Host resistances to Aphanomyces trifolii root rot of subterranean clover: first opportunity to successfully manage this severe pasture disease

Abstract

Subterranean clover (Trifolium subterraneum) is an important pasture legume in Australia (29 million ha) and elsewhere. However, severe pasture decline occurs in association with several root pathogens, including Aphanomyces trifolii, that has been misidentified for decades as A. euteiches until recently confirmed as A. trifolii. A series of controlled environment experiments was undertaken to identify host resistance to A. trifolii in subterranean clover and to compare virulence and phylogeny of isolates. In experiment 1, Dalkeith, Bacchus Marsh, Riverina and Yarloop were the most resistant of 38 cultivars with a percentage disease index (PDI) ≤10 for both tap and lateral roots. Experiment 2 confirmed resistance of Yarloop, but a change in some relative varietal resistances suggested physiological specialization among A. trifolii isolates. Experiment 3 confirmed extensive variation in virulence and physiological specialization across 23 isolates of A. trifolii, with three distinct clades, two of which were distinct from isolates collected previously. Experiment 4 identified host resistance(s) effective against a mixture of 20 A. trifolii isolates, but the most resistant cultivars (Antas, Uniwager, Leura) still showed significant disease. This is the first study to show physiological specialization in A. trifolii and to identify host resistance. This study defines A. trifolii as a significant but largely unknown contributor to severe root disease of subterranean clover in southern Australia. Finally, development and calibration of a new soil commercial DNA test not only enables field quantification of the disease, but development of appropriate breeding, selection and farm management strategies to reduce its impact.

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

“…That heat maps confirmed least disease severity under warmer temperatures is significant. Other studies have also shown less root disease from A. trifolii at 22/17 °C, day/night (You et al ., ) and at 25/20 °C, day/night (O'Rourke et al ., ). It is possible that at warmer temperatures more rapid root growth assists plants to both ‘escape’ and/or compensate for already diseased roots, and this is in line with Wong et al .…”

“…50% for A. trifolii in the same field samples (You et al ., ). The highest level of A. trifolii DNA was at a high rainfall site where subterranean clover forages had to be resown every 3–4 years due to collapse and decline from severe root disease (You et al ., ). For another oomycete pathogen, P. clandestina , You & Barbetti () also showed significant interactions across similar multiple explanatory variables in terms of pre‐emergence damping‐off.…”

“…Isolation details are as provided in You et al . (). Aphanomyces trifolii cultures were then subcultured onto fresh cornmeal agar (CMA) and grown for 1 week in preparation for inoculum production as described below.…”

“…Varieties Riverina, Seaton Park and Woogenellup were used. While Riverina does show resistance to one or more specific individual isolates of A. trifolii , it is susceptible to a mixture of isolates that is more representative of the physiological variation existing in the pathogen population (You et al ., ). In contrast, both Woogenellup and Seaton Park varieties were highly susceptible to aphanomyces damping‐off and root disease in the same experiments (You et al ., ).…”

Plant genotype and temperature impact simultaneous biotic and abiotic stress‐related gene expression in Pythium‐infected plants

“…Subterranean clover is attacked by various foliar (Barbetti and You, ; Nichols et al ., ) and, in particular, soilborne pathogens (Barbetti et al , ; ; ; Nichols et al , ). Soilborne oomycete pathogens like Pythium irregulare , Phytophthora clandestina , and Aphanomyces trifolii , together with the fungal pathogen Rhizoctonia solani , have been identified as responsible for most of the severe damping‐off and root disease observed (Barbetti et al , ; ; O'Rourke et al , ; You et al , ). Despite being widespread, subterranean clover root diseases have historically been largely unrecognized by farmers as the primary cause of deterioration in forage legume pastures (Barbetti et al , ; Foster et al , ).…”