The pot test

By Anna Mouton.

One of the challenges of apple replant disease is identifying which soils are affected. “We’ve learnt the hard way that not all replanted soils will have severe replant disease, which is problematic for evaluating the efficacy of new treatments” says Prof. Adéle McLeod, of the Department of Plant Pathology at Stellenbosch University. “We’ve learnt to test the soil before we establish any orchard trials, otherwise the untreated control may not differ from the standard soil fumigation treatment. The results from all novel treatments will thus be inconclusive.”

McLeod’s team uses the pot test to assess soils. They collect a soil sample and split it in two equal parts. One part is treated with heat to destroy any soil organisms. The other is left as is. Apple seedlings are grown in the treated and untreated soils. If replant disease is present, the seedlings in the treated soil will significantly outperform those in the untreated soil.

“At present this isn’t a service that we offer to growers, because we haven’t conducted enough tests to know how reliable the pot test is ” says McLeod.” Therefore, more research would be needed before rolling out a commercial version of the pot test. I think we need to look at aspects such as whether it matters at what time of year you collect the soil samples, and how many samples per hectare should be collected, and where soil should be collected.”

Soil fumigation prior to planting is the conventional approach to controlling apple replant disease. Having a test for replant could save growers the expense of unnecessary fumigation. For now, however, McLeod recommends playing it safe. “I’d be concerned about the risk to the producer of relying on the pot test, since an incorrect test result will have a major negative impact on yield for the lifespan of the orchard.”