Fusiform Rust Fungus Research Articles

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

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.

Performance of the loblolly pine fusiform rust disease resistance gene (Fr1) in a slashXloblolly pine hybrid family

Unexpectedly high levels of field susceptibility to the fusiform rust fungus observed for slashXloblolly hybrid families in the Cooperative Forest Genetics Research Program Pine Hybrid Trials led to several hypotheses concerning causation of the observed susceptibility. One of these hypotheses involved the failure of major resistance genes to appropriately function in this particular hybrid combination. This current work, involving the loblolly pine resistance gene Fr1 and a fusiform rust inoculum avirulent against Fr1 resistance in a greenhouse trial, delineates the investigation of major gene resistance for a particular slashXloblolly hybrid family. In this trial, the Fr1 resistance allele, derived from the heterozygous (Fr1/fr1) loblolly male parent and transferred to hybrid offspring that should have been segregating 1:1 for resistance, was fully penetrant. Likewise, in the pure-species loblolly control, the Fr1 resistance allele was again fully penetrant, and the performances of our hybrid family and the loblolly control family (both of which segregated for Fr1 resistance) were comparable. These results by inductive reasoning refute the hypothesis that major resistance genes are not appropriately functional in a slashXloblolly hybrid background.

Effects of elevated atmospheric CO2 on two southern forest diseases

Research into the effects of rising atmospheric carbon dioxide (CO2) on plant diseases remains limited despite the economic importance of this subject. Loblolly pine (Pinus taeda) seedlings were exposed to ambient and twice ambient levels of atmospheric CO2 prior to inoculation with the fusiform rust fungus (the obligate pathogen Cronartium quercuum f.sp. fusiforme, CQF) or the pitch canker fungus (the facultative pathogen Fusarium circinatum, FC). Additionally, northern red oak seedlings (Quercus rubra; an alternate host of CQF) were exposed to ambient or elevated levels of atmospheric CO2 prior to inoculation with CQF. In all cases, disease incidence (percent of plants infected) and disease severity (proportion of each plant affected) were determined; with the oak seedlings, the latent period (time to sporulation) was also monitored. In general, disease incidence was decreased by exposure to elevated CO2. This exposure also increased the latent period for CQF on oak seedlings. In no instance did exposure to elevated CO2 affect disease severity. This research demonstrated that plants may benefit from exposure to the increasing concentration of CO2 in the atmosphere through decreases in fungal disease incidence.

Genetic Models of Host–pathogen Gene Interaction Based on Inoculation of Loblolly Pine Seedlings with the Fusiform Rust Fungus

Loblolly pine (Pinus taeda) seedlings from 12 full-sib families obtained from a six-parent half diallel mating design were challenged in a greenhouse using two basidiospore inocula of the fusiform rust fungus (Cronartium quercuum f.sp. fusiforme) at extremely high spore density. Each basidiospore inoculum originated from a mixed gall collection of aeciospores obtained from field-infected trees. Assessments at 4.5 months after inoculation showed that rust disease levels were high for every full-sib family and were typically above 90% for most full-sib families for both inocula. However, disease (% galled) for family E by A progeny, even at 9 months post inoculation, was lower, around 75%. A genetic model for interaction of two pairs of genes was proposed to explain the observed infection levels (% galled) in this diallel based on a gene-for-gene hypothesis. The putative genotypes of host parents and virulence compositions of mixed inocula were postulated. A bulk-segregant analysis approach based on phenotype (gall vs. no gall) was used to search for dominant molecular markers associated with the potential resistance genes in the host parents. A few candidate marker polymorphisms were observed between the gall vs. no gall bulks; however, none of the candidates appropriately co-segregated with phenotype when tested across the progeny set. An alternative model involving recessive resistance controlled by a single locus was also considered, but as with the two gene model, no markers to support the appropriateness of the recessive resistance model were observed.

Genetic Interaction of the Fusiform Rust Fungus with Resistance Gene Fr1 in Loblolly Pine

ABSTRACT We propose a method for defining DNA markers linked to Cronartium quercuum f. sp. fusiforme avirulence (Avr) genes. However, before this method can be successfully employed, a spore competition study was needed to determine the genetic composition of single pycnial drops and multiple drops on single galls when using the standard inoculation procedure, whether virulent (avr1) basidiospores ever predispose some resistant (Fr1/fr1) trees to infection by avirulent (Avr1) basidiospores, and whether avr1 and Avr1 basidiospores equally infect susceptible (fr1/fr1) trees. Results of this study suggest that multiple infections within a single gall are common using the concentrated basidiospore system, resulting on average in >4 infection events per tree. Due to multiple infections within a single gall, an individual pycnial drop cannot be assumed to consist of spores from only a single haploid pycnium. Roughly 57% of the drops harvested were found to consist of more than one haploid genotype, most likely due to the physical mixing of spores from genetically different pycnia. Most importantly, although multiple infections do occur in the formation of a single gall, there is no evidence to suggest that the genetics of the proposed gene-for-gene interaction are compromised. Only avr1 basidiospores were observed to cause infection on Fr1/fr1 trees, whereas both avr1 and Avr1 basidiospores were observed to cause infection on fr1/fr1 trees, albeit not at equal frequencies.

Direct Mechanical Dispersion and in Vitro Culture of Fusiform Rust Fungus Single Basidiospores

Single basidiospores of Cronartium quercuum f. sp. fusiforme were cast from telia suspended over a solidified nutritional medium affixed to an operating orbital shaker. Spores thus mechanically dispersed and isolated, germinated to develop single-genotype colonies.

Direct mechanical dispersion and in vitro culture of fusiform rust fungus single basidiospores

Single basidiospores of Cronartium quercuum f. sp. fusiforme were cast from telia suspended over a solidified nutritional medium affixed to an operating orbital shaker. Spores thus mechanically dispersed and isolated, germinated to develop single-genotype colonies.

Prescreening Slash Pine and Cronartium Pedigrees for Evaluation of Complementary Gene Action in Fusiform Rust Disease.

Single-urediniospore cultures of the fusiform rust fungus were used to inoculate seedlings from 10 full-sib families of a five-parent slash pine diallel at two different times in 1994. The presence or absence of fusiform rust galls was recorded for each inoculated seedling at 9 months postinoculation, and percent infection levels for each family-inoculum-time combination were used for detecting differences among host families and fungal cultures and for identifying differential interactions. The existence of differential interactions between two or more fungal cultures and two or more host families verifies that complementary gene action does exist in this pathosystem. Some host families may be excluded from more detailed interaction studies on the basis of their redundancy and lack of participation in differential interactions.

Detection of DNA Polymorphisms in a Single Urediniospore-Derived Culture ofCronartium quercuumf. sp.fusiforme

PCR (polymerase chain reaction) with single, arbitrarily chosen primers was used to detect DNA polymorphisms among 34 sibling gametothalli (single-basidiospore hyphal cultures) and spermatia (single drops of pycniospores) derived from a single-urediniospore culture of the fusiform rust fungus, Cronartium quercuum f. sp. fusiforme. Ten single-basidiospore hyphal cultures were distinguished by comparing nine polymorphic segments produced by four primers. Polymorphisms also were detected between 24 single drops of pycniospores collected from four different galls and drops within the same gall, revealing that phenotypes of galls were all different, and some contained more than one phenotype []

Indirect Prediction of Breeding Values for Fusiform Rust Resistance of Slash Pine Parents Using Greenhouse Tests

Abstract Six-week-old greenhouse-grown seedlings of 100 open-pollinated slash pine families were artificially inoculated with fusiform rust fungus at the USDA Forest Service Resistance Screening Center (RSC). All families had been tested for rust resistance in four or more field tests. Selection indices were developed to use symptoms of rust infection observed in the greenhouse to predict parental breeding values for field resistance to fusiform rust. Of the 100 families, 75 represented an unimproved population, while 25 represented a population improved for rust resistance. A large number (56,688) of indirect selection indices were considered in an attempt to identify efficient indices which use a small number of traits. Indices were developed using three different sets of variance and covariance estimates: (1) from the unimproved population, (2) from the improved population, and (3) from the combined population. Separate indices were necessary for the two distinct populations. The new indices were more correlated with field resistance and result in more efficient indirect selection for field resistance than the equation currently in use by the RSC. However, the greenhouse test to indirectly assess field rust resistance (using the new indices) is somewhat less efficient than a single field progeny test on average. For. Sci. 38(1):45-60.

Genetic Parameter Estimates for Greenhouse Traits of Slash Pine Artificially Inoculated with Fusiform Rust Fungus

Abstract Six-week-old greenhouse-grown seedlings of 100 open-pollinated slash pine families were artificially inoculated with fusiform rest fungus in three testing periods at the USDA Forest Service Resistance Screening Center. SIX months after inoculation, seedlings were scored for symptoms of rest infection. Genetic parameters (heritabilities, genetic correlations and family x test interactions) were estimated for 16 traits. Eleven traits appear highly promising for inclusion into a selection index to predict breeding values for rest resistance in the field. For these 11 traits, family x test interaction was not important, family heritabilities were high (>0.78), and differences among the four groups of families were minor for family variance, variance of family means, and heritability estimates. The other five traits had measurable amounts of family x test interaction and lower heritability estimates (<0.70). For. Sci. 37(3):836-848.

Cytokinin activity in southern pines inoculated withCronartium quercuumf.sp.fusiforme

Cytokinin activity in tissue extracts from southern pine seedlings susceptible, moderately resistant, and resistant to the fusiform rust fungus was determined six months after inoculation. Qualitative and quantitative differences in cytokinin activity were detected among all rust-reaction groups and between inoculated and uninoculated trees within each pine species. Cytokinin activity in slash pines was detected at relatively low levels in 2 of 10 Rf zones bioassayed. After inoculation, cytokinin activity increased significantly within these zones, as well as in additional zones where activity had not appeared before inoculation with the rust fungus. Extracts from uninoculated resistant tissues were positive for cytokinin activity in all 10 Rf zones, whereas in extracts from inoculated tissues, activity was not detected in 3 zones and was diminished in the remaining 7 zones. Fresh weights of cocklebur cotyledons incubated in extracts from inoculated susceptible and moderately resistant seedlings were sign...

Alteration in auxin levels in selected southern pines inoculated with the fusiform rust fungus

Compounds with auxin-like activity were extracted from 3- and 6-month-old stem and needle tissues from uninoculated and inoculated southern pine trees susceptible, tolerant and resistant to the fusiform rust fungus, Cronartium quercuum f. sp. fusiforme. Compared to uninoculated seedlings, auxin-like activity increased in susceptible, but decreased in tolerant and resistant seedlings. Both qualitative differences and transitions in substances with auxin activity were found in extracts from seedlings of the three reaction groups, sampling periods and inoculated and uninoculated treatments.

Wounds on Loblolly Pines are Sites of Infection for Fusiform Rust Fungus

Abstract Wounds on loblolly pines are shown to be infection sites for fusiform rust fungus (Cronartium quercuum f. sp. fusiforme). Recommendations are proposed for preventing infection of wounds.

Axenic culture of Cronartium fusiforme from three spore forms

Axenic cultures of the fusiform rust fungus (Cronartium fusiforme) were initiated from basidiospores, aeciospores, and uredospores. The morphology, cytology, and nuclear condition of the mycelium from the three spore stages are described and illustrated. Cultures derived from aeciospores and uredospores of C. fusiforme are reported for the first time. An improved culture medium is described. Inoculation of slash pine (Pinus elliottii var. elliottii) seedlings with suspensions of mycelium from basidiospores induced some typical galls which produced pycnial exudates and aeciospores. On synthetic medium, cultures from basidiospores were monokaryotic. Mostly dikaryotic mycelium was produced from aeciospores and uredospores, but some monokaryotic hyphae were observed.