C. S. van Deventer

Breeding for resistance to ear rot of maize caused by Stenocarpella maydis (Berk) Sutton. 1. Evaluation of selection criteria

The objective of this study was to provide a better understanding of the interrelationship between methods of screening for resistance to ear rot, caused by Stenocarpella maydis. The inheritance of resistance as well as genetic and phenotypic correlation of various selection criteria were investigated. A complete diallel cross of 10 inbred lines, ranging from resistant to susceptible, was evaluated at each of three localities, representative of different environments within the South African maize production area. Plants were artificially inoculated twice during the vegetative stages of plant development. Resistance to ear rot was found to be controlled by additive gene effects, with low dominance and interaction effects. Significant genetic correlations existed between various characteristics relating to disease incidence, with high heritability and highly correlated responses. The incidence of rotted ears was the most practical and reliable method to select for resistance. However, the simultaneous use of some measurement of kernel infection was necessary to ensure detection of internal ear infection. An increase in the incidence of upright ears was indicated to result from S. maydis infection, rather than that upright ears predisposed ears to the disease.

A six week screening method for mechanisms of host plant resistance to Diuraphis noxia in wheat accessions

The Russian wheat aphid Diuraphis noxia (Kurdjumov) is a serious pest of wheat in South Africa. The use of D. noxia-resistant cultivars may reduce the impact of this pest on wheat production and at the same time reduce environmental risks and control costs. The mechanisms of resistance in two new sources of resistance were compared by using various methods, in order to establish a rapid and relatively accurate screening protocol. The resistant lines OSU ID 2808 and Aus 22498 were compared to the susceptible cultivars Betta and Tugela. The predominant mechanism of resistance in OSU ID 2808 was antibiosis, although a low level of antixenosis may also be present. The line Aus 22498 was primarily a tolerant type, with a moderate level of antibiosis and a low level of antixenosis. A six week screening procedure is described using the colony count technique for antibiosis, a completely random free choice experiment for antixenosis and a three week test measuring initial and final plant height, initial and final D. noxia infestation, damage rating, leaf area and dry plant mass for tolerance.

Genetic variability of preharvest sprouting – the South African situation

Preharvest sprouting resistance is a major breeding criterion in many regions. Screening for preharvest sprouting tolerance is difficult owing to the low heritability of the trait and its tendency to be expressed as a quantitatively inherited character. The screening procedure used currently at the ARC-Small Grain Institute is designed to assess variation of sprouting in intact heads. Five winter wheat cultivars were crossed in a complete diallel fashion. The objectives of the study were to determine the genetic variability of sprouting resistance in the progeny of five winter wheat cultivars, including Elands, a cultivar with excellent sprouting resistance. The preharvest sprouting response and α-amylase activity of these cultivars and their ensuing progeny were compared. Elands was identified as contributing positively towards preharvest sprouting tolerance in various combinations. Elands also had the best overall combining ability for the improvement of sprouting tolerance.

Biscuit-making quality of backcross derivatives of wheat differing in kernel hardness

Kernel texture in wheat has been found to be directly controlled by one or two major genes. Generally, good biscuit-making wheats are those with soft endosperm texture, lower protein content, more breakflour and a smaller particle size. The aim of this study was to determine the effect of the dominant soft endosperm genes on biscuit-making quality. Backcross derivatives were developed with a backcrossing procedure. The backcross derivatives and parents were planted in a randomised block design with six replications. Fourteen quality characteristics were measured. Except for hectolitremass and mixing time, the presence of the soft endosperm genes had a major effect on all characteristics used to predict biscuit-making quality. In the soft backcross derivatives there was a significant decrease in alkaline water retention capacity (AWRC), alveograph stability, alveograph strength, alveograph P/L ratio, flour extraction, and protein content. There was a significant increase in alveograph extensibility, biscuit diameter and breakflour yield. In this study, the presence of the soft endosperm genes was associated with good biscuit-making quality characteristics.

New generation maize inbred lines resistant to diplodia ear rot, caused by Stenocarpella maydis (Berk) Sutton

In an attempt to obtain superior levels of resistance to Stenocarpella maydis ear rot, three elite maize inbred lines (DO620Y, E739 and B37) from different genetic backgrounds that had previously been shown to have useful levels of resistance were crossed in all combinations. Additionally, two susceptible experimental maize populations were crossed to a resistant experimental inbred line as donor parent. Progeny in each of the five populations was selfed and selected for resistance over five successive generations, following artificial inoculation of plants at the localities Potchefstroom, Cedara and Hazyview. The five populations progressed from the S1 during 1997/98 to the S5 during 2000/01. A progressive improvement in resistance occurred over seasons in all progenies. After three cycles of selection, approximately 50% of the selections showed less than 10% infected ears, with a total of 84 selections showing no infection. Selections displaying the lowest disease incidence at any one locality were not necessarily the most resistant at another locality, possibly owing to significant pathogen × environment interactions. It is therefore possible that resistance in new generation inbred lines may be more stable than in the donor parents. Using I137TN as tester, a number of resistant selections were identified during 2001/02 that had combining ability for yield that compared favourably with commercial standards.

