Pyramiding and dissecting disease resistance QTL to barley stripe rust.

Quantitative resistance (QR) to disease is usually more durable than qualitative resistance, but its genetic basis is not well understood. We used the barley/barley stripe rust pathosystem as a model for the characterization of the QR phenotype and associated genomic regions. As an intermediate step in the preparation of near-isogenic lines representing individual QTL alleles and combinations of QTL alleles in a homogeneous genetic background, we developed a set of QTL introgression lines in a susceptible background. These intermediate barley near-isogenic (i-BISON) lines represent disease resistance QTL combined in one-, two-, and three-way combinations in a susceptible background. We measured four components of disease resistance on the i-BISON lines: latent period, infection efficiency, lesion size, and pustule density. The greatest differences between the target QTL introgressions and the susceptible controls were for the latter three traits. On average, however, the QTL introgressions also had longer latent periods than the susceptible parent (Baronesse). There were significant differences in the magnitudes of effects of different QTL alleles. The 4H QTL allele had the largest effect, followed by the alleles on 1H and 5H. Pyramiding multiple QTL alleles led to higher levels of resistance in terms of all components of QR except latent period.

Molecular marker-based genetic diversity assessment of Striga-resistant maize inbred lines.

Striga-resistant maize inbred lines are of interest to maize breeding programs in the savannas of Africa where the parasitic weed is endemic and causes severe yield losses in tropical maize. Assessment of the genetic diversity of such inbred lines is useful for their systematic and efficient use in a breeding program. Diversity analysis of 41 Striga-resistant maize inbred lines was conducted using amplified fragment length polymorphism (AFLP) and simple sequence repeat (SSR) markers to examine the genetic relationships among these lines and to determine the level of genetic diversity that exists within and between their source populations. The two marker systems generated 262 and 101 polymorphic fragments, respectively. Genetic similarity (GS) values among all possible pairs of inbred lines varied from 0.45 to 0.95, with a mean of 0.61+/-0.002 for AFLPs, and from 0.21 to 0.92, with a mean of 0.48+/-0.003, for SSRs. The inbred lines from each source population exhibited a broad range of GS values with the two types of markers. Both AFLPs and SSRs revealed similar levels of within population genetic variation for all source populations. Cluster and principal component analysis of GS estimates with the two markers revealed clear differentiation of the Striga-resistant inbred lines into groups according to their source populations. There was clear separation between early- and late-maturing Striga-resistant inbred lines. Considering the paucity of germplasm with good levels of resistance to Striga in maize, the broad genetic diversity detected within and among source populations demonstrates the genetic potential that exists to improve maize for resistance to Striga.

A comparison of two methods of assessment of maize varietal resistance to the maize weevil, Sitophilus zeamais Motschulsky, and the influence of kernel hardness and size on susceptibility.

Fifty-two maize varieties were screened for resistance to infestation by the maize weevil, Sitophilus zeamais, using assessment methods proposed by Dobie (J. Stored Products Res. 10 (1974) 183-197) and Urrelo et al. (J. Stored Products Res. 26 (1990) 100). The two methods gave similar assessments of maize susceptibility to S. zeamais. The Dobie method is preferred due to the lower total time required for assessment of relative susceptibility of maize varieties. The greatest disadvantage of the Urrelo method is the intensive labour requirements in the early stages of a trial, when numbers of eggs have to be counted, although it has the advantage that the assessment may be terminated upon emergence of the first F(1) adult. Two explanatory variables, kernel size and hardness, were investigated to determine whether they may be used as indicators of resistance. Results suggested that kernel size is the more important in determining resistance to attack by S. zeamais, with large kernels appearing to show greater resistance than small ones. Contrary to expectations, of the varieties tested, including local, hybrid and improved open pollinated (IOP) varieties, the local varieties were generally more susceptible. This may be related to kernel size, as all IOPs and hybrids tested had large kernels, whereas the majority of the local varieties had small ones. No clear relationship between weevil susceptibility and kernel hardness could be detected, although there was an indication that differences associated with kernel size varied depending on kernel hardness. None of the varieties tested showed high levels of resistance to attack by S. zeamais.

Interactions Between Fusarium verticillioides, Aspergillus flavus, and Insect Infestation in Four Maize Genotypes in Lowland Africa.

ABSTRACT An experiment was designed to compare cycles of selection of four maize genotypes for ear- and grain-quality characteristics, interactions with Aspergillus flavus and Fusarium verticillioides infection, and insect ear infestation in two seasons. Mean infection levels by A. flavus and F. verticillioides were significantly higher in inoculated rows than in the controls. The F. verticillioides-inoculated rows had significantly more coleopteran beetles and lepidopteran borers per ear than the controls and A. flavus-inoculated rows. Genotypes and cycles of selection within genotype were not different with respect to number of insects or percent fungal incidence in the ear, but they were different for husk extension, field weight, 100-grain weight, and grain density. Inoculation with either fungus resulted in significantly higher percentage of floaters (i.e., loss of grain density) and lower grain weight than the controls. Aflatoxin (B1 and B2) in A. flavus-inoculated rows averaged 327 ppb in the first season and 589 ppb in the second (dryer) season. Fumonisin levels in F. verticillioides-inoculated rows did not differ between seasons, with an average of 6.2 ppm across seasons. In the noninoculated control rows, fumonisin was significantly higher in the first (5.3 ppm) than in the second (3.1 ppm) season. For all genotypes, husk extension and yield parameters decreased in the fungal-inoculated treatments. General ear-rot scoring was significantly correlated with incidence of F. verticillioides in kernels and grain-weight loss but not with A. flavus in the grain.