In-field heterogeneity of apple replant disease: Relations to abiotic soil properties

Orchards affected by apple replant disease may show a distinct in-field variability of the severity of apple replant disease (ARD) symptoms. As abiotic soil properties can affect pathogen survival but show considerable variability at the scale of a few meters, we hypothesized that the variability of abiotic soil properties may be used to explain parts of the in-field variability of ARD. We sampled soil and rhizosphere of 32 apple trees with different degree of ARD symptoms after two years of growth at the horticultural research station Heidgraben, Germany. The sandy soils correspond to Entic Podzols. Soil analyses comprised soil aggregate-size fractionations, the assessment of the stocks of nitrogen and essential macro- and micronutrients, as well as a tracing of stable delta N-15 natural abundance both in rhizosphere and non-rhizosphere soil. The results showed that there was indeed a huge in-field variability of tree growth, following a Gaussian distribution, but classified here in shoot heights of < 40, 40-80, 80-120, and > 120 cm, respectively. An effect of soil aggregation on ARD or vice versa was not detected. Elevated tree growth, however, went along with a consumption of soil macronutrients (R-2: = 0.3 - 0.8 for the n = 4 ARD classes; significant only for K on individual plant basis). Intriguingly, opposite correlations were observed for micronutrients, where reduced tree growth occurred at subsites short in micronutrient supply (R-2 = 0.9 for Mn and Co, significant for Mn and Co for ARD infected plants up to 100 cm shoot length on individual plant basis). Moreover, the plants showing most stunted shoot growth revealed lowest delta N-15 values in the rhizosphere, as typical for reduced N cycling. Our data therefore confirm that at least parts of the in-field variability of ARD severity is correlated to reduced micronutrient supply and N cycling, possibly via effects on habitat properties of pathogenic soil microbiota.