Epidemiology of lentil rust in Ethiopia with special reference to disease progress and yield loss assessment

English: Rust [Uromyces viciae-fabae (Schroet.)] is one of the major diseases of lentil (Lens culinaris Medik.) in the world causing major crop losses when conditions are conducive for disease development. Effective lentil rust management depends on knowledge of, e.g., disease - yield relationships, dynamics of rust epidemics, accurate phenotyping of resistance, and components of resistance. Sequential analyses of lentil rust epidemics were performed by assessing crop and disease parameters in five field rust epidemics. The five epidemics produced similarshaped disease progress curves that varied significantly in the rate of temporal progression (rL) and area under the disease progress curve (AUDPC). The epidemics did not affect crop growth; however, they had significant influences on seed yield, pods per plant and seed mass at crop growth stages between early flowering and early pod formation. The rust also significantly affected harvest index and days to maturity. Results demonstrated that a seed yield loss of up to 41.7% could occur and the effect of rust on seed yield of lentil can be predicted with AUDPC and critical-point models using disease severity assessed on the upper canopy layer in the early flowering stage. Every 1% increase in rust severity reduced seed yield by 8.39%. Rust severity ≥ 4.7% at the critical stage will significantly reduce seed yield. To assess the effect of rust on the value of infected lentil straw as animal feed, a trial was conducted to evaluate its degradability in rumens of Zebu cows. Following 6 h of incubation, samples of all treatments lost ≥ 40% of the total dry matter, degradation reaching ca. 65% at 72 h. Rust did not decrease degradability of dry matter in the rumen. The nitrogen (N), phosphorus (P) and the total crude protein (TCP) content in rust infected straw was higher than those of healthy straw. Seeds from healthy plants contained more P than the seeds from rusted plants. The rust had no effect on seed N content. Inoculation and incubation techniques resulting in uniform spore deposition and infection, thereby obtaining reproducible and accurate data on host responses, are required for quantitative studies. To achieve this, a spore-settling tower was developed, and uniformity of spore deposition was assessed by dispersing 1, 2, 4 and 8 mg urediniospores of U. viciae-fabae into the tower. Uniform spore deposition was obtained when 8 mg spore quantity was discharged into the tower and allowed a settling period of 3 min. A linear relation was found between mass of spores discharged into the tower and spores deposited/cm2. Uniformity of spore deposition increased as the spore amount discharged increased. In experiments quantifying the effects of temperature on germination of U. viciaefabae, a high percentage (≥ 80%) of spore germination was observed after 3 h of incubation on 1.5% water agar at 10, 15, 20 and 25°C, with an optimum (99%) at 20°C. At this sampling time the length of germ tubes ranged from 66 μm (10°C) to 196 μm (20°C). For minimum infection of lentil cultivar EL-142 at 20°C, a dew period of at least 3 h was required, whereas maximum infection occurred with a dew period of 24 h. Components of resistance to U. viciae-fabae, namely, latent period, infection efficiency, pustule size and spore production were evaluated in the lentil cultivars Gudo, R-186, FLIP-87-66L and FLIP-89-60L and EL-142 (susceptible check) in a glasshouse. The cultivars were also compared for area under the disease progress curve (AUDPC), area under the pustule density curve (APDC), apparent infection rate (rG), and disease severity under field conditions. Gudo and R-186 had significantly smaller and fewer pustules, lower spore yield and longer latent period than EL-142. FLIP87-66L was intermediate for infection efficiency and pustule size. In addition, significant differences were found between cultivars for AUDPC, APDC, disease severity and rG. Estimates of AUDPC, APDC, disease severity and rG were reduced in Gudo, R-186 and FLIP-87-66L compared with the susceptible check EL-142. FLIP-89-60L also showed low AUDPC, APDC and disease severity. Some of the components obtained in the field were significantly correlated with each other and those measured in the glasshouse. Most of the components studied in the glasshouse were correlated with AUDPC and disease severity. Data indicated the existence of incomplete (partial) resistance in the test cultivars, and the possibility of using infection efficiency, latent period, spore production and pustule size as selection criteria in the evaluation of partial resistance to rust in lentil. Since there was an interdependence of the components, selection based on more than one component helps obtain lines with higher levels of partial resistance. The AUDPC, disease severity and rG could also be used for selecting lines with partial resistance in the field. Histological studies showed that the resistance mechanism in the lentil cv. Gudo against U. viciae-fabae is a combination of hypersensitive and pre-penetration types. Furthermore, cv. Gudo had a higher percentage of early aborted colonies than the susceptible cultivar.