by using next-generation sequencing (NGS) technologies. Soil
sampling was carried out when the difference between fumigated and untreated plots became significant
in terms of shoot growth and fruit yield and specifically at the end of the second growing season. Total soil
DNA was extracted and two target regions (ITS for fungi and 16S rDNA for bacteria), were pyrosequenced
with Roche’s 454 Platform. Both bacterial and fungal communities differed significantly in fumigated and
untreated soils of our study. Bacillus sp. (r = 0.64), Streptomyces sp. (r = 0.64), Pseudomonas sp. (r = 0.59),
and Chaetomium sp. (r = 0.85) were some of the taxa positively correlated with asymptomatic apple trees.
Although a cause-effect relation with ARD cannot be proven, our results confirm that, fumigation with
dazomet reduces ARD symptoms, and also modifies soil microbial communities at length, in particular by
increasing the presence of some beneficial microorganisms known for their action against plant
pathogens.