Characterization of Pyrenophora tritici-repentis in Tunisia and Comparison with a Global Pathogen Population

Abstract

Pyrenophora tritici-repentis (Ptr) causes tan spot, an important foliar disease of wheat. A collection of Ptr isolates from Tunisia, located in one of the main secondary centers of diversification of durum wheat, was tested for phenotypic race classification based on virulence on a host differential set, and for the presence of the necrotrophic effector (NE) genes ToxA, ToxB , and toxb by PCR analysis. While races 2, 4, 5, 6, 7, and 8 were identified according to their virulence phenotypes, PCR testing indicated the presence of ‘atypical’ isolates that induced necrosis on the wheat differential ‘Glenlea’, but lacked the expected ToxA gene, suggesting the involvement of other NEs in the Ptr/wheat interaction. Genetic diversity and the Ptr population structure were explored further by examining 59 Tunisian isolates and 35 isolates from Algeria, Azerbaijan, Canada, Iran, and Syria using 24 simple sequence repeat markers. Average genetic diversity, overall gene flow and percentage polymorphic loci were estimated as 0.58, 2.09 and 87%, respectively. Analysis of molecular variance showed that 81% of the genetic variance occurred within populations and 19% between populations. Cluster analysis by the unweighted pair group method indicated that ToxB- isolates grouped together and were distantly related to ToxB+ isolates. Based on Nei’s analysis, the global collection clustered into two distinct groups according to their region of origin. The results suggest that both geographic origin and the host-specificity imposed by different NEs can lead to differentiation among Ptr populations.

Characterization of Mediterranean Durum Wheat for Resistance to Pyrenophora tritici-repentis

“…As a part of the evaluation process, infected wheat leaves and debris were collected and analyzed morphologically to confirm the presence of Ptr . The identification of typical conidia of Ptr from collected leaf samples confirmed the infection with tan spot ( Figure S3 ), and several obtained isolates have been published in Laribi et al [ 24 , 25 ]. In addition, the collected debris contained spherical black pseudothecia visible to naked eyes.…”

“…Recent studies on Pyrenophora tritici-repentis in Tunisia revealed high phenotypic and genotypic diversity of Ptr in Tunisia. Hence, phenotyping wheat populations from the Mediterranean region under favorable conditions can lead to the discrimination between resistant and susceptible accessions [ 14 , 19 , 24 , 25 ]. In this study, 549 wheat accessions that comprised mainly landraces from five Mediterranean countries were assessed for resistance/susceptibility to TS disease in Tunisia over two cropping seasons at the adult stage.…”

“…This disease has been reported in all major wheat-growing regions, including North and South America, North and South Asia, Europe, and North Africa [ 19 , 20 ]. Tan spot is increasingly gaining importance in the Mediterranean region and has been reported in Algeria, Morocco, and Tunisia [ 11 , 21 , 22 , 23 , 24 , 25 ]. To date, Ptr is known to produce three well characterized Necrotrophic effectors (NEs)—one NE inducing necrosis termed Ptr ToxA and two NEs inducing chlorosis termed Ptr ToxB and Ptr ToxC [ 26 , 27 , 28 ].…”

“…In Tunisia, tan spot disease is an emerging threat to wheat production [ 11 , 14 ]. Although no studies were carried out to assess the extent of yield losses, recent studies about Ptr in Tunisia revealed a high morphological, phenotypic, and genotypic diversity [ 19 , 24 , 25 , 39 ]. Six races, namely, 2, 4, 5, 6, 7, and 8, were identified in addition to atypical isolates that were able to cause necrosis on the differential line ‘Glenlea’ but lacked the expected ToxA gene [ 19 , 24 , 25 ].…”

“…Although no studies were carried out to assess the extent of yield losses, recent studies about Ptr in Tunisia revealed a high morphological, phenotypic, and genotypic diversity [ 19 , 24 , 25 , 39 ]. Six races, namely, 2, 4, 5, 6, 7, and 8, were identified in addition to atypical isolates that were able to cause necrosis on the differential line ‘Glenlea’ but lacked the expected ToxA gene [ 19 , 24 , 25 ]. The homothallic nature of the pathogen, its virulence and the wide adoption of conservation practices, as well as the wide cultivation of susceptible cultivars, contributed to the high genetic diversity of the pathogen [ 11 , 25 ].…”