An evaluation of the specific apple replant problem in Western Cape orchard soils

[摘要] ENGLISH ABSTRACT:Apple replant disease (ARD) is one of the major impediments to the establishment ofan economically viable apple orchard on sites previously planted to apple. In spite ofextensive research on ARD, the etiology remains to be fully elucidated. A possiblebiological origin of ARD etiology in South Africa was investigated by the dilution ofreplant field soil with sterilised soil. Commercial orchards with ARD were selected foruse in pot trials and disease severity evaluated after three months, by measuringshoot length, dry mass of plants as well as root discolouration. Although dilutingreplant soil to 25 and 50% (v/v) significantly reduced the effects of ARD, symptomswere only absent in 0% replant soil. It was clear that seedlings planted in anymixture containing replant soil, even only 25% replant soil, consistently exhibitedsymptoms of stunted growth and root discolouration similar to those seedlings grownin 100% replant soil. This indicates that ARD in South Africa is primarily of abiological nature.As an initial step in formulating sustainable disease control alternatives to replacemethyl bromide, pot trials were conducted to assess the impact of composttreatments as well as biological control products on ARD. Compost as well assterilised and unsterilised compost teas (compost extract) significantly increasedseedling growth even under optimum nutrient conditions when compared to thecontrol, suggesting that they negate the effects of ARD. Results also indicated thatapplying high concentrations of compost does not necessarily provide additionalgrowth benefits compared to lower concentrations. Results with biocontrolformulations were less favourable. Only one of the biocontrol formulations, acombination of Bacillus spp. (Biostart®) improved growth significantly compared tothe control. There was, however, some inconsistency with results for the differenttrials conducted using this product.Fungal as well as nematode populations associated with ARD soils werecharacterised to the generic level to get a clearer understanding of the etiology ofARD in South Africa. Pythium and Cylindrocarpon spp. were consistently isolatedfrom all six replant soils in all trials that formed part of this study, indicating that thesefungi may have a role in ARD etiology in South Africa. Nematodes implicated in ARDdevelopment were inconsistently associated with ARD soils used in these studies.This suggests that nematodes do not have a primary causal role in ARD etiology inSouth Africa. Field trials were conducted in commercial orchards to assess the impact of organicamendments and promising biological control products, as indicated by the pot trials,on ARO severity under field conditions. These biological soil amendments were alsocompared with the standard chemical control methods for ARO, methyl bromide andchloropicrin. In all three trials established, compost and mulch as well as manureand mulch, consistently increased growth to the same extent as the standardchemical treatments and by combining these chemical treatments with organicamendments a significant, additional growth increase could be attained. Biocontrolformulations evaluated in field studies gave variable results. Biostart® improvedgrowth when applied on its own, but not in combination with the chemical Herbifume(metham-sodium). Inoculating soil with effective microorganisms (EM), consistingprimarily of photosynthetic bacteria, had no significant effect on growth.Results from this study indicate that application of organic amendments couldpossibly substitute for soil fumigation in replanted apple orchards. However, compostquality standards need to be implemented and because few types of compost areuniversally effective, different types of composts should be compared in specific soilenvironments before recommendations can be made. Oue to variable results withbiocontrol products, ARO management with these biological soil amendments cannotbe guaranteed at this stage and further studies are recommended.

[发布机构] Stellenbosch University