Studies On Genetic Variability Of Stylosanthes Accessions Resistant Or Susceptible To The Stylo Anthracnose

The genus Stylosanthes contains over 44 species and subspecies, which are naturally distributed in South and Central America, tropical North America, Africa, and Southeast Asia, including India. Many of them are well adapted to the drier and less fertile areas between 30癗 and 30癝, where the average growing season is 63 to 190 d long. In 1962, Stylosanthes was firstly introduced to Hainan of China as a cover crop in rubber tree plantation from Malaysia. Many species are promising tropical legumes and used as pasture for grazing cattle and for making leaf meal for livestock. They are also used as cover crops in fruit-tree plantations, and for improving soil fertility.Anthracnose, caused by the fungus Colletotrichum gloeosporioides (Penz.) Sacc, is a major constraint to the extensive use of S. guianensis as tropical forage. Stylosanthes dry matter loss, with consequent reduced nutritive value, was reported in Australia, Colombia and Southern China because of stylo anthracnose. The development of anthracnose resistant varieties is the major method of stylo anthracnose control. The genetic resources available for Stylosanthes improvement include improved breeding lines and landraces. Currently, more than 2961 accessions are available in world 5. guianenesis collection held at CIAT (Centra Internacional de Agriculture Tropical). Information about the structure of these gene pools is important in the development of sound strategies for 5. guianensis germplasm conservation and utility. Resistance has been found in 5. guianensisboth in South America and China. To date, the genes involved in the resistance response have not been identified. Genetic studies showed that isolates of C. gheosporioides originated from S. guianensis exhibited genetic and pathogenic diversity in South America, Australia, and China. Up to date, no clear interactions between stylo cultivars and strains have been established. In these studies, the aims are to: (1) describe the distribution of genetic variability within a group of accessions representing the genetic diversity available in S. guianensis germplasm; (2) assess their resistance or susceptibility to anthracnose in order to determine if resistance is general or specific; and (3) help in identifying accessions that could be selected for improvement programs and in studies of host-pathogen interaction. The main results are shown as following:1. Disease responses of 45 Stylosanthes germplasms to two strains of Collectotrichum gheosporioides (Penz.) Sacc. CATAS292 (Type A) and CATAS100 (Type B) isolated from China were presented. Plants inoculation and their detached leaves were tested in the experiment. The results are shown as that 37.8% of all Stylosanthes germplasms were resistant to CATAS292, 31.1% to CATAS100, and 64.4% were susceptible to both strains. Some of Stylosanthes plants were resistant to Collectotrichum gheosporioides while their detached leaves were susceptible. There were differences between the resistant and the susceptible Stylosanthes germplasms in average number of infected and defoliated leaves per plant, average number of sclerotia per plant, and average upward progress of disease per plant.2. The RAPD reaction system for Stylosanthes was developed after optimizing the concentrations of Mg2+, dNTPs, Taq DNA polymerase and template DNA. RAPD analysis for 45 accessions of S. guianensis was performed using twenty primers screened from 200 arbitrary oligonucleotides, and a 71.5% level of polymorphism was found. Based on the RAPD data, the dendrogram was constructed with unweighted pair group method of averages (UPGMA) and showed genetic similarity from 50% to 94% among all stylo accessions, and fourteenclusters were defined at 66.5% genetic similarity. SG907-1, a specific DNA fragment containing 837 bp presenting in anthracnose resistant accession 907, while absent in susceptible accession CIAT184, was cloned. The largest open reading frame (ORF) of this fragment is 325 bp in size and encodes a peptide of 114 amino acid residuces. This peptide shows 63.1%...