Triticale powdery mildew: population characterization and wheat gene efficiency.

Powdery mildew has emerged on triticale in the early 2000s in many locations, probably due to a host range expansion of the wheat formae speciales, Blumeria graminis f.sp. tritici. Many triticale cultivars are highly susceptible to powdery mildew, mainly in seedling stage, revealing a probably narrow genetic basis for powdery mildew resistance genes (Pm). Moreover, as Blumeria graminis is an obligate biotrophic fungus, it is very time consuming and difficult to maintain powdery mildew isolates for a non-specialized laboratory and populations can evolve. In order to identify wheat Pm genes efficient against natural populations of powdery mildew, wheat differential hosts and triticale seedlings were inoculated below susceptible triticale crop naturally contaminated by mildew, in several locations and several years. Symptoms on seedlings were measured after approximately two weeks of incubation in favorable fungus growth conditions. According to these data, we classified the Pm genes presents in our wheat differential hosts set in 3 classes: Pm already overcame by triticale powdery mildew, Pm having variable effects and Pm still efficient against triticale mildew. Data on triticale seedlings allowed us to identify some few triticale cultivars resistant to Blumeria graminis in seedling stage. We will try to identify Pm genes present in those cultivars next year by testing them with the characterized isolates of powdery mildew from Gent University. Nevertheless, interspecific crossing of wheat, resistant to powdery mildew in seedling stage, and rye have been initiated to introduce potentially interesting genes for resistance in triticale.

A single-stranded conformational polymorphism (SSCP)-derived quantitative variable to monitor the virulence of a Barley yellow dwarf virus-PAV (BYDV-PAV) isolate during adaptation to the TC14 resistant wheat line.

A standardized single-stranded conformational polymorphism (SSCP) procedure is proposed as an alternative to the time-consuming biological characterization of Barley yellow dwarf virus-PAV (BYDV-PAV) isolates. Using this procedure, six of 21 overlapping regions used to scan the viral genome gave patterns specific to '4E' (avirulent) or '4T' ('4E'-derived virulent) isolates. The calibration of samples and integration of SSCP patterns corresponding to the nucleotide region 1482-2023 allowed the estimation of P(T) values that reflect the proportions of a '4T'-specific band. Analysis of the biological (area under the pathogen progress curve) and molecular (P(T)) data suggested a positive linear relation between these variables. Moreover, sequence analysis of the nucleotide region 1482-2023 highlighted the presence of a nucleotide polymorphism (C/A(1835)) which can be considered as a candidate for virus-host interactions linked to the monitored virulence. According to these parameters, P(T) values associated with '4E'- and '4T'-derived populations show that: (i) long-term infection of a BYDV-PAV isolate on the 'TC14' resistant host leads to the fixation of virulent individuals in viral populations; and (ii) the introduction of susceptible hosts in successive 'TC14' infections results in the maintenance of low virulence of the populations. Thus, the presented study demonstrates that SSCP is a useful tool for monitoring viral populations during the host adaptation process. The described impact of host alternation provides new opportunities for the use of the 'TC14' resistance source in BYDV-resistant breeding programmes. This study is part of the global effort made by the scientific community to propose sustainable alternatives to the chemical control of this viral disease.