|NYONI, M - Stellenbosch University|
|MCLEOD, A - Stellenbosch University|
Submitted to: Meeting Abstract
Publication Type: Proceedings
Publication Acceptance Date: 11/30/2014
Publication Date: 1/18/2015
Citation: Nyoni, M., Mazzola, M., Mcleod, A. 2015. Evaluating systemic semi-selective chemicals for the management of apple replant disease in fumigated and non-fumigated orchards systems. 49th Congress of the Southern Africa Society for Plant Pathology. p. 121.
Technical Abstract: Apple Replant Disease (ARD) is a phenomenon where apple trees are stunted when replanted onto old apple soil, as the result of apple monoculture resulting in soil microbial changes where pathogenic and parasitic organism s predominate. The main soilborne organisms that cause ARD include oomycetes, fungi and nematodes. Preplant soil fumigation with broad spectrum fumigants, mainly mixtures of chloropicrin (Chl) and I,3-dichloropropene( I,3-D), has been the standard management method for control of ARD. Soil fumigation is only applied to the planting row, which results in recolonization of tree roots once the root system expands into untreated soil. In South Africa, oomycetes (certain Pythium spp. and Phytophthora cactorum) and Pratylenchus spp. have been identified as the most virulent pathogens, along with Cylindrocarpon spp. and binucleate Rhizoctonia spp. that are generally less virulent. Semi-selective chemicals that target oomycetes and nematodes might thus have potential for suppressing ARD in South Africa. The aims of the study were to (i) evaluate in two orchard trials the efficacy of two fumigants varying in Chl and 1,3-D content(33.3% Chl:60.8% 1,3-D vs. 57% Chl:38% 1,3-D), and semi-selective chemicals (fenamiphos, metalaxy potassium phosphonate) applied to fumigated and non-fu mi gated soil and (ii) to elucidate the ARD etiology of the two orchard soils under glasshouse conditions. Evaluation of tree growth responses (increases in trunk diameter, total shoot length and leader length)after the first growing season showed similar trends for both trials. All fumigant treatments had significantly better growth than the untreated controls. The semi-selective chemicals applied to non-fumigated soil did not significantly increase tree growth relative to the un treated control. However, in both trials there was a trend for this treatment to contribute towards improved growth over that of the untreated control. The semi-selective chemicals in combination with a fumigant also did not significantly enhance growth relative to the fumigant treatment alone. Fumigants containing different Chl and I,3-D contents perf01med equally well. Glasshouse trials have been completed for the one orchard soiI, whereas that of the other soil is still underway. The trial confirmed that the one orchard soil had a severe ARD status, since soil pasteurization significantly enhanced seedling growth by 72% (weight) and 34, 5% (length). Furthermore, soil dilution of 1 5% (v/v) non-treated soil into pasteurized soil caused pathogen re-infestation of soil and significant seedling growth reductions. Isolation studies showed that the etiological agents in this soil were comprised of Cylindrocarpon spp., Pythium vexans, Pythium irregulare, Phytophthora cactorum and Pratylenchus spp.