The effect ofGlu-B1 high molecular weight glutenin subunits on biscuit-making quality of wheat

SummaryThe aim of this study was to assess the effect of specificGlu-B1 HMW-GS on biscuit-making quality. Three soft spring wheat cultivars with the sameGlu-A1 andGlu-D1 HMW-GS, but differentGlu-B1 HMW-GS were used in crosses. F2∶4 derived lines were developed from these crosses.Glu-B1 HMW-GS 6+8 and 17+18; and 7+9 and 17+18 were compared. Lines with HMW-GS 6+8 versus those with HMW-GS 17+18 had a higher flour protein- and alveograph P/L ratio, shorter mixograph mixing time, more vitreous kernels, and a lower alveograph distensibility and strength (all values significant at p=0.05). Lines with HMW-GS 7+9 compared to those with 17+18 showed significant differences for flour extraction and biscuit diameter. The presence of HMW-GS 17+18 was significantly correlated with several biscuit-making quality characteristics in the Dirkwin/Zaragosa F2∶4 lines but not in the Waverley/Zaragosa F2∶4 lines, therefore the effect of HMW-GS 17+18 was modified by the genetic background in which they were expressed.

Comparison between methods for estimating sprout damage in wheat

Sprouting in wheat is common when rain occurs during harvest. This causes the moisture content of the grain to increase to a level at which germination takes place and α-amylase increases. Various methods can be used to determine α-amylase in cereals. The Falling Number method is currently widely used in the milling and baking industry. As well as being time-tested and reliable, this method is very sensitive to the presence of low levels of enzyme. However, it is often influenced by factors other than α-amylase, such as the nature of the starch, starch damage and environment. In this study various methods to determine preharvest sprouting and α-amylase activity were compared. All the methods evaluated showed significant correlations to each other and any of these methods could therefore be used to determine sprouting. Results indicated that the Stirring Number and Falling Number methods are the most reliable for the determination of preharvest sprouting and α-amylase activity.

Yield retention of resistant wheat cultivars, severely infested with Russian wheat aphid, Diuraphis noxia (Kurdjumov), in South Africa

Russian wheat aphid, Diuraphis noxia (Kurdjumov), has been a serious pest of wheat in South Africa since 1978. Wheat producers observed that resistant cultivars, developed for control of this aphid, differed in resistance and questioned whether insecticide treatment would be economically justifiable. This study was undertaken to confirm and quantify the observed differences in sixteen resistant cultivars under field conditions. A split-plot field trial with four replicates was planted near Bethlehem, South Africa. The yield of each aphid- infested plot was expressed as a percentage of the yield of the corresponding aphid-free plot giving percentage yield retained after infestation for each cultivar. The percentage yield retained varied in 2000 from 33.0% to 119%; in 2001 from 28.3% to 150.0% and in 2003 from 28.4% to 109.2 %. Cultivars were ranked and classified into more resistant and less resistant groups for each year and an AMMI analysis was conducted. Some cultivars compensated for D. noxia infestation with infested plots giving higher yield than aphid-free plots. It was shown that the level of resistance was not the same in all resistant cultivars and varied dramatically between cultivars and years when measured under severe D. noxia infestation.

Inheritance of resistance to Russian wheat aphid, Diuraphis noxia (Homoptera: Aphididae) in two wheat lines

The Russian wheat aphid, Diuraphis noxia, is a serious pest of wheat grown in the summer rainfall regions of South Africa. The use of D. noxia resistant cultivars may reduce the impact of this pest on cereal production, at the same time reducing environmental risks and minimizing control costs. The objective of the study was to determine the inheritance of D. noxia resistance present in the lines OSU ID 2808 (Triticum aestivum) and Aus 22498 (Triticum aestivum var. vavilovi) in order to ensure their judicious use in a backcross breeding programme. Resistant lines OSU ID 2808 and Aus 22498 were crossed with the susceptible cultivar Tugela after being screened with D. noxia. Resistance reactions of the F1, BC1 and F2 plants, and individual F2 plant derived F3 families indicated that the resistance in both genotypes is controlled by single dominant genes. It is unknown whether these genes are identical to any known, designated resistance genes or each other. In Suid-Afrika is die Russiese koringluis, Diuraph...

Stability analysis of kernel and milling characteristics in winter and facultative wheat

Consistency in quality of wheat grain and flour over seasons is critical to the manufacture of high quality products. The aim of this study was to measure the stability of kernel and milling quality characteristics. Thirteen hard red winter and facultative pure line and hybrid bread wheat varieties were evaluated at two planting dates for three seasons. IPC1 stability scores were compared with the AMMI stability values. Environment was found to be the most important contributor to variance in all traits. Genotype was a large contributor of variance for thousand kernel weight, kernel diameter and hardness index. GXE interaction contributed more to variation than genotype for test weight, percentage vitreous kernels, flour yield, flour colour and flour protein content at both planting dates. Molen was the most stable cultivar overall for the first planting date. At the second planting date PAN3235 and Limpopo were the most stable. The quality of the hybrids was the most inconsistent.