Specific suppression of plant diseases provided by compost-amended substrates

Natural potting mixes prepared with Sphagnum peat and/or composted pine bark may induce suppression to Rhizoctonia diseases and induce systemic resistance (SR) in plants. Unfortunately, these effects are variable in nature and the SR-active microflora remains unknown. The objectives of this work were to (1) determine the frequency by which natural potting mixes control these diseases, (2) determine the impacts of organic components used in potting mixes on the efficacy of biocontrol agents against Rhizoctonia diseases and (3) identify the most SR-active microorganisms for preparation of potting mixes inducing SR consistently in plants. The results show that <20% of natural composted pine bark mixes suppressed Rhizoctonia damping-off of radish and that only 12.5% reduced the severity of bacterial spot of radish caused by Xanthomonas campestris pv. armoraceae (Xca). The biocontrol agents Trichoderma hamatum 382 (T382) and Chryseobacterium gleum 299R2 (C299R2) consistently induced suppression to Rhizoctonia damping-off in composted pine bark mixes. They were less effective in light, slightly decomposed peat and ineffective in more decomposed dark peat mixes. Similar results were obtained for control of Rhizoctonia crown and root rot of poinsettia.; Several bacterial strains were isolated from the rhizosphere of radish seedlings grown in SR-active compost-amended potting mixes that suppressed the severity of bacterial spot. None of these strains provided consistent control. T382 was the most SR-active biocontrol agent recovered from compost-amended mixes. Bacterial spot severity was significantly ( P ≤ 0.05) reduced on radish seedlings grown in the composted pine bark mix fortified with T382 compared to seedlings grown in the control mix. T382 fortified-composted pine bark mix also reduced the population of Xca in infected plants relative to the control mix. However, it was not as effective as acibenzolar-s-methyl which induces SAR in radish. Spatial separation of T382 and the pathogen on the host plant was maintained suggesting that a systemic response was induced.