Replant research overview

A summary of current, completed, and new research. Compiled by Grethe Bestbier.

Hortgro Science has supported and funded several research projects on apple replant disease, and continues to do so. Apple replant disease is caused by a complex of organisms that stunt young apple trees when planted on soil where apples grew previously. Extensive research is needed to better understand the disease, and to develop management strategies.

This summary brings you highlights of completed and ongoing research, and of new projects starting in 2021. For more details and the complete findings of each study, contact Hortgro Science to access the project reports.

What we know: completed projects

1. Expansion of the specific replant disease biotest (Van Schoor, 2003)

The reliability of the biotest for apple replant disease that existed at the time was improved by refining the methodology and revising the existing soil-sampling procedure. The results of the new-and-improved biotests showed a good correlation to that of field trials, successfully predicting replant disease in 75% of cases investigated.

Furthermore, it was found that the primary cause of apple replant disease in South Africa is biological. Replant soils retain their negative impact on apple seedling growth even when diluted 1:3.

The study also suggested that the application of organic amendments could possibly substitute for methyl bromide fumigation in replanted orchards, while a fumigant containing chloropicrin and 1,3-dichloropropene was identified as a possible alternative to replace methyl bromide.

2. The aetiology of apple replant disease (McLeod, 2010)

The cause of apple replant disease remained unknown for many decades, with progress in clarifying its origins only occurring in the past ten years. Its reported origins have varied substantially, including both abiotic and biotic factors. In South Africa, research has shown that the disease is primarily caused by biological agents.

This study aimed to investigate the aetiology of apple replant disease in six South African orchard soils. It was found that replant is caused by a complex that includes various oomycetes, fungi, and nematodes. These include species that are widespread and virulent. Evidence suggested that actinomycete bacteria are not involved in causing apple replant disease.

3. Apple replant rootstock trials for determining a screening technique (Vermeulen, 2017)

This study investigated the resistance or tolerance of various apple rootstocks to apple replant disease. Measurements of percentage increase in trunk and total shoot growth, and plant weight, clearly indicated that rootstock tolerance was site-specific.

A possible exception to the rule is MM.109, which seemed to be most susceptible to replant at both trial sites. However, a previous round of trials (2010–2012) found that MM.109 was the only rootstock that consistently showed tolerance in the field.

These seemingly contradictory results demonstrate that there are a number of site-specific factors that contribute to rootstock performance. These factors need to be taken into account when selecting rootstocks for apple replant disease management.

4. Identification of inoculum sources of oomycetes, a major contributor to apple replant disease, and the management thereof (McLeod and Mazzola, 2019)

Certain oomycetes, including Phytophthora and Pythium species, are important contributors to apple replant disease in South African orchards, and have been associated with the death of young trees. Other replant-disease agents include plant-parasitic nematodes and certain fungi.

This study found organisms associated with apple replant disease on apple nursery trees. Agents of replant disease were also found in irrigation water, but less frequently.

The researchers showed that a higher dosage chloropicrin fumigant was generally more effective than a lower dosage for managing apple replant disease. A combination of semi-selective chemicals and a low dosage of chloropicrin was also an effective treatment. Both treatments significantly increased yields over a three-year period.

What we hope to learn soon: ongoing projects

1. Managing apple replant disease (McLeod and Wessels)

This project aims to evaluate cost-effective alternatives to the nematicide fenamiphos for the management of apple replant disease. A proposed alternative treatment of compost, mulch, and oomyceticides could not provide consistent control of replant disease across all trial sites. Fumigation of these soils only significantly improved tree growth in two of the five orchard trials, which could indicate that some of the soils were not affected by replant disease.

2. Evaluation of apple rootstock tolerance to specific apple replant disease (Labuschagne)

Rootstocks that are tolerant to apple replant disease hold the potential for use within an integrated disease management strategy. A tolerant rootstock would result in the development of low pathogen populations because of fast recognition and activation of defence responses by the host. This project aims to evaluate a variety of new apple rootstocks to compare and identify replant-tolerant rootstocks under local orchard conditions.

3. Effect of organic amendments on apple replant disease (McLeod and Wessels)

Apple replant disease is currently managed in South Africa by fumigation, but this strategy is unsustainable and costly. The use of compost and mulches could potentially reduce the build-up of replant pathogens during the lifespan of an apple orchard, which in turn could reduce the severity of the disease. This study aims to investigate the effect of long-term organic amendments on the severity of apple replant disease in apple orchard re-plantings. The project will investigate compost and mulch, alone and in combination, for preventing replant disease. A further aim is to develop a chemical mix to reduce the risk of introducing apple replant disease with nursery trees.

What’s to come: new projects

1. Effect of mulching, compost, and rootstock genotype on white root rot of apple (McLeod)

White root rot, caused by a fungus called Rosellinia, can cause tree death, and is a growing concern for the apple industry due to its increasing prevalence. Furthermore, there are concerns that organic amendments such as mulch and compost could increase white root rot development.

Currently, there is no effective management strategy for the disease other than containment through tree removal. While no resistant rootstocks have been found, rootstocks that are tolerant and exhibit delayed symptom development have been identified, and may present a viable management strategy.

This study aims to determine the tolerance of apple rootstocks to white root rot. It will also examine whether Rosellinia grows in the mulch around infected trees, and how its growth is affected by mulch and compost.

2. Tolerance of apple rootstocks against Phytophthora root rot (McLeod)

This project aims to determine the tolerance of apple rootstocks against Phytophthora root rot, which often causes reduced tree growth and tree losses in young orchards. Current apple rootstocks in South Africa are all susceptible to the pathogen, but there have been reports that some Geneva rootstocks offer tolerance. However, data are not available for all of the Geneva rootstocks, and the relative performance of these rootstocks is not known. Therefore, these rootstocks will be challenged using South African strains of Phytophthora.