Recrystallization as the governing mechanism of ion track formation.

Response of dielectric crystals: MgO, Al2O3 and Y3Al5O12 (YAG) to irradiation with 167 MeV Xe ions decelerating in the electronic stopping regime is studied. Comprehensive simulations demonstrated that despite similar ion energy losses and the initial excitation kinetics of the electronic systems and lattices, significant differences occur among final structures of ion tracks in these materials, supported by experiments. No ion tracks appeared in MgO, whereas discontinuous distorted crystalline tracks of ~2 nm in diameter were observed in Al2O3 and continuous amorphous tracks were detected in YAG. These track structures in Al2O3 and YAG were confirmed by high resolution TEM data. The simulations enabled us to identify recrystallization as the dominant mechanism governing formation of detected tracks in these oxides. We analyzed effects of the viscosity in molten state, lattice structure and difference in the kinetics of metallic and oxygen sublattices at the crystallization surface on damage recovery in tracks.

Bacterial populations on the surfaces of organic and conventionally grown almond drupes.

AIMS: To compare the bacterial populations on organically and conventionally grown almond drupes before and after hull split. METHODS AND RESULTS: We constructed 16S rRNA gene libraries, containing approx. 3000 sequences each, from the bacteria from organically and conventionally grown drupes before and after hull split. We observed that before hull split both conventionally and organically grown drupes were colonized by relatively few types of bacteria that were mostly common phyllosphere-associated Proteobacteria. However, the organically grown drupes contained significantly more Alphaproteobacteria and the conventionally grown drupes contained significantly more Gammaproteobacteria. The conventionally grown drupes also contained significantly more sequences associated with the phylum Actinobacteria. After hull split, we observed a significant increase in bacterial diversity, with many newly appearing sequences that were not normally associated with the phyllosphere. CONCLUSIONS: Organic and conventional growing methodologies influence the types of bacteria on almond drupes and hull split results in a burst of microbial diversification. SIGNIFICANCE AND IMPACT OF THE STUDY: Production of organic produce is increasing due to consumer preferences, but it was unknown how this methodology affects the bacterial populations on almond drupes. This is the first study to compare the bacterial populations of organically and conventionally grown almond drupes.

Bacterial population structure and dynamics during the development of almond drupes.

AIMS: To describe the bacterial populations and their dynamics during the development of almond drupes. METHODS AND RESULTS: We examined 16S rRNA gene libraries derived from the bacterial populations on almond drupes at three stages of development: (i) when the drupes were full sized, but before embryo development, (ii) when the drupe hulls first began to split and (iii) when the drupes were fully mature, but before harvesting. Our data revealed that the immature drupes were colonized by relatively few types of bacteria, belonging mostly to common phyllosphere-associated bacteria within the genera Pseudomonas, Pantoea, Methylobacterium and Sphingomonas. However, after the hulls first began to split, the level of bacterial diversity increased and continued to do so until the drupes were fully mature. At the last sampling period, we observed several sequences belonging to bacteria that are not usually associated with the phyllosphere, including some identical to Salmonella enterica. CONCLUSIONS: The bacterial populations on almond drupes before hull split were composed of relatively few types, most of which were commonly associated with the phyllosphere. However, after hull split, the level of microbial diversity increased, which was mostly due to increased levels of bacteria that are not normally associated with the phyllosphere, including Salm. enterica. SIGNIFICANCE AND IMPACT OF THE STUDY: This is the first report of the bacterial populations associated with almond drupes and their dynamics during development. Of specific significance is the observation that Salm. enterica was present on the drupes just prior to harvesting, which may represent a critical control point.

Etiology and Management of a Mandarin Rind Disorder in California.

Recent reports of a preharvest rind disorder of Satsuma mandarin fruit resulted in severe crop losses in some growing areas of California (Butte, Fresno, Tulare, and Kern Counties). Symptoms were more frequently observed on fruit on the outer perimeter of the tree and on the exposed side of the fruit. Fungal isolations from affected fruit were inconsistent among orchard sites and growing seasons. Isolations resulted in species of Alternaria, Fusarium, Macrophomina, Ulocladium, and Cladosporium. In laboratory studies, mandarin fruit that were soaked in water for 6 h at 15 to 35°C, air dried, and incubated at 20°C for 3 days developed lesions similar to those observed in the field. When fruit were treated with an agricultural summer spray oil or the antitranspirant di-1-p-menthene prior to water soaking, the incidence and severity of the disorder were significantly reduced. Similar results were obtained in 2 years of field trials at two sites using one to three preharvest applications with the agricultural antitranspirant or summer oil starting after fruit color break. In most trials, a single application of these treatments at 1 or 2 weeks after color break was highly effective. Fungicides were inconsistent in their efficacy, whereas trees that were physically protected from rain did not develop the disorder. These data suggest that this mandarin rind disorder is a physiological, abiotic disorder rather than a pathological problem caused by fungi and can be economically managed with water-repellant treatments.

Almond Replant Disease and Its Management with Alternative Pre-Plant Soil Fumigation Treatments and Rootstocks.

