Host status and reaction of <Emphasis Type="Italic">Medicago truncatula</Emphasis> accessions to infection by three major pathogens of pea (<Emphasis Type="Italic">Pisum sativum</Emphasis>) and alfalfa (<Emphasis Type="Italic">Medicago sativa</Emphasis>)

Ditylenchus dipsaci, the stem nematode of alfalfa (Medicago sativa), Mycosphaerella pinodes, cause of Ascochyta blight in pea (Pisum sativum) and Aphanomyces euteiches, cause of pea root rot, result in major yield losses in French alfalfa and pea crops. These diseases are difficult to control and the partial resistances currently available are not effective enough. Medicago truncatula, the barrel medic, is the legume model for genetic studies, which should lead to the identification and characterization of new resistance genes for pathogens. We evaluated a collection of 34 accessions of M. truncatula and nine accessions from three other species (two from M. italica, six from M. littoralis and one from M. polymorpha) for resistance to these three major diseases. We developed screening tests, including standard host references, for each pathogen. Most of the accessions tested were resistant to D. dipsaci, with only three accessions classified as susceptible. A very high level of resistance to M. pinodes was observed among the accessions, none of which was susceptible to this pathogen. Conversely, a high level of variation, from resistant to susceptible accessions, was identified in response to infection by A. euteiches.     

Controlled environment assessment of partial resistance to Mycosphaerella pinodes in pea (Pisum sativum L.) seedlings

We evaluated the effect of various genotypes of pea (Pisum sativum L.) to delay the development of ascochyta blight, caused by the fungus Mycosphaerella pinodes, at the seedling stage under controlled conditions. We assessed leaflets, stipules and stems, separately. We compared genotypes on the basis of four variables assessing resistance: incubation period (IP), appearance of disease scoring 3 (DS3), disease progress rate (r_d) and area under the disease progress curve (AUDPC).For leaves, a continuous variation was observed between pea lines in disease severity at all times and disease progress curves were similar for the various genotypes. Partial resistance was defined as a delay in first symptom appearance and disease development. Resistance factors are thought to be involved in the very early stages of the interaction. Similar results were obtained for stipules and leaflets, suggesting similar mechanisms of resistance in these two organs. Stem and leaf resistances were also strongly correlated. We found that IP could discriminate genotypes but that factors associated with disease progress (DS3, AUDPC and r_d) were more informative. We suggest that further genetic analyses should be based on growth-chamber methods, with AUDPC as the variable measured. This revised version was published online in August 2006 with corrections to the Cover Date.     

Cultural, molecular and pathogenic variability of Mycosphaerella pinodes and Phoma medicaginis var. pinodella isolates from dried pea (Pisum sativum) in France

The characteristics of 50 isolates of Mycosphaerella pinodes and 17 isolates of Phoma medicaginis var. pinodella , originating from several regions of France where ascochyta blight is prevalent, were investigated using cultural, physiological, molecular and pathogenicity analyses. M. pinodes was distinguished from P. medicaginis var. pinodella on the basis of presence of pseudothecia, a higher proportion of larger, bicellular conidia, compared with the smaller, predominantly unicellular conidia of P. medicaginis var. pinodella , and a slower linear growth rate on agar under a 12-h light regime. RAPD analysis clearly distinguished the two species, which had low intraspecific variability. Although both species gave identical symptoms, they could be distinguished by their incubation period and aggressiveness, respectively, shorter and higher for M. pinodes . Virulence tests gave no definitive evidence for the existence of pathotypes among the M. pinodes isolates. Two unidentified isolates had similar characters to both M. pinodes and P. medicaginis var. pinodella in some features but were distinguished from them by their RAPD patterns.     

Biotic factors affecting the expression of partial resistance in pea to ascochyta blight in a detached stipule assay

The expression of partial resistance in pea to ascochyta blight (caused by Mycosphaerella pinodes) was studied in a detached stipule assay by quantifying two resistance components (fleck coalescence and lesion expansion) using the method of point inoculation of stipules. Factors determining optimal conditions for the observation of partial resistance are spore concentration, the age of the fungal culture prior to spore harvest and the pathogenicity of the isolate used for testing. Partial resistance was not expressed when spore concentration was high or when the selected isolate was aggressive. Furthermore, assessments of components of partial resistance were highly correlated with disease severity in a seedling test. A screening protocol was developed based on inoculations of detached stipules to study partial resistance in pea. To simplify the rating process, a more comprehensive disease rating scale which took into account fleck coalescence and lesion expansion was tested by screening a large number of genotypes.