Pression d’inoculum de la maladie des raies noires (MRN) causée par Mycosphaerella fijiensis Morelet, dans les plantations industrielles de bananiers, en Côte d’ivoire

The Black Leaf Streak Disease (BLSD) is one of main parasitic constraints of the agro-industrial plantations of banana in the world. Caused by a fungus ascomycete (Mycosphaerella fijiensis Morelet), the losses of yield are up to 50 %, without of resistant cultivars and effective management. This study relative to the level of the disease has been conducted in 12 industrial plantation in 6 production area: Aboisso, Grand- Bassam, Dabou, Agboville, Azaguié and Tiassalé; for a total surface of 2576 ha. The observations concerned the ranks of Youngest Leaf Spotted (YLS), of the Youngest Leaf Necrosed (YLN), the Number of Standing Leaves (NSL) and the gap between the Youngest Leaf Spotted and the Number of Standing Leaves. The results showed that chronic re-infestation of industrial plantation during the last five years, except CDBCI Tiassalé-Songon in 2011 and 2012. An acceptable level of re-infestation was observed in 2012 on other plantations, with the rank of YLS greater than or equal to 3, except BATIA Tiassalé and CDBCI Azaguié. The internal inoculum pressure (YLS) varied a lot from one plantation to another and from one year to another. Except plantation SBMK Aboisso and BATIA Tiassalé, other plantations had at least one value of the rank of the top YLS or equal to 8, during the study indicating a relatively low severity of the BLSD. From 2009 to 2012, all plantations have shown over 8 standing leaves, the minimum required for proper filling of the bunch of banana regime at the time of flowering

Historical Contingencies Modulate the Adaptability of Rice Yellow Mottle Virus

The rymv1-2 and rymv1-3 alleles of the RYMV1 resistance to Rice yellow mottle virus (RYMV), coded by an eIF(iso)4G1 gene, occur in a few cultivars of the Asiatic (Oryza sativa) and African (O. glaberrima) rice species, respectively. The most salient feature of the resistance breaking (RB) process is the converse genetic barrier to rymv1-2 and rymv1-3 resistance breakdown. This specificity is modulated by the amino acid (glutamic acid vs. threonine) at codon 49 of the Viral Protein genome-linked (VPg), a position which is adjacent to the virulence codons 48 and 52. Isolates with a glutamic acid (E) do not overcome rymv1-3 whereas those with a threonine (T) rarely overcome rymv1-2. We found that isolates with T49 had a strong selective advantage over isolates with E49 in O. glaberrima susceptible cultivars. This explains the fixation of the mutation T49 during RYMV evolution and accounts for the diversifying selection estimated at codon 49. Better adapted to O. glaberrima, isolates with T49 are also more prone than isolates with E49 to fix rymv1-3 RB mutations at codon 52 in resistant O. glaberrima cultivars. However, subsequent genetic constraints impaired the ability of isolates with T49 to fix rymv1-2 RB mutations at codons 48 and 52 in resistant O. sativa cultivars. The origin and role of the amino acid at codon 49 of the VPg exemplifies the importance of historical contingencies in the ability of RYMV to overcome RYMV1 resistance