An integrated multivariate approach to net blotch of barley: Virulence quantification, pathotyping and a breeding strategy for disease resistance

Abstract

Knowledge of pathotype diversity and virulence in local populations of Pyrenophora teres is a prerequisite to screening for durable resistance to net blotch. The current study aimed to quantify the virulence level of Moroccan isolates, identify and designate existing pathotypes, and select resistant genotypes. We developed a method for virulence quantification of P. teres isolates based on a conversion of infection responses into frequencies for use in correspondence analysis. Coordinates of the first axis of this analysis had a virulence spectrum and ranked isolates from virulent to avirulent. Mixed model analysis was also devised for virulence quantification. Coordinates of the first dimension of correspondence analysis were linearly correlated to BLUPs (Best Linear Unbiased Predictors) of the mixed model. A genotype by genotype by environment model (GGE) coupled with cluster analysis differentiated P. teres isolates into ten and nine pathotypes for net-and spotforms respectively. Populations of these two forms were dissimilar in terms of classes of virulence. For P. teres f. maculata, avirulent, moderately virulent and highly virulent isolates represented one-third of the population, whereas 90% of P. teres f. teres population was composed of avirulent to moderately avirulent isolates. Barley differential sets were subsequently reduced to two new sets that simplified pathotyping through a key code based on resistant or susceptible reactions. Dendrograms of cluster analysis based on GGE analysis depicted the stability of a genotype's reactions across all isolates, and using only resistant cultivars as sources of resistance to control net blotch disease would, based on this analysis, fail to control all pathotypes. Therefore, we propose an alternative breeding strategy to control net blotch effectively.

Virulence of MoroccanPyrenophora teresf.teresRevealed by International Differential Barley Genotypes

“…In contrast, in our study we recorded 5.8 IR for Rabat071. Similarly, Rabat071 was reported to be resistant with a mean IR of 3.4 when challenged with 61 Moroccan Ptt isolates collected during the field survey of (Jebbouj and El Yousfi 2010. In our study, however, Rabat071 was one of the most susceptible genotype (susceptible to 11 out of 15 Ptt isolates).…”

“…Very recently, Bentata (2015) reported highly virulent Ptt isolates to Rabat071 and argued that dominance of these isolates in Moroccan Ptt population might lead to an epidemic of net blotch. In contrast to Jebbouj and El Yousfi (2010), during the disease survey of 2015 in Morocco, Rehman and El Yousfi (unpublished data) found that ~60% of Moroccan farmers grew Rabat071 in survey area and the disease severity of Ptt on Rabat071 was recorded between 35-75%. According to McDonald and Linde (2002), pathogens with mixed reproduction system (pathogen with both sexual and asexual Cereal Research Communications 44, 2016 cycles per growing season) pose greater threat to erosion of resistance in host.…”

“…Resistance to the majority of Ptt isolates is still lacking in the North Africa (Harrabi and Kamel 1990;Douiyssi et al 1998;Yousfi and Ezzahiri 2002;Manninen et al 2006;Jebbouj and El Yousfi 2010;Bouajila et al 2012). Harrabi and Kamel (1990) reported that none of the ten barley genotypes used in their study were resistant to 33 Ptt isolates, which originated from the Mediterranean region, though they reported four major virulence groups based on virulence spectrum of these isolates.…”

“…Seed of the international differential set was obtained from Genetic Resource Unit of ICARDA. In addition, four widely grown Moroccan barley varieties (Aglou, Arig8, Rabat071, and Taffa) were also included in the current study (Jebbouj and El Yousfi 2010 otype, at least 10 seeds were sown in 10 cm 2 plastic pots containing a soil mixture of peat (1/3) and natural soil (2/3). Plants were grown in greenhouse till three leaves were fully developed at temperature regime of 20-22 °C and photoperiod of alternating 12/12 h of light/dark period.…”

“…Helminthosporium teres) causing the net form of net blotch (NFNB) [P. teres f. teres] (Ptt) and spot form of net blotch (SFNB) [P. teres f. maculata] (Ptm) of barley (Rau et al 2015), are the most important factors limiting successful barley production. Net blotch has become a serious disease in the dry areas of North Africa (Jebbouj and El Yousfi 2010;Bouajila et al 2013), Europe (Tuohy et al 2006), Nordic region (Jonsson et al 1997), USA (Liu et al 2012) and Australia (Gupta and Loughman 2001). Specifically in Morocco, the net form of net blotch (Ptt) is reported as more prevalent than the spot form of net blotch (Ptm) (Harrabi and Kamel 1990;Yousfi and Ezzahiri 2001).…”