Modeling Effects of Temperature, Soil, Moisture, Nutrition and Variety As Determinants of Severity of Pythium Damping-Off and Root Disease in Subterranean Clover

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.

Critical factors driving aphanomyces damping‐off and root disease in clover revealed and explained using linear and generalized linear models and boosted regression trees

“…The outcomes from the study by You et al . () and the current study are widely applicable across soilborne oomycete pathogens of forage legumes. These studies contrast with previous investigations of other pathogens, such as P. clandestina and R. solani , where only analyses of variance were used in an attempt to decipher these complex relationships (You & Barbetti, ,).…”

“…To the authors’ knowledge, this is the first study to use modelling approaches to reveal the complex nature of how fluctuating soil temperature, moisture and nutrition conditions, and soil type and variety, determine aphanomyces damping‐off and root disease severity in subterranean clover, as well as the resultant adverse impacts on productivity of subterranean clover forages and their long term persistence. Such an approach for soilborne forage legume diseases has only been used once previously for a different forage legume pathogen, P. irregulare (You et al ., ). In both that study and the current study, linear and generalized linear models and BRTs proved ideal for apportioning and quantifying the different environmental influences on root disease, damping‐off and on plant productivity and interactions.…”

“…There have been a number of attempts to relate environmental factors to the severity of root disease in subterranean clover involving its other oomycete pathogens such as P. clandestina (Wong et al ., ; You & Barbetti, ) and P. irregulare , both historically (Wong et al ., ) and, recently, in a more comprehensive way (You et al ., ). However, little is known about environmental influences for A. trifolii except that it is most damaging in areas that receive relatively high annual rainfall (>700 mm per annum), particularly where forages are subject to brief periods of transient waterlogging (O'Rourke et al ., ).…”

“…The conditions used were representative of the variable environmental conditions across southern Australian subterranean clover forages. Statistical modelling, using linear and generalized linear models and boosted regression trees (BRTs), has recently been shown to be the best approach towards highlighting and explaining the role of different environmental conditions on forage legume soilborne disease caused by P. irregulare (You et al ., ). Therefore, similar statistical modelling was used to test the results of the present investigation and elucidate the complex interactions found.…”

“…The important oomycete soilborne pathogens on subterranean clover include Aphanomyces trifolii , various Pythium species, particularly Pythium irregulare , Phytophthora clandestina and Rhizoctonia solani (Wong et al ., ,; Barbetti et al ., ,, , ; You et al ., , ; Ma et al ., ; O'Rourke et al ., ; Nichols et al ., ; You & Barbetti, ). Recently, Foster et al .…”