Abstract
Subterranean clover (Trifolium subterraneum) is a critical pasture legume in Mediterranean regions of southern Australia and elsewhere, including Mediterranean-type climatic regions in Africa, Asia, Australia, Europe, North America, and South America. Pythium damping-off and root disease caused by Pythium irregulare is a significant threat to subterranean clover in Australia and a study was conducted to define how environmental factors (viz. temperature, soil type, moisture and nutrition) as well as variety, influence the extent of damping-off and root disease as well as subterranean clover productivity under challenge by this pathogen. Relationships were statistically modeled using linear and generalized linear models and boosted regression trees. Modeling found complex relationships between explanatory variables and the extent of Pythium damping-off and root rot. Linear modeling identified high-level (4 or 5-way) significant interactions for each dependent variable (dry shoot and root weight, emergence, tap and lateral root disease index). Furthermore, all explanatory variables (temperature, soil, moisture, nutrition, variety) were found significant as part of some interaction within these models. A significant five-way interaction between all explanatory variables was found for both dry shoot and root dry weights, and a four way interaction between temperature, soil, moisture, and nutrition was found for both tap and lateral root disease index. A second approach to modeling using boosted regression trees provided support for and helped clarify the complex nature of the relationships found in linear models. All explanatory variables showed at least 5% relative influence on each of the five dependent variables. All models indicated differences due to soil type, with the sand-based soil having either higher weights, greater emergence, or lower disease indices; while lowest weights and less emergence, as well as higher disease indices, were found for loam soil and low temperature. There was more severe tap and lateral root rot disease in higher moisture situations.
Highlights
Subterranean clover (Trifolium subterraneum) is critical pasture legume in Mediterranean regions of southern Australia and elsewhere, including Mediterranean-type climatic regions in Africa, Asia, Australia, Europe, North America, and South America (Nichols et al, 2014)
The second approach to modeling using boosted regression trees provided support for the complex nature of the relationships found in linear models
There was a pattern for both tap and lateral root with higher levels of disease found in plants under high moisture (Figure 4). These are the first studies to use a comprehensive modeling approach to highlight the importance of environmental conditions, as occur across southern Australia in upon the severity of Pythium damping-off and root disease and subterranean clover forage productivity
Summary
Subterranean clover (Trifolium subterraneum) is critical pasture legume in Mediterranean regions of southern Australia and elsewhere, including Mediterranean-type climatic regions in Africa, Asia, Australia, Europe, North America, and South America (Nichols et al, 2014). Subterranean clover is attacked by a diverse range of soilborne pathogens, especially oomycetes. Oomycete soilborne pathogens cause severe pre- and post-emergence damping off and root disease in seedlings (Wong et al, 1984; Barbetti et al, 1986a,b, 2006b, 2007) and root disease in subterranean clover on mature plants later in spring (O’Rourke et al, 2009), resulting in severe reduction of forage crop (Gillespie, 1983; Barbetti et al, 1986a). In-field losses in subterranean clover forages include up to 90% of seedlings from damping-off (Wong et al, 1985a), ≥70% productivity across the 6–17 weeks period post-germination (Barbetti, 1984a). Widespread deterioration of subterranean clover forages from damping-off and root disease seriously and adversely curtails both livestock carrying capacity per se and overall profitability (Barbetti et al, 1986b; Nichols et al, 2014), with some farmers no longer remaining economically viable in worst-affected areas
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