Trials were conducted in orchards near Chico, CA and microplots near Parlier, CA to examine symptoms and control measures for a replant disease (RD) on almond (Prunus dulcis). In the orchard trials, areas with a recent history of severe RD were cleared, given soil fumigation treatments in the fall, and replanted with almond trees on various rootstocks the following winter. The replants in nonfumigated soil developed severe RD (stunting, wilting, chlorosis, defoliation) by the following summer, while those in most fumigated treatments remained healthy. Trees in nonfumigated soil developed smaller trunk diameters and fewer healthy roots ≤1 mm diameter, compared with the healthy trees. Almond developed RD on all rootstocks evaluated (Marianna 2624, Lovell, and Nemaguard), but the trees on Marianna 2624 were the most severely affected. Pre-plant tree-site (spot) fumigation treatments with methyl bromide (MB), chloropicrin (CP), 1,3-dichloropropene (1,3-D), 1,3-D + CP, iodomethane, and iodomethane + CP all prevented severe RD. Broadcast soil fumigation with CP also was effective, but broadcast MB and 1,3-D were ineffective. In microplots filled with RD-conducive soil, CP was more potent than MB for prevention of RD on Nemaguard peach. There was no association between nematodes and RD in orchard or microplot trials. The RD apparently was mediated by a biological agent(s) other than nematodes and can be prevented by appropriate fumigation with CP or other MB alternatives.

First Report of Fruit Rot and Associated Branch Dieback of Almond in California Caused by a Phomopsis Species Tentatively Identified as P. amygdali.

A fruit rot of almond (Prunus dulcis (Mill.) D. Webb.) was observed in an orchard in Durham, CA (Butte County), in June of 1998 after an unusually wet spring with a total precipitation of 17.2 cm for April and May. Disease incidence on fully developed fruit of almond cv. Sonora was nearly 90% in the lower tree canopy by July. Almond cv. Nonpareil grown in alternate rows in the same orchard was much less affected. Fruit symptoms included extensive grayish brown discolored and shriveled hulls, often associated with a clear gum secretion and shriveled kernels. Affected fruit frequently abscised. Leaf symptoms and branch dieback were not associated with the disease in 1998. In May of 1999, however, extensive twig dieback was observed on almond cv. Sonora in the same orchard. Isolations from more than 100 symptomatic fruit were conducted from 9 sampling sites in the 9-ha orchard. Based on morphological characteristics, the same fungus was isolated from 93% of the fruit. The fungus also was isolated consistently from samples exhibiting twig dieback. During a major disease survey conducted in 1998, the fungus was only incidentally isolated from almond fruit from other California orchards. Ascomata were not observed in vivo or in vitro. The fungus produced alpha and beta spores in pycnidia when cultured on potato dextrose agar. Spore measurements were obtained from 10 spores for each of 3 isolates obtained from fruit or twig dieback of almond cv. Sonora. Conidial dimensions of fruit and twig isolates were very similar. Based on spore sizes, with alpha spores measuring 5.3 to 7.5 (to 8) × 1.7 to 2.5 µm and beta spores measuring12.8 to 29.8 × 0.6 to 0.7 µm, the fungus was tentatively identified as Phomopsis amygdali (Del.) Tuset & Portilla (2). Previous reports on this fungus (2), however, indicated that beta spores are not produced in culture, and disease symptoms have not been observed on fruit. The fungus was morphologically different from other species of Phomopsis reported from almond and other Prunus species, including P. mali Roberts, P. padina (Sacc. & Roum.) Died., P. parabolica Petrak, P. perniciosa Grove, P. pruni (Ellis & Dearn.) Wehm., P. prunorum (Cooke) Grove, P. ribetejana Camara, and P. stipata (Lib.) Sutton (3). Field inoculation studies were performed in May of 1999 on almond cvs. Carmel and Mission. Almond fruit were wounded (2 × 2 × 2 mm) or left unwounded and were sprayed with water (control) or a suspension of alpha spores (105 spores per ml). Branches were bagged for 4 days to maintain high humidity. Fruit symptoms on cv. Carmel were observed after 4 weeks on wounded and nonwounded inoculated fruit, and P. amygdali was successfully reisolated from diseased tissue. No symptoms were observed in the control treatment for almond cv. Carmel or in any treatment for cv. Mission. This is the first report of P. amygdali causing a late spring and summer fruit rot and associated branch dieback of almond in North America (1). References: (1) D. F. Farr et al. 1989. Fungi on Plants and Plant Products in the United States. The American Phytopathological Society, St. Paul, MN. (2) J. J. Tuset and M. T. Portilla. Taxonomic status of Fusicoccum amygdali and Phomopsis amygdalina. Can. J. Bot. 67:1275, 1989. (3) F. A. Uecker. 1988. A World List of Phomopsis Names with Notes on Nomenclature, Morphology, and Biology. Mycologia Memoir No. 13. J. Cramer, Berlin.

The evolutionary ecology of corals.

Corals display a wide range of complex life histories. The evolutionary consequences of factors such as clonality, indeterminate growth, asexual reproduction coupled with various (sexual) breeding systems, different levels of gene flow, and strongly overlapping generations have only just begun to be explored. We identify a series of problems and areas for new research that may be resolved b y the application of novel theoretical approaches (including nonequilibrium population genetic models and demographic models incorporating modular processes such as colony fission and polyp mortality), greater in situ experimentation, long-term monitoring of population dynamics and the use of new genetic techniques.

Diversity in tropical rain forests and coral reefs.

The commonly observed high diversity of trees in tropical rain forests and corals on tropical reefs is a nonequilibrium state which, if not disturbed further, will progress toward a low-diversity equilibrium community. This may not happen if gradual changes in climate favor different species. If equilibrium is reached, a lesser degree of diversity may be sustained by niche diversification or by a compensatory mortality that favors inferior competitors. However, tropical forests and reefs are subject to severe disturbances often enough that equilibrium may never be attained.