Towards the intergrated management of apple replant disease using knowledge on disease etiology

Authors: MCCLEOD, ADELE - University Of Stellenbosch; Mazzola, Mark

Submitted to: IOBC/WPRS Bulletin (Abstract for Conference Proceedings)
Publication Type: Abstract Only
Publication Acceptance Date: 10/2/2014
Publication Date: 11/25/2014
Citation: Mccleod, A., Mazzola, M. 2014. Towards the intergrated management of apple replant disease using knowledge on disease etiology. IOBC/WPRS Bulletin (Abstract for Conference Proceedings). 110/73-75.

Interpretive Summary:

Technical Abstract: Young apple orchards that are cultivated on old apple soils often suffer from apple replant disease (ARD). ARD symptom expression is characterized by tree stunting, shortened internodes and discoloured roots, which appears throughout the orchard shortly after orchard establishment when trees are most susceptible. The disease is economically important since it results in a delay in initial fruit production, and a reduction in fruit yield and quality throughout the lifespan of the orchard. The etiology of the disease has been debated, but a biological cause is supported by the alleviation of symptom expression through soil fumigation or pasteurization. In South Africa, Italy and the USA, the causative biological agents have been identified as several species of Pratylenchus, Pythium, Phytophthora, Rhizoctonia and Ilyonectria. However, site specific differences exist in the occurrence and relative abundance of these causative agents. Among the Ilyonectria species, isolates within the same species can have low virulence or are non-pathogenic. More than 20 Pythium species have been associated with ARD, but only some species are highly virulent, whereas others have low virulence or are non-pathogenic. Rhizoctonia solani AG-5 and AG-6 are prominent highly virulent ARD pathogens in some countries. Knowledge on the etiology of ARD is imperative for formulating integrated management strategies, and to understand the mechanisms which function in disease control. Disease etiology knowledge has not been used extensively in studies that have examined the tolerance/resistance of Geneva apple rootstocks, which currently show promise for use within an integrated ARD management strategy. The “resistance” of these rootstocks has exclusively been assessed based upon relative growth in response to fumigation rather than examination of root infection/colonization by causal pathogens. As a result, significant variability in rootstock performance across sites has been reported with an individual genotype receiving ratings from “resistant” to susceptible based upon horticultural performance. Future research therefore needs to take a more focused approach towards elucidating the rootstock genotype response to the causal pathology at any given experimental site in terms root-pathogen interactions. Knowledge on the response of ARD pathogens to management practices, specifically Brassica seed meals, has elegantly shown how important it is to understand and monitor the etiology of the disease at experimental sites. This has resulted in the formulation of a Brassica juncea/Sinapsis alba seed meal mix that is able to effectively provide long-term suppression of the ARD pathogen complex, whereas single Brassica species are never effective. Knowledge on the disease etiology is also useful for investigating alternative inoculum sources, specifically nursery material and irrigation water. In South Africa, most nursery materials are a source of inoculum of Pratylenchus and the highly virulent Pythium irregulare, whereas irrigation water seems less important.