Increase in soil pH due to Ca-rich organic matter application causes suppression of the clubroot disease of crucifers

Abstract

Clubroot disease of cruciferous plants caused by the soil-borne pathogen Plasmodiophora brassicae is difficult to control because the pathogen survives for a long time in soil as resting spores. Disease-suppressive and conducive soils were found during the long-term experiment on the impact of organic matter application to arable fields and have been studied to clarify the biotic and abiotic factors involved in the disease suppression. The fact that a large amount of organic matter, 400 t ha−1 yr−1 farmyard manure (FYM) or 100 t ha−1 yr−1 food factory sludge compost (FSC), had been incorporated for more than 15 yr in the suppressive soils and these soils showed higher pH and Ca concentration than the disease conducive soil led us to hypothesize that an increase in soil pH due to the long-term incorporation of Ca-rich organic matter might be the primary cause of the disease suppression. We have designed a highly reproducible bioassay system to examine this hypothesis. The suppressive and conducive soils were mixed with the resting spores of P. brassicae at a rate of 106 spore g−1 soil, and Brassica campestris was grown in a growth chamber for 8 d. The number of root hair infections was assessed on a microscope. It was found that the incorporation of FYM and FSC at 2.5% (w/w) to the conducive soil suppressed the infection and that the finer particles (5 mm) of FSC inhibited the infection and increased soil pH more effectively. Neutralization of the conducive soil by Ca(OH)2, CaCO3 and KOH suppressed the infection, but the effectiveness of KOH was less than those of Ca(OH)2 and CaCO3. Acidification of the suppressive soils by H2SO4, promoted the infection. The involvement of soil biota in the disease suppression was investigated using the sterilized (γ-ray irradiation) suppressive soils with respect to soil pH. The γ-ray irradiation promoted the infection at pH 5.5, but no infection was observed at pH 7.4 irrespective of the sterilization status. All these observations suggest that soil pH is a major factor in disease suppression by organic matter application and that Ca and soil biota play certain roles in the suppression under the influence of soil pH

Suppression of clubroot disease under neutral pH caused by inhibition of spore germination of Plasmodiophora brassicae in the rhizosphere

“…The application of food factory sludge compost (FSC) to soil increased soil pH, calcium, carbon, nitrogen and phosphorus levels, and there could be additional effects of these elements on disease suppression. However, this is unlikely because of the fact that the suppressive effect in FSC‐treated soils was cancelled out by acidification with sulphuric acid (Niwa et al ., 2007).…”

“…The objective of this study was to clarify the mechanism of disease suppression at neutral soil pH in the assessment of spore germination. The reliability and reproducibility of the germination assay system were examined in conjunction with the soil‐based plug‐pot bioassay (Niwa et al ., 2007), with particular emphasis on the validity of nuclear staining for the germination assay. It was uncertain, however, whether the assay system was sensitive enough to detect germination, because if spores germinated in response to root exudates (Macfarlane, 1970; Suzuki et al ., 1992), only a small proportion of spores in the vicinity of roots would germinate.…”

Composted Oyster Shell as Lime Fertilizer Is More Effective Than Fresh Oyster Shell

“…21) Neutralization of conducive soil by Ca(OH) 2 , CaCO 3 , and KOH suppressed clubroot disease of cruciferous plants caused by the soil-borne pathogen Plasmodiophora brassicae, and Ca compounds were more effective than KOH. 28) In this study, we studied variations in the physical and chemical properties of fresh and composted oyster shell samples. We also studied the efficacy of fresh and composted oyster shell meal as a lime fertilizer in comparison with commercial lime for soybean cultivation as a test crop.…”

Assessment of the response of Plasmodiophora brassicae in contaminated horticultural land, using lime‐based fertilizer concentrations

“…Niwa et al. () showed that calcium‐rich organic matter suppressed disease. This was due to the effect of increasing pH rather than the presence of calcium.…”