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  • Author or Editor: D. M. Benson x
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Several fungicides, including benomyl, flutolanil, iprodione, metalaxyl, and a mixture of metalaxyl and benomyl that control rhizoctonia stem rot (Rhizoctonia solani Kuhn) of poinsettia (Euphorbia pulcherriman Willd. ex Kl.) were evaluated for inhibition of poinsettia root initials and suppression of root elongation. Fungicides were applied as either foliar sprays to poinsettia cuttings in rooting cubes or as soaks of rooting cubes before sticking of cuttings. Rooting cube soaks of iprodione and benomyl and fungicide sprays of iprodione, benomyl, and chlorothalonil inhibited root initiation as measured by root counts 28 days after sticking cuttings. However, root elongation as measured by root rating was similar for all fungicides and the untreated control at 28 days. Plant height of rooted poinsettia cuttings transplanted to 400-cm3 pots was significantly less (P = 0.05) only for cuttings sprayed initially with iprodione, but not for rooting cubes soaked in iprodione 58 days after transplanting. The inhibitory effect of other fungicides on root initiation did not appear to affect plant growth once plants were transplanted. Chemical names used: methyl 1-(butyl-carbamoyl)-2-benzimidazolecarbamate (benomyl); tetrachloroisophthalonitrile (chlorothalonil); N -[3-(methylethoxy)phenyl]-2-(trifluoromethyl)benzamide (flutolanil); 3-(3,5dichlorophenyl)N -(lmethylethyl)-2,4-dioxo-l-imidazolidinecarboxamide (iprodione); N -(2,6-dimethylphyenyl)N -(methoxyacetyl) alanine methyl ester (metalaxyl).

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Control of preemergence damping-off caused by Phytophthora parasitica Dastur was investigated on three bedding plant species in a 1 peat: 1 vermiculite medium (v/v) limed at 3 kg·m–3 and drenched with aluminum at 10, 25, or 50 meq Al/100 cm3 medium. Aluminum as Al2(SO4)3 was applied as a drench at 0.75, 1.9, or 3.75 g/150 ml water to the surface of infested medium in 650-cm2 plug trays (1300-cm3 tray volume). All concentrations of aluminum were effective in controlling preemergence damping-off of snapdragon (Antirrhinum majus L.) and vinca (Catharanthus roseus G. Don, Madagascar periwinkle), but only 50 meq Al+3/100 cm3 medium was effective for petunia (Petunia ×hybrida Hort. Vilm.-Andr.). At 4 days after seeding and drenching with aluminum sulfate, exchangeable aluminum was 0, 0.5, and 2.03 meq Al+3/100 g medium, respectively, for the three concentrations used. Control of damping-off of snapdragon and vinca with 10 meq Al+3/100 cm3 medium with no detectable exchangeable aluminum 4 days after application suggests that P. parasitica was suppressed by aluminum early in the host–pathogen interaction, whereas petunia was susceptible to damping-off for a longer period before seedling emergence. Aluminum was not phytotoxic to vinca, snapdragon, or petunia grown in a limed medium.

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Seventeen-month-old seedlings from three fraser fir (Abies fraseri [Pursh] Poir.) seed sources (Mount Mitchell, Richland Balsam and Roan Mountain) were inoculated in an outdoor lath house with five genotypes of Phytophthora cinnamomi Rands. After 122 days, overall mortality was 90.5% with significant (p ≤ 0.07) differences among seed sources. The Mount Mitchell source had lower mortality (83.2%) than the Roan Mountain source (95.8%), while the Richland Balsam source (92.5%) was intermediate. Mortality curves were developed using nonlinear regression (Richards' function). Due to a significant seed source × inoculum genotype interaction (p ≤ 0.0001), equations were developed for each combination of seed source and inoculum genotype. Results suggest that while the overall frequency of resistance in fraser fir is low, seed sources differ in their frequency of resistance and that more than one resistance gene may be present. Survivors from this or similar inoculations could be cloned via grafting or rooted cuttings for further resistance testing and/or grafted into a Phytophthora-resistant fraser fir seed orchard.

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Wheat bran inoculum of Penicillium janthinellum (Biourge) [1% w/w added to pine bark (PB) container medium] suppressed “`root rot of azalea (Rhododendron obtusum Planch.) caused by Phytophthora cinnamomi Rands in greenhouse experiments. Shoot fresh weight was increased by 31% to 91% and mortality reduced by 30% to 50% for azaleas planted in natural (nonsterile) PB amended with P. janthinellum compared with the infested control. The population densities of P. janthinellum exceeded 105 to 106 cfu/g dry PB within 7 days and remained stable over time. Penicillium janthinellum, a natural colonizer of PB container media, shows potential as a biological control of phytophthora root rot of azalea.

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Abstract

‘Autumn Sun’, ‘Cochran's Lavender’, ‘Pink Camellia’, and ‘Wolfpack Red’ azaleas [Rhododendron obtusum (Lindl.) Planch.] were released in 1981 by the North Carolina Agricultural Research Service. Stock plants were made available to commercial nurserymen for the purposes of propagation and distribution in the nursery industry.

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Abstract

A single 4 hour exposure of shore juniper, Juniperus conferta Parl., to 0.3 ppm O3, alone or in combination with 0.15 ppm nitrogen dioxide and/or sulfur dioxide, produced a significant number of small (<3 mm), elongate, tan foliar lesions 2 to 4 days after exposure. The injury symptoms were not identical to those associated with shore juniper decline.

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Containerized seedlings of Fraser fir [Abies fraseri (Pursh) Poir.], momi fir (A. Firma Sieb. & Zucc.), and Siberian fir (A. siberica Ledeb. var. argentea) were tested for resistance to Phytophthora cinnamomi Rands (two experiments). Fraser fir was very susceptible, with many plants dead after 4 to 5 weeks. Symptoms developed more slowly in Siberian fir, but most plants were dead or dying after 10 weeks. Momi fir appeared somewhat resistant to the pathogen. Though undesirable as a Christmas tree, momi fir might be useful as a rootstock for grafting better quality species in areas affected by root rot.

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One-year-old Rhododendron L. `Nova Zembla' were grown in four container media infested with Phytophthora cinnamomi Rands. The media (all v/v) were pine bark, 3 pine bark:1 sand, 3 pine bark:1 peat, and 1 peat: 1 sand: 1 soil. After 20 weeks, plants were evaluated for root rot symptoms and the total porosity, air space, moisture-holding capacity, and bulk density were determined for all media. All media provided adequate moisture-holding capacity for container production of rhododendron in noninfested media. Shoot fresh weight in noninfested media was positively correlated with bulk density and water (percent by volume) held in the 1.0- to 5.0-kPa matric tension range and negatively correlated with total porosity and air space. Root rot severity was greatest in peat: sand: soil, intermediate in pine bark: peat, and least in pine bark and pine bark: sand. Root rot severity was negatively correlated with total porosity and air space and positively correlated with bulk density and water (percent by volume) held in the 5.0- to 10.0-kPa matric tension range.

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