Abstract
Didymella pinodes is the principal causal agent of ascochyta blight, one of the most important fungal diseases of pea (Pisum sativum) worldwide. Understanding its host specificity has crucial implications in epidemiology and management; however, this has not been clearly delineated yet. In this study we attempt to clarify the host range of D. pinodes and to compare it with that of other close Didymella spp. D. pinodes was very virulent on pea accessions, although differences in virulence were identified among isolates. On the contrary, studied isolates of D. fabae, D. rabiei, and D. lentil showed a reduced ability to infect pea not causing macroscopically visible symptoms on any of the pea accessions tested. D. pinodes isolates were also infective to some extend on almost all species tested including species such as Hedysarum coronarium, Lathyrus sativus, Lupinus albus, Medicago spp., Trifolium spp., Trigonella foenum-graecum, and Vicia articulata which were not mentioned before as hosts of D. pinodes. On the contrary, D. lentil and D. rabiei were more specific, infecting only lentil and chickpea, respectively. D. fabae was intermediate, infecting mainly faba bean, but also slightly other species such as Glycine max, Phaseolus vulgaris, Trifolium spp., Vicia sativa, and V. articulata. DNA sequence analysis of the nuclear ribosomal internal transcribed spacer region (ITS) was performed to confirm identity of the isolates studies and to determine phylogenetic relationship among the Didymella species, revealing the presence of two clearly distinct clades. Clade one was represented by two supported subclusters including D. fabae isolates as well as D. rabiei with D. lentil isolates. Clade two was the largest and included all the D. pinodes isolates as well as Phoma medicaginis var. pinodella. Genetic distance between D. pinodes and the other Didymella spp. isolates was not correlated with overall differences in pathogenicity. Based on evidences presented here, D. pinodes is not specialized on pea and its host range is larger than that of D. fabae, D. lentil, and D. rabiei. This has relevant implications in epidemiology and control as these species might act as alternative hosts for D. pinodes.
Highlights
Cool season legumes play an important role in farming systems worldwide (Siddique et al, 2012)
The highest levels of susceptibility were found in P. sativum (DR = 5; Disease severity (DS) = 67%) confirming expectations (Fondevilla et al, 2005), followed by L. albus (DR = 4.7; DS > 20%), Trifolium spp., Medicago spp., V. articulata, C. arietinum, and L. culinaris (2 < disease rating (DR) < 3; DS > 15%)
Some infection was observed on G. max, V. sativa, L. sativus, H. coronarium, P. fulvum, S. muricatus, V. faba, and T. foenum-graecum at the level of resistance (DR < 2; DS < 20%)
Summary
Cool season legumes play an important role in farming systems worldwide (Siddique et al, 2012). Yield losses caused by aschochyta blight are in order of 40% in lentil (Gossen and Derksen, 2003), but in severe cases losses higher than 90% have been reported in faba bean (Omri Benyoussef et al, 2012) and chickpea (Pande et al, 2005). In pea, this disease is caused by a complex of fungi formed by Ascochyta pisi Lib., Didymella pinodes (Berk and Blox) Petrak, Phoma medicaginis var. Jones) is the most predominant and damaging pathogen and under some conditions can cause yield losses up to 70% (Tivoli and Banniza, 2007)
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