Submitted to: Soil Biology and Biochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 9, 2012
Publication Date: February 2, 2012
Citation: Mazzola, M., Reardon, C.L., Brown, J. 2012. Initial Pythium species composition and Brassicaceae seed meal type influence extent of Pythium-induced plant growth suppression in soil. Soil Biology and Biochemistry. 48:20-27. Interpretive Summary: Members of the genus Pythium are important pathogens to a diversity of crop plants, however species within this group vary greatly in the degree of plant damage they can cause. Typically, much attention is placed to estimating populations of this pathogen when assessing the need for soil treatment but relatively little effort is given to determination of species composition. Brassicaceae seed meals (SM) have different effects on Pythium population densities, ranging from stimulation to suppression but again little information does not exist as to how seed meal amendments influence species composition and subsequent plant health. Pythium populations of similar density in two soils exhibited dramatic differences in the resulting ability to damage apple and wheat. This difference was related to the Pythium species composition within the two soil systems. When Pythium populations were increased 100-fold in response to Brassica napus SM amendment, there was still little damage to plants grown in CV soil, but extensive damage to plants grown in RF soil. This resulted from the fact that weakly virulent Pythium species, Pythium heterothallicum, dominated CV soil while highly virulent Pythium ultimum dominated the RF soil population. However, although Sinapis alba SM amendment elevated Pythium pouplations to a similar degree as B. napus SM in CV soil, extensive plant damage was detected. This result was associated with a transformation of the Pythium population from one dominated by P. heterothallicum in B. napus treated CV soil to one dominated by P. ultimum in the same soil treated with S. alba SM. Thus, knowledge of Pythium population species composition and not just densities is needed to effectively predict the importance of this pathogen in limiting crop productivity. Such information will also be of value in determining an effective strategy in optimizing the use of brassicaceae seed meals as a pest management tool.
Technical Abstract: The composition of Pythium spp. populations differs significantly among agronomic soils resulting in site to site variation in the relative importance of these pathogens to growth and development of agronomically important crop species. Brassicaceae seed meal (SM) amendments differentially influence quantitative dynamics of Pythium populations resident to soil systems. Studies were conducted to determine the effect of initial species composition and Brassicaceae and non-Brassicaceae SM type on qualitative attributes of Pythium spp. populations and resulting effects on plant growth and disease development. Quantitatively, Pythium populations were similar between the RF and CV study soils, but apple seedling mortality and suppression of wheat emergence was only observed in the RF soil. Correspondingly, application of Pythium suppressive Brassica juncea SM only improved emergence and reduced seedling mortality in the RF soil. Although Brassica napus, Glycine max and Sinapis alba SM amendments consistently elevated Pythium spp. soil densities, the corresponding effect on plant growth was more deleterious in RF than CV soil. These differences corresponded with the initial Pythium species composition resident to the soils, with highly virulent Pythium ultimum var. ultimum dominant in RF soil and less virulent Pythium heterothallicum most prevalent in CV soil. Although Pythium populations were lower in CV soil amended with S. alba than B. napus SM, plant damage consistently was greater in S. alba amended soil. This result corresponded with the finding that S. alba SM application preferentially elevated recovery of P. ultimum var. ultimum from CV soil or plants grown in this soil while less virulent species continued to dominate B. napus SM treated soil. Although the effect of SM type on Pythium soil densities is important when considering its use as a soil amendment, our findings indicate that the Pythium spp.-selective nature of a seed meal is of greater consequence. Our findings also demonstrate that the determination of Pythium spp. soil densities without verification of species composition can be insufficient to predict the potential for plant damage from this pathogen population.