Interactions of Fr genes and mixed-pathogen inocula in the loblolly pine-fusiform rust pathosystem

Abstract

Open-pollinated loblolly pine seedlings derived from seven maternal parents were inoculated in a greenhouse with 10 different bulked inocula of the fusiform rust fungus and assessed for disease incidence. The maternal parents are heterozygous (Rr) for one or two of nine known pathotype-specific Fr genes (fusiform rust resistance genes). Progeny were genotyped to identify carriers of known R and r alleles inherited from the maternal parents. The R alleles condition resistance to specific genotypes of the fungal pathogen, while r alleles do not condition for resistance. Interactions were tested among different host genotypes and different bulked inocula. Significant differences in virulence against R genotypes were observed in the bulked inocula. Likewise, the inocula were significantly different with regard to their ability to incite disease at the family level and in r genotypes. Across the inocula, disease levels differed significantly among families. Within each family, r genotype seedlings typically exhibited higher disease rates than did R genotype seedlings. The magnitude of difference (odds ratio) between the R versus r genotypes for disease incidence within each family varied from 1 to 32 times.Significant interactions between host and pathogen genotypes were observed in four of the seven families. These greenhouse assessments using bulked inocula sources revealed wide ranges of pathogen virulence levels against the different R alleles. Barring virulence masking by unknown resistance genes, similar virulence assessments should be effective guides for the field deployment of seedlings carrying specific R alleles to regions where inocula samples show low or no corresponding virulence.

Stability of Slash Pine Families with Major Gene and Partial Resistance to Single-Gall and Mixed-Gall Inocula of Cronartium quercuum fusiforme in Greenhouse Studies

“…In this sense, partially resistant families were more stable temporally and spatially than was family R1. The instability of resistant slash pine Family R1 (with major gene resistance) to various pathogen inocula (virulence) is similar to that reported for loblolly pine [21].…”

Narrative History of the Resistance Screening Center: It’s Origins, Leadership and Partial List of Public Benefits and Scientific Contributions

“…We then submitted seed from these 25 half-sib families for evaluation at the RSC using a mixed inoculum derived from 30 different rust galls at 10 geographical locations across the southern US. From our comparison of results from both field measurements and RSC tests [34], we learned that: (a) all of the families that were susceptible in the field were susceptible in controlled inoculations, and most (12 of 17) of the field-resistant families were resistant in response to controlled inoculations; (b) significant pathogenic variability was observed among the different bulk inocula, although this accounted for only 1.9% of the total variation; (c) genetic differences among families within field-resistant or field-susceptible groups accounted for 13.7% of the total variation; and (d) the family by inocula interaction was highly significant, but a single field-resistant family contributed 44% of the total family by inocula interaction variance, and two other field-resistant families also showed significant interactions.…”

“…Fortunately, RSC staff had the flexibility necessary to fulfill the requirements of these specialized objectives. Most "Research" tests at the RSC were conducted for one or more of the following four kinds of client-defined scientific purposes: (a) To identify possible histological, physiological, or biochemical mechanisms of resistance to fusiform rust [32]; (b) To determine the number and types of genes that control resistance and susceptibility to fusiform rust in loblolly and pine seedlings [33,34]; (c) To determine the number and types of genes that control virulence and avirulence in the fusiform rust pathogen [35]; and (d) To determine if the gene-for-gene theory of evolution discovered in cereal rust pathosystems also exists in the fusiform rust pathosystem [33] and explore the possibility that additive effects of minor genes may also be involved [36,37]. (4) Other developmental tests performed included more complicated procedural issues such as optimizing the basidiospore inoculum density to maximize discrimination between slash and/or loblolly pine seed lots that should be classified as "susceptible", "moderately resistant" or "highly resistant" to a given geographic source of inoculum, or, alternatively, to a mixed basidiospore inoculum derived from aeciospores collected from several different geographic regions [29][30][31].…”

Identification of Nine Pathotype-Specific Genes Conferring Resistance to Fusiform Rust in Loblolly Pine (Pinus taeda L.)

“…Neither of the two Family B matrix cells (Family B × Isolates 3327-13-3 and NC2-40) tentatively classified as C interactions for both Fr5 and Fr9 were subjected to marker-phenotype analysis. Further evidence that the second resistance gene in Family B could not be Fr1 is provided by Isik et al [42], where 10 field-collected inocula were used to challenge the families having resistance genes Fr1 through Fr9. In Isik et al [42] Table 4 shows that disease incidence values for progeny carrying the resistance allele of Fr1 (these being designated R1 in that table) vs. those carrying only the Fr9 resistance allele (these being designated r5R9 progeny in that table) were very different for multiple inocula and on average across the 10 inocula.…”

“…That should not have been the case if Fr1 and Fr9 were one and the same. For correlative purposes, the Isik et al [42] family designations F1, F2, F3, F4, F5&9, F6&7, and F8 respectively correspond to selections/families 10-5, A, 152-329, 29R, B, C, and D in the current work.…”