Managing apple replant disease: the effect of biological soil amendments on tree performance
Louise van Schoor, ARC Infruitec-Nietvoorbij, Stellenbosch.
Introduction
Apple replant disease (ARD) is a disorder associated with the poor growth of young apple trees
planted on sites where apples were planted previously. It is a major impediment in establishing an
economically viable apple industry and an increasingly important problem due to limited
availability of suitable virgin soil sites. The problem is exacerbated by the demand for new
cultivars and improved rootstocks which necessitate new plantings. Although ARD is not lethal, it
reduces the success of an intensive planting system which depends on the rapid development of the
orchard canopy and early production.
The etiology of ARD is biological in origin, is complex and site-specific, and involves a shift in soil
microbial community composition towards pathogens dominating the soil microbial profile
(Mazzola, 1999). ARD may be controlled by the application of broad spectrum fumigants, but
environmental concerns necessitate that biological alternatives are investigated. Since ARD mainly
develops through changes in soil microbial populations, the integration of biological soil
amendments into production systems may shift the population balance towards a more beneficial
microbial community (Barea et al., 2005; Cohen et al., 2005; Yao et al., 2006). Due to the adverse
effects of ARD on root proliferation and development, stimulation of root growth can also play an
important role in managing ARD.
Various trials were established to investigate the potential of biological soil amendments to reduce
the growth retarding effects of ARD. Compost, compost extracts, a Bacillus bacteria inoculant
mixture (Biostart®), humates and combinations of these amendments were investigated relative to
untreated controls. Compost and compost extracts contain a diverse group of microorganisms, and
easily available nutrients. Mechanisms by which beneficial microorganisms primarily affect plant
growth include solubilisation and increased uptake of nutrients, phytohormone production and
biological control of soil pathogens (Dobbelaere et al., 2003). Compost and humates were applied
annually from orchard establishment and soil inoculants were applied monthly throughout the
growing season.
Results and Discussion
Application of Biostart®, in combination with an activator carbon (C) source and low dosage of
humate, as well as compost extract applied with compost, significantly improved tree performance
in ARD sites. Continuous application of these amendments over five seasons to ‘Fuji’ and ‘Ruby
Gala’ trees on M793, increased growth by 20-30%, and cumulative yield by 15-40%, compared to
untreated plots. No significant differences were found in fruit quality. This orchard showed high
infestation of root lesion nematode, even in fumigated plots. It is possible that plants were protected
from nematode attack by changes in the soil microbial populations through biological amendment.
Positive effects on growth were also observed with compost extract application in ARD orchards in
another field study in South Africa (Van Schoor et al., 2009). In pot trials using apple seedlings, the
application of compost, as well as sterilised and unsterilised compost extracts, significantly
increased growth of apple seedlings in six ARD soils.
In three additional ARD field sites (consisting of ‘Fuji’ on M793 and M7 and ‘Royal Gala’ on
M793), the effects on tree growth using compost and either Biostart® (with C additions) or
compost extract, were only significant in orchards showing mild ARD effects but not those with
severe cases of ARD. In the site where ARD symptoms were most severe, biological amendments
resulted in little improvement in yield relative to untreated plots. In the other two sites ARD was
less severe and biological amendment improved tree growth to the same extent as methyl bromide
fumigation over three growing seasons. There was no clear indication of which amendment, in
addition to compost, resulted in the best tree performance.
Although biological amendments showed promise in reducing ARD symptoms, fumigation gave
the most significant and consistent response in tree performance. Effects resulting from fumigation
seem to be dramatic and immediate, allowing trees to establish in an environment with little
microbial competition, and in the absence of soil pathogens. Due to its broad-spectrum activity
methyl bromide is also effective under various soil biological conditions. In contrast to this, effects
with biological amendments are more gradual and long term and regular application is essential.
Early application is also important, in order for trees to be colonised by beneficial organisms
protecting them from pathogen attack. Furthermore, young trees still trying to establish an efficient
root system can benefit from improved nutrient solubilisation and microbial plant growth hormone
production in the rhizosphere. With continued application, long term changes can be induced in soil
microbial communities, making it more difficult for soilborne pathogens to dominate. This can
possibly also have positive implications for the next orchard to be established on this site.
In this study, application of compost or humates on its own did not result in significant changes in
tree performance. Variability in amendments, as well as soil conditions, is one of the biggest
impediments in the wide scale use of organic amendments. It has been suggested that controlled
inoculation of composts can induce more consistent effects especially in disease suppression
(Hoitink et al., 1997). Our results suggest that regular application of a diverse group of microbial
inoculants in combination with organic material, may reduce symptoms of ARD. However, effects
are site-specific and therefore intensive management and increased knowledge of the plant-soil
system is required. Biological approaches to ARD control ultimately should form part of an
integrated biological management system to maximise benefits. The use of biological management
practices in combination with ARD tolerant rootstocks (Geneva® Rootstock range) may show more
consistent effects over a wide range of soil conditions.
Acknowledgements
The project is funded by the ARC and the DFPT under the Soil Health Programme.
For further information contact Louise van Schoor: e-mail, vschoorL@arc.agric.za; Tel., (021) 809
3367.
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