Mefenoxam Sensitivity, Aggressiveness, and Identification of Pythium Species Causing Root Rot on Floriculture Crops in North Carolina.

Herbaceous ornamental plants exhibiting symptoms of Pythium root rot were collected from 26 greenhouses in 21 counties in North Carolina (NC) from 2010 to 2012. Plant symptoms ranged from mild stunting to severe wilting, root rot, and death. Roots were plated on selective media, and 356 isolates of Pythium were recovered from 34 host species. Selected isolates were identified by sequencing of the internal transcribed spacer (ITS) rDNA gene region. Seventeen Pythium species were identified, with P. aphanidermatum, P. irregulare, and P. myriotylum comprising 75% of the 320 isolates sequenced. Twelve of the 26 greenhouses had more than one species present. Mefenoxam sensitivity was tested in vitro by growing isolates in wells of microtiter plates containing clarified V8 agar amended with 100 µg a.i./ml mefenoxam. Colonization was scored after 24 to 48 h using a scale of 0 (no growth) to 5 (entire well colonized). Fifty-two percent of the isolates were resistant to mefenoxam (mean score ≥4). All 32 isolates of P. myriotylum were sensitive, whereas sensitivity varied among isolates of P. aphanidermatum and P. irregulare. Resistant and sensitive isolates of the same species were found within the same greenhouses. The aggressiveness of P. aphanidermatum and P. irregulare isolates was evaluated on poinsettia, Gerbera daisy, and petunia. P. aphanidermatum was more aggressive than P. irregulare on poinsettia and petunia; symptoms were mild and no differences in aggressiveness were observed on Gerbera daisy. Sensitivity to mefenoxam was not related to aggressiveness.

Phytophthora Populations in Nursery Irrigation Water in Relationship to Pathogenicity and Infection Frequency of Rhododendron and Pieris.

Phytophthora spp. are waterborne plant pathogens that are commonly found in streams, rivers, and reclaimed irrigation water. Rhododendron and Pieris trap plants at two commercial nurseries were irrigated with water naturally infested with Phytophthora spp. during the 2011 and 2012 growing seasons to assess the frequency of disease. Phytophthora spp. were consistently recovered from water samples at every collection time but detected on only 2 of the 384 trap plants during the two growing seasons. Pathogenicity assays proved that Phytophthora hydropathica and Phytophthora taxon PgChlamydo, commonly recovered taxa in irrigation water at the nurseries, were foliar pathogens of Rhododendron and Pieris; however, neither species was able to cause root rot on these same hosts. Overall, Phytophthora spp.-infested irrigation water did not act as a primary source of infection on Rhododendron and Pieris, even though foliar pathogenic species of Phytophthora were present in the water.

Diversity and Mefenoxam Sensitivity of Phytophthora spp. Associated with the Ornamental Horticulture Industry in the Southeastern United States.

Phytophthora isolates associated with ornamental plants or recovered from irrigation water in six states in the southeastern United States (Georgia, North Carolina, South Carolina, Tennessee, Texas, and Virginia) were identified and screened for sensitivity to mefenoxam. Isolates from forest and suburban streams in Georgia and Virginia were included for comparison. A new in vitro assay, utilizing 48-well tissue culture plates, was used to screen for mefenoxam sensitivity; this assay allowed high throughput of isolates and used less material than the traditional petri plate assay. In total, 1,483 Phytophthora isolates were evaluated, and 27 species were identified with Phytophthora nicotianae, P. hydropathica, and P. gonapodyides, the most abundant species associated with plants, irrigation water, and streams, respectively. Only 6% of isolates associated with plants and 9% from irrigation water were insensitive to mefenoxam at 100 µg a.i./ml. Approximately 78% of insensitive isolates associated with plants were P. nicotianae, and most of these (67%) came from herbaceous annual plants. Most of the insensitive isolates recovered from irrigation water were P. gonapodyides, P. hydropathica, P. megasperma, and P. pini, and 83% of the insensitive isolates from streams were P. gonapodyides. Overall, this study suggests that mefenoxam should continue to be a valuable tool in the management of Phytophthora diseases affecting ornamental plants in the southeastern United States.

First Report of Boxwood Blight Caused by Cylindrocladium pseudonaviculatum in the United States.

