The efficacy of semiselective chemicals and chloropicrin/1,3-dichloropropene-containing fumigants in managing apple replant disease in South Africa

Authors: NYONI, M - Stellenbosch University; Mazzola, Mark; WESSELS, JP - Procrop; MCLEOD, A - Stellenbosch University

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/1/2019
Publication Date: 4/15/2019
Citation: Nyoni, M., Mazzola, M., Wessels, J.B., McLeod, A. 2019. The efficacy of semiselective chemicals and chloropicrin/1,3-dichloropropene-containing fumigants in managing apple replant disease in South Africa. Plant Disease. 103(6):1363-1373. https://doi.org/10.1094/PDIS-10-18-1844-RE.
DOI: https://doi.org/10.1094/PDIS-10-18-1844-RE

Interpretive Summary: Control of soil borne pathogens and parasites is crucial to the successful establishment of new orchards on old orchard sites. These pathogens when acting together incited a phenomenon termed apple replant disease which results in severe economic losses in orchard systems. Although methyl bromide soil fumigation, historically a primary measure used to control this disease, has been removed as a disease management option in developed countries for over a decade it has only recently been abolished in developing countries including South Africa. Studies were conducted to assess the utility of alternative fumigants used independently or in conjunction with specific semi-selective biocides known to possess activity toward one or more of the replant plant pathogens for the control of apple replant disease. Two different fumigant formulations containing the fungicidal chemistry chloropicrin and the nematicidal chemistry 1,3-dichloropropene were evaluated in this study. These fumigants were utilized with and without the addition of the semi-selective biocide treatment. Each of these alternative disease control options significantly enhanced tree growth in three orchard trials conducted on replant sites in South Africa. Among the evaluated treatments, the fumigant containing 33% chloropicrin and 67% 1,3-dichloropropene (Pic33-1,3-D)used in concert with semi-selective biocide treatment provided the most consistent increase in cumulative fruit yields. When used independently, the fumigant formulation 57% chloropicrin and 43% 1,3-dichloropropene was more effective than Pic33-1,3-D in enhancing tree growth and yield on replant sites.

Technical Abstract: Apple replant disease (ARD) is a biological phenomenon that is encountered when old apple orchards are replanted, resulting in tree growth and yield reductions in young trees. Three ARD orchard trials were conducted, which showed that semi-selective chemicals (fenamiphos, metalaxyl, imidacloprid and phosphonates) used independently, two fumigant formulations (33.3% chloropicrin/60.8% 1,3-dichloropropene [Pic33-1,3D] and 57.% chloropicrin/38% 1,3 dichloropropene [Pic57-1,3D]) and semi-selective chemicals combined with fumigation all contributed to significant increases in tree growth (trunk diameter and shoot length) relative to the untreated control, 3- to 4-years post-planting. All treatments improved tree growth in a similar manner. Yield was more indicative of treatment efficacy, but treatment efficacy varied between orchards. The Pic33-1,3D fumigant in concert with semi-selective chemistries was the most consistent in significantly increasing cumulative yields. The Pic57-1,3D treatment significantly increased cumulative yields in comparison to the Pic33-1,3D treatment in one orchard, and in another orchard resulted in a significant increase in yield relative to the control treatment whereas the Pic33-1,3D did not. Phytophthora cactorum contributed to disease development in all three orchards; significant negative correlations existed between the quantity of P. cactorum DNA detected in roots and tree growth, and less often yield. In two orchards, certain treatments that significantly reduced the quantity of P. cactorum DNA in tree roots relative to the control, resulted in a significant increase in tree growth. Some of the same trends were evident for Pratylenchus spp. root densities in two of the orchards. There was a significant positive correlation between P. cactorum root DNA quantities and Pratylenchus spp. root densities. Pythium spp. and ‘Cylindrocarpon’-like DNA quantities detected in tree roots typically were not indicative of treatment efficacy. However, a significant positive correlation existed between these two pathogen groups, suggesting complex interactions not associated with pathogen quantities per se.