Ergot resistance in sorghum in relation to flowering, inoculation technique and disease development

Abstract

Ergot is an important disease of sorghum (Sorghum bicolor) in parts of Africa and Asia. Studies were conducted to determine the relationship between flowering biology and ergot infection, and to develop an artificial field‐screening technique to identify ergot resistance in sorghum. Spikelets resisted infection after anthesis, but each day's delay in anthesis after inoculation supported 8‐3% more ergot. The screening technique consisted of three components: trimming of panicles to remove pollinated spikelets before inoculation, a single inoculation of trimmed panicles, and panicle bagging for 7‐10 days. Inoculated panicles were evaluated by a qualitative visual rating method (on a 1‐5 scale) and a quantitative spikelet counting method. Selected accessions from the world collection of sorghum germplasm were screened at Karama Research Station, Rwanda, for two seasons and 12 ergot‐resistant lines were identified. These were also resistant at ICRISAT Centre, India.

Inoculation Techniques for Identifying Resistance in Sorghum Genotypes to Sorghum Ergot

“…In previous studies, ergot severity assessments were generally conducted between 20 and 40 DAI (McLaren, 1992;Tegegne et al, 1994;Musabyimana et al, 1995). This study has shown that ergot severity per panicle generally increased from 10 to 15 DAI, and thereafter, only negligible or no increases in the number of ergot-infected florets were observed (Figures 2, 3, and 4).…”

“…A single spray or sponge inoculation before flower initiation, followed by bagging of the panicles for 7 days, was the most effective in identifying genotypes that may possess genes for ergot resistance. Similarly, research conducted in Africa and Asia by Tegegne et al (1994) and Musabyimana et al (1995) showed that a single inoculation at the beginning of anthesis, followed by bagging, was most suitable in screening sorghum for resistance to ergot in those environments.…”

“…This technique tended to obviate the influence of flowering characteristics by ensuring that the non-pollinated stigmas are available for infection. Bagging of the inoculated panicle increases the humidity and thereby enhances the ability of the pathogen to cause infection, exclude external pollen (Musabyimana et al, 1995), and extend the dew period around the panicles. When this technique is use, disease assessments can be completed within 15 or 20 DAI.…”

“…Inoculation techniques previously used by other workers include: single or multiple spray inoculation at (1) 10% pollen shed in male-fertile; or stigma emergence in male-sterile non-bagged panicles, (2) anthesis only when the tip of the panicles or 25% or more spikelets had completed flowering followed by bagging for 7-10 days (McLaren, 1992;Tegegne et al, 1994;Musabyimana et al, 1995). Musabyimana et al (1995) also adopted the dipping method, in which panicles are immersed in a spore suspension. This method is feasible when dealing with tall sorghum genotypes but will be difficult to use with the dwarf grain sorghum breeding lines and hybrids planted in the US.…”

Synchronicity of pollination and inoculation with Claviceps africana and its effects on pollen–pistil compatibility and seed production in sorghum

“…Hence, synchronous flowering among parental lines, deposition time of pollen and conidia on the stigma, and the relative growth rates of pollen tubes and conidia determines whether the ovary will be fertilized by the pollen nuclei or colonized by the fungus (Ryley et al 2002;Mendoza-Onofre et al 2006). Environmental factors may also predispose fertile sorghum genotypes to infection by decreasing pollen viability (Musabyimana et al 1995;McLaren 1997) or compatibility between pollen and the stigma (Moran et al 2002).…”