In September and October 2011, a new disease was observed on Buxus spp. in North Carolina and Connecticut, respectively. In North Carolina, over 10,000 containerized Buxus sempervirens (American boxwood) were affected at one location. A few weeks later, the disease was found in Connecticut on entire plantings of B. sempervirens 'Suffruticosa' (English boxwood) at two residential properties, and shortly thereafter on over 150,000 plants at two production nurseries. Initial foliar symptoms appeared as light to dark brown spots, often with dark borders. Spots enlarged and coalesced, often with a concentric pattern, and black streaks or cankers developed on stems. Infected leaves became brown or straw colored and dropped quickly after foliar symptoms were visible. Branch dieback and plant death were also observed in Connecticut. Cultures were isolated from symptomatic leaves and stems and identified as Cylindrocladium pseudonaviculatum Crous, Groenewald & Hill 2002 (1) (syn. Cylindrocladium buxicola Henricot 2002 [2]) on the basis of morphological characteristics. Macroconidiophores were single or in groups of up to three and comprised a stipe, stipe extension, and a penicillate arrangement of fertile branches. The stipe extension was septate, hyaline (89 to 170 × 2 to 4.5 µm), and terminated in an ellipsoidal vesicle (6 to 11 µm diameter) with a papillate or pointed apex. Conidia were cylindrical, straight, hyaline, and one septate (48 to 62 × 4 to 6 µm), occurring in slimy clusters. No microconidiophores were observed. Chlamydospores were medium to dark brown, thick walled, and smooth to rough. Microsclerotia were observed on PDA (1). A portion of ß-tubulin gene sequence from two Connecticut (Genbank Accession Nos. JQ866628 and JQ866629) and two North Carolina isolates showed 100% similarity with only C. pseudonaviculatum strains. USDA-APHIS-PPQ confirmed this new United States record on October 24, 2011. Pathogenicity was confirmed by inoculating three 1-gallon container plants of B. sempervirens 'Suffruticosa' in North Carolina and four liners of B. sinica var. insularis × B. sempervirens 'Green Velvet' in Connecticut with a spore suspension of approximately 5.0 × 106 conidia (North Carolina) or 1.0 × 106 conidia (Connecticut) on the foliage of each plant; untreated control plants were sprayed with water. After incubation at ambient temperature, all inoculated plants developed foliar and stem lesions within 3 to 4 days and blighting occurred within 2 weeks; control plants remained asymptomatic. C. pseudonaviculatum was reisolated from inoculated plants. To our knowledge, this is the first report of C. pseudonaviculatum in the United States. C. pseudonaviculatum causes a serious disease of Buxus spp. in the United Kingdom and several other European countries as well as New Zealand (1). Confirmation of boxwood blight in the United States is significant because of the popularity of boxwood as a landscape plant, and because of the potential economic impact this disease may have on commercial growers; boxwood production in the United States has an annual wholesale market value of approximately $103 million (3). References: (1) P. Crous, et al. Sydowia 54:23, 2002. (2) B. Henricot and A. Culham Mycologia 94: 980, 2002. (3) USDA-NASS, Census of Horticulture, 2010.

Microbial Profiling of Cultural Systems for Suppression of Phytophthora Root Rot in Fraser Fir.

Phytophthora root rot of Fraser fir, caused by several Phytophthora spp., is a severe problem in Christmas tree production. Since fungicides are not economically viable for disease management in field plantings and host resistance is not available, cultural control methods were investigated. Mulches, dairy compost, and soil pH adjustment were tested at five field sites in North Carolina. Treatments included wood chips, wood chips plus compost, or pine bark as raised beds, and compost or sulfur tilled into soil. Soil and mulch microbial populations were characterized by dilution plating and calculation of a log series diversity index, and by enzyme analyses at 5, 12, 17, and 24 months after planting. Bacterial and fungal counts, microbial activity, and cellulase activity were higher in mulch than in soil at all sites and times (P < 0.01), and generally did not differ among mulch types or among soils. Treatments significantly affected disease ratings and tree survival at three of five sites, with one or more mulch treatments yielding lower disease ratings and greater survival than controls. Tree mortality at each time point varied significantly with cellulase activity in the upper root zone (P = 0.005). Other biological variables did not show significant relationships with disease ratings or mortality.

First Report of Foliar Infection of Maianthemum racemosum by Phytophthora ramorum.

In May 2003, Phytophthora ramorum S. Werres & A.W.A.M. de Cock was isolated from the leaf tips of a single plant of false Solomon's seal (Maianthemum racemosum (L.) Link, formely known as Smilacina racemosa (L.) Desf.), a native, herbaceous perennial of the Liliaceae family, at the Jack London State Park in Sonoma County, California. Affected leaves had cream-to-brown lesions on the tips that were delimited by a yellow chlorotic zone. Lesions on the stems were not observed. The isolate (American Type Culture Collection [ATCC], Manassas, VA, MYA-3280; Centraal Bureau voor Schimmelcultures, Baarn, the Netherlands, CBS 114391) was typical of P. ramorum with large chlamydospores and caduceus, semipapillate sporangia, and the sequence (GenBank Accession No. AY526570) of the internal transcribed spacer region of the rDNA matched those published previously (4). The site, from which wood rose (Rosa gymnocarpa) was recently identified as a host, is a mixed forest containing confirmed P. ramorum-infected coast redwood (Sequoia sempervirens), California bay laurel (Umbellularia californica), and tanoak (Lithocarpus densiflora) trees (2,3). Two leaves per asymptomatic, pesticide free, potted plant of false Solomon's seal were inoculated with zoospores of the P. ramorum isolate obtained from infected false Solomon's seal (1). Five plants were inoculated in trial 1, and the following day, three plants were inoculated in trial 2. A control leaf of each plant was dipped in sterile deionized water. Plants were enclosed in plastic bags, misted regularly with sterile distilled water, and maintained at 16 to 21°C in the greenhouse. In both trials, plants did not have lesions on the leaves after 16 days and were reinoculated on separate days for each trial with higher concentrations of zoospores (1 × 105 [trial 1] and 2 × 105 [trial 2] zoospores/ml). Cream-colored lesions, similar to those observed in the field, were evident 1 week after the second inoculation and stopped progressing in both trials by 17 days. Lesions starting from the leaf tips averaged 13 mm (range 8 to 24 mm) long, and P. ramorum was reisolated on Phytophthora-selective agar medium modified with 25 mg of pentachloronitrobenzene from 44% (trial 1) and 83% (trial 2) of all lesions (4). Control leaves had no lesions, and P. ramorum was not reisolated. Sporangia were not observed on any leaves when examined with the dissecting microscope. The fact that lesions developed only after a second inoculation with higher concentrations of zoospores, and these lesions stopped progressing after 17 days, suggests that false Solomon's seal is much less susceptible than other hosts such as western starflower (Trientalis latifolia) (1) and wood rose (2). To our knowledge, this is the first report of a plant from the Liliaceae as a natural host for P. ramorum, although Smilax aspersa was identified as being susceptible in artificial inoculations of detached leaves (E. Moralejo and L. Hernández, personal communication). False Solomon's seal is popular in the horticultural industry. References: (1) D. Hüberli et al. Plant Dis. 87:599, 2003. (2) D. Hüberli et al. Plant Dis. 88:430, 2004. (3) P. E. Maloney et al. Plant Dis. 86:1274, 2002. (4) D. M. Rizzo et al. Plant Dis. 86:205, 2002.

Effects of Mesocriconema xenoplax on Vitis vinifera and Associated Mycorrhizal Fungi.

Previous surveys of vineyards had indicated that Mesocriconema xenoplax was present in 85% of vineyards in western Oregon, but yields were not depressed in established vines. Microplot studies were initiated in 1997 in a Willamette Valley vineyard to determine the impact of M. xenoplax on vine establishment. Plots were infested with 0.03, 0.6, and 3.0 M. xenoplax g(-1) soil and planted with self-rooted Chardonnay and Pinot Noir vines. In November 2000, four growing seasons after planting, pruning weights, fine root weights, and fruit yield of vines planted in infested soil were reduced by 58%, 75%, and 33%, respectively, relative to control vines (planted in noninfested soil). In 1998 with ca 2000 degree-day base 9 degrees C accumulation, population densities increased 32-fold and 44-fold on 1-year-old Chardonnay and Pinot Noir vines, respectively. Nematode population dynamics and pruning data suggested that the carrying capacity of vines in microplots was 5 to 8 M. xenoplax g(-1) soil. In November 2000, more than 80% of the fine root length was colonized by arbuscular mycorrhizal fungi in all treatments. The frequency of fine roots containing arbuscules (the site of nutrient transfer between plant and fungus), however, was depressed from 5% to 65% in plants infested initially with M. xenoplax as compared to controls. Competition for photosynthate within the root system is proposed as a possible mechanism by which nematodes suppressed arbuscule frequency.

The Use of Soil Solarization for the Management of Soilborne Plant Pathogens in Strawberry and Red Raspberry Production.

Root rot caused by Phytophthora fragