Management of apple replant disease

Soil fumigation is the cornerstone of replant control. Can research provide growers with more options? By Anna Mouton.

Apple replant is a soilborne disease of apples established on soils previously occupied by apples. The condition primarily stunts the growth of young trees. “Affected trees take longer to come into production,” says plant pathologist Prof. Adéle McLeod. “And nowadays, growers need to harvest their first crop in year three.”

McLeod has been studying apple replant disease since she joined the Department of Plant Pathology at Stellenbosch University nearly 20 years ago. Researching replant in the orchard is slow because trees are evaluated from planting to harvest for each experiment.

“We assess replant on a site by comparing the growth and yield of trees planted on fumigated and unfumigated soil,” says McLeod.

According to McLeod, fumigation is the standard control measure for replant. “Most growers will fumigate because they assume they have replant soils. Replant disease has such a big impact on yields that it’s too risky not to fumigate.”

Together with MSc student Doré de Villiers, McLeod recently completed a Hortgro-funded project examining the effects of soil amendments and a mix of semi-selective chemicals on replant disease. This work built on previous trials of semi-selective chemicals. She spoke to Fresh Quarterly about the results.

A many-headed monster

Apple replant disease is caused by a mix of different species of nematodes, fungi, and oomycetes — oomycetes are microorganisms that resemble fungi but are more closely related to algae, such as kelp. The microbial makeup of each replant soil is unique, which is why broad-spectrum interventions tend to be more successful than targeted treatments.

“Soil fumigation is broad-spectrum — it kills or suppresses everything in the soil. Semi-selective chemicals are aimed at specific pathogens,” explains McLeod.

Trials in Grabouw and Koue Bokkeveld orchards planted in 2013 and 2014 compared various permutations of soil fumigation and semi-selective chemicals to an untreated control. The semi-selectives included fenamiphos, imidacloprid, metalaxyl, and potassium phosphonate. Soil fumigants included chloropicrin and 1,3-dichloropropene combinations, and methyl bromide.

Fenamiphos is a nematicide, and metalaxyl and potassium phosphonate are oomyceticides. Imidacloprid was added to control woolly apple aphids. “Woolly apple aphids can really wreck your trials,” says McLeod, “so we apply imidacloprid in all our treatments.”

Results showed that semi-selective chemicals with or without fumigation were associated with significantly greater tree growth than the untreated control 3–4 years after planting. However, fumigation increased yields relative to the untreated control more than did semi-selective chemicals.

McLeod points out that the lack of a fungicide might have contributed to the inconsistent performance of the semi-selective chemicals. “We know that certain fungi play a role in replant disease, but most crop protectants don’t move well through the soil, and there isn’t a fungicide that we think will work as a soil application.”

Adding organics to the mix

Four new orchard trials were established in Grabouw and the Koue Bokkeveld in 2017 and 2018. Different combinations of soil fumigation and semi-selective chemicals were again compared to an untreated control. The semi-selectives included fluopyram, imidacloprid, mefenoxam, and potassium phosphonate. Soil fumigants included chloropicrin and 1,3-dichloropropene combinations, and metham sodium.

Mefenoxam replaced metalaxyl as one of the oomyceticides in the new trials. Fluopyram replaced fenamiphos as the nematicide.

“Fluopyram is an interesting product because it’s a nematicide with activity against fungi, although it’s not broad-spectrum,” comments McLeod. “We’ve seen it suppress certain replant fungi at some sites when used at high doses.”

In addition to testing chemical treatments, the experiments included combinations of compost, mulch and oomyceticides. “The literature tells you that compost and mulch alone don’t provide reliable control,” says McLeod. “That’s why we didn’t test it as a standalone treatment.”

The results of the new trials were mixed. Two of the four orchards turned out not to have a replant problem, as there was no difference in the performance of trees on fumigated and unfumigated soils.

The third orchard showed inconsistent treatment effects. “I suspect there was something in that orchard that our semi-selective mix failed to suppress,” speculates McLeod. “Possibly nematodes because we used fluopyram instead of fenamiphos in these trials.”

In the fourth orchard, however, combining semi-selective chemicals, compost, and mulch was as effective as fumigation in mitigating replant disease. McLeod is planning new trials to confirm and refine these results.

“We are going to include a semi-selective mix with fenamiphos and combine it with compost and mulch,” she says. “And we’re going to apply the fluopyram more than once to see if we get better results.”

Best practices for replant

McLeod worries that growers will note how well the fourth orchard performed after applying semi-selective chemicals, compost, and mulch, and jump to conclusions. “Replant differs between sites — you can’t make a recommendation based on data from a single orchard,” she cautions, pointing out that the treatment appeared ineffective in one of the other orchards.

For now, fumigation with chloropicrin and 1,3-dichloropropene combinations remains the safest option. Although many growers mourn the loss of methyl bromide, McLeod demonstrated that chloropicrin and 1,3-dichloropropene work as well, provided growers follow best practices for replant management.

“The recommendation is that you shouldn’t pull out an orchard and replant in the same year because it’s not possible to remove all the roots, and those that remain don’t have a chance to decay,” she says. “With methyl bromide, you could get away with planting sooner as it penetrated the roots better, but the new products don’t move as strongly in soil.”

McLeod sees semi-selectives as supplementing fumigation, especially when nursery trees may be infected. Surveys have confirmed replant pathogens in nursery trees, and replant-infected nursery trees are unavoidable as long as trees are produced in layer beds and open fields instead of in tissue culture and containerised sterile media.

Going forward, McLeod will be testing anaerobic soil disinfestation, which involves incorporating large amounts of organic matter in the soil, saturating it with water, and covering it with plastic to allow anaerobic microorganisms to proliferate. The anaerobic conditions and related changes in the soil microbiome destroy replant pathogens.

She hopes that multiple orchard trials demonstrating the efficacy of anaerobic soil disinfestation will eventually allow it to replace fumigation. Although anaerobic disinfestation is more complex and may be more costly than fumigation, it is more environmentally friendly and increases soil organic matter.

Meanwhile, McLeod thinks fumigation is here to stay for now. “Replant has been around for decades, and we don’t currently have another solution because you need such a broad-spectrum treatment to control all the different organisms.”

What are people saying about this research?

“Apple replant disease is a complex issue that affects apple growers globally. The trouble is that the replant-disease complex comprises a number of plant pathogens that are not exactly the same in any site, let alone globally. The more local knowledge generated on apple replant disease, the better for the industry, especially considering ever-mounting pressure on chemical remedies for plant diseases.”

Graeme Krige. General Manager, Fruitmax Agri.