Mohamed Béchir Allagui

Molecular and Pathogenicity Characteristics of Phytophthora nicotianae Responsible for Root Necrosis and Wilting of Pepper (Capsicum annuum L.) in Tunisia

Isolates of Phytophthora from pepper, produced in Tunisia, were characterised according to molecular and pathogenicity criteria. Polymerase chain reaction amplification of the ITS1 region in the ribosomal DNA resulted in different sized fragments. The pepper isolates and P. nicotianae yielded a fragment of 310 bp that distinguished it from P. capsici with a fragment of 270 bp. The ribosomal RNA gene amplicons of both internal transcribed spacers and the 5.8 S of the pepper Phytophthora and P. nicotianae were digested with 8 endonucleases. The patterns generated, with the 2 enzymes that cut, were identical for both taxa. This molecular analysis corroborated the morphological and biological characteristics and suggests strongly that the isolates of Phytophthora from pepper belong to the species P. nicotianae. Inoculation of pepper, tomato, eggplant and tobacco plants with the isolates of P. nicotianae from pepper showed they were highly pathogenic on pepper but not on tobacco, while their pathogenicity was weak on tomato and eggplant and was associated with atypical symptoms not observed in the field. These pathogenicity tests suggest that pepper isolates of P. nicotianae are particularly adapted to their host and may thus constitute a forma specialis of P. nicotianae.

Relationship between epistasis and aggressiveness in resistance of pepper (Capsicum annuum L.) to Phytophthora nicotianae

This study evaluated the types of gene action governing the inheritance of resistance to Phytophthora nicotianae necrosis in populations derived from two crosses involving two susceptible (Beldi and Nabeul II) and one resistant (CM334) cultivars of pepper (Capsicum annuum L.). Populations, composed of Pr, Ps, F1 , F 2 , BC 1 Pr, and BC 1 Ps generations, were inoculated with six P. nicotianae isolates. Generation means analysis indicated that an additive-dominance model was appropriate for P. nicotianae isolates Pn Ko1 , Pn Ko2 and Pn Kr1 , which showed low aggressiveness in the two crosses. For the more aggressive isolates Pn Bz1 , Pn Bz2 and Pn Kr2 , epistasis was an integral component of resistance in the two crosses. The presence of epistasis in the resistance of pepper to P. nicotianae was dependent on the level of aggressiveness of the isolates. Selection in pepper with less aggressive isolates was efficient, but not with more aggressive isolates; on the other hand, selection with more aggressive isolates was more stable. The minimum number of genes controlling resistance was estimated at up to 2.71. In the majority of cases, the additive variance was significant and greater than the environmental and dominance variance.

Rhamnus lycioides in Tunisia is a new aecial host of oat crown rust

Puccinia coronata was not previously described on Rhamnus spp. in Tunisia. Three sites in the northwest of Tunisia, where Rhamnus is reported to be abundant, were surveyed for the presence of pycnia and aecia of oat crown rust caused by Puccinia coronata f. sp. avenae. Two Rhamnus species (R. lycioides and R. alaternus) were encountered in the sites. Pycnia with viable pycniospores and aecia with viable aeciospores were found on R. lycioides. However, no characteristic structures of crown rust were found on R. alaternus. Aeciospores collected from leaves of R. lycioides were used to inoculate oat plants usually susceptible to oat crown rust. Typical uredinia containing oat crown rust urediniospores appeared on the leaves of these plants. Moreover, the sixteen Pc-gene differential oat lines, used by oat researchers to study the virulence pattern in oat crown rust populations, were artificially inoculated with aeciospores from R. lycioides. These inoculated lines showed resistance/susceptibility similar to the registered resistance level of these lines to crown rust under field conditions in Tunisia. These results indicate that R. lycioides, a common and endemic part of the vegetation in the northwest of Tunisia, is a new aecial host of oat crown rust. The aeciospores produced on this forest plant could constitute the source of the virulence diversity already detected via the Pc-gene line trials.

Epistasis and maternal effect in resistance to Puccinia coronata Cda. f. sp. avenae Eriks in oats (Avena sp.).

The objective of this paper was to investigate the mode of heredity for resistance in oats (Avena sp.) to crown rust caused by Puccinia coronata Cda. f. sp. avenae Eriks. Eight generations of 2 crosses were used to estimate genetic effects and narrow-sense heritability (NSH). Separate generation means analysis (GMA) indicated a complex gene action controlling this trait with additive, dominance, epistatic and maternal effects (ME). The genetic model which assumed no epistasis and no ME did not accurately describe the resistance to P. coronata. In both crosses, the digenic epistatic model with ME was sufficient to explain variation in generation means for isolate CRPc58 and isolate CRPc46. Additive dominance and epistatic components were negative in most cases, suggesting that gene effects contributed more to the resistance than to the susceptibility. The estimated values of NSH were 15-99% depending upon the cross and isolates. The results indicated that appropriate choice of maternal parent and recurrent selection would increase resistance to crown rust in oats.

Natural population of oat crown rust in Tunisia.

Oat crown rust, caused by Puccinia coronata f. sp. avenae , is considered the most widespread and damaging disease of oat ( Avena sativa ) in Tunisia . The virulence structure of the natural oat rust population in Tunisia was studied in four areas from 2002 to 2007 using Puccinia coronata resistance genes ( Pc-genes ). The areas are located in northern Tunisia : Afareg, Bourbia, Sedjnen and Ariana. In this survey, Pc38 , Pc39 and Pc68 showed a high level of resistance to natural oat crown rust. But the most important finding in 2004 in Ariana was that the rust was virulent on Pc68 (IT ‘4’). Moreover, in 2002 Sedjnen survey, there was a high degree of virulence to Pc39 (IT ‘3’). In the other areas and other years, Pc68 and Pc39 were highly resistant to natural oat crown rust. Only Pc38 showed a stable high level of resistance to natural oat crown rust in all four areas and during the six years of the study. Areas showing a high degree of similarity were Sedjnen and Afereg (SI=4.5). Ariana and Bourbia showed little similarity to the other areas, and had the greatest dissimilarity to each other (SI=11.30). The virulence phenotypes of the P . coronata natural population in Tunisia are certainly influenced by the alternate host, Rhamnus lycioides which is abundant in the mountains of northwestern Tunisia . A combination of the Pc38 , Pc39 and Pc68 genes will provide a high level of durable protection from crown rust in Tunisia .