Summary Since introduced a century ago, Cronartium ribicola has devastated many populations of North American white pines. However, significant genetic resistance to white pine blister rust occurs naturally and can be exploited. In this review, we discuss the progress and different approaches to breeding for resistance in North American white pines. Three broad categories of resistance are: (1) ontogenetic resistance, (2) R‐gene resistance and (3) partial resistance. Ontogenetic resistance is associated with increased host age and indicated by higher susceptibility to infection in primary needles and young seedlings then in grafts and older trees. R‐gene resistance (major gene resistance) is an example of the classic gene‐for‐gene system common in many rust diseases. R‐gene resistance provides immunity but may not be durable. Host resistance and the corresponding rust virulence which defeats it are well described for sugar pine and western white pine. Host plants with partial resistance are able to retard or tolerate disease development without eliminating the pathogen. Partial resistance is also called slow‐rusting resistance or low‐level resistance and is revealed in seedlings by several responses, including slow‐canker‐growth, difficult‐to‐infect, needle‐shed and bark‐reaction. Most of these seedling responses are presumed to be multigenic; but needle‐shed may be controlled by recessive genes. Long‐term, field trials for verification of screening and selection results are sparse. Although 100% higher survival of selected material over unselected occurs in some trials, mortality is high under conditions of high hazard and heavy inoculum load. In several, long‐term trials, some full‐sib crosses expressed a strong phenotypic resistance that indicates specific combining ability between complimentary parents. These and other observations suggest that we might yet find strong and durable resistance. Study of Eurasian white pines infected by blister rust fungi could help us better understand endemic pathosystems. Different strategies are identified for deploying material selected for either R‐gene or partial resistance. Current research suggests that resistance is more complex than previously modelled, but new molecular techniques offer useful methods for investigating the white pine blister rust pathosystem.
White pine blister rust, Cronartium ribicola, has plagued the forests of North America for almost a century. Over past decades, eastern white pine (Pinus strobus) that appear to tolerate the disease have been selected and incorporated into breeding programs. Seeds from P. strobus with putative resistance were collected from Oconto River Seed Orchard, Nicolet National Forest, WI. Seedlings were grown for 5 months and artificially inoculated with basidiospores of C. ribicola in two replicated greenhouse experiments. Needles from infected seedlings were fixed, sectioned, and stained with a variety of histological reagents, and rate of mortality for the remaining seedlings was monitored. The most susceptible families suffered 50% mortality in approximately half the time of the more resistant families. Extensive inter- and intracellular hyphae were observed in needles from seedlings of susceptible families, whereas hyphal proliferation was restricted in needles of resistant seedlings. Needles from resistant families had pronounced responses to infection. Phenolics, observed with phloroglucinol-HCl staining, were deposited around infection sites where dense mycelial masses were present. Abnormal host cell growth and rapid cell death in the immediate area of infection were also observed in some eastern white pine families.
Epicuticular wax on needles was evaluated for its influence on Cronartium ribicola infection of resistant and susceptible selections of Pinus strobus. Environmental scanning electron microscopy comparisons revealed that needles from a resistant selection of eastern white pine, P327, had a significantly higher percentage of stomata that were occluded with wax, fewer basidiospores germinating at 48 h after inoculation, and fewer germ tubes penetrating stomata than needles from a susceptible selection H111. In addition, needles from seedlings that failed to develop symptoms 6 weeks after inoculation, from a cross between P327 and susceptible parent H109, had a significantly higher percentage of stomata occluded with wax compared with needles from seedlings that developed symptoms. In experiments where epicuticular waxes were removed from needles before seedlings were infected, resistant seedlings without wax developed approximately the same number of infection spots (as measured by spot index) as susceptible seedlings with wax intact. Gas chromatography/mass spectrometry comparisons of extracted epicuticular waxes revealed several peaks that were specific to P327 and not found in susceptible H111 suggesting biochemical differences in wax composition. These results implicate the role of epicuticular waxes as a resistance mechanism in P. strobus selection P327 and suggest a role for waxes in reducing spore germination and subsequent infection through stomatal openings.
Aloe vera, a common ingredient in cosmetics, is increasingly being consumed as a beverage supplement. Although consumer interest in aloe likely stems from its association with several health benefits, a concern has also been raised by a National Toxicology Program Report that a nondecolorized whole leaf aloe vera extract taken internally by rats was associated with intestinal mucosal hyperplasia and ultimately malignancy. We tested a decolorized whole leaf (DCWL) aloe vera, treated with activated charcoal to remove the latex portion of the plant, for genotoxicity in bacteria, acute/subacute toxicity in B6C3F1 mice, and subchronic toxicity in F344 rats. We found this DCWL aloe vera juice to be nongenotoxic in histidine reversion and DNA repair assays. Following acute administration, mice exhibited no adverse signs at 3- or 14-day evaluation periods. When fed to male and female F344 rats over 13 weeks, DCWL aloe led to no toxicity as assessed by behavior, stools, weight gain, feed consumption, organ weights, and hematologic or clinical chemistry profiles. These rats had intestinal mucosal morphologies—examined grossly and microscopically—that were similar to controls. Our studies show that oral administration of this DCWL aloe juice has a different toxicology profile than that of the untreated aloe juice at exposures up to 13 weeks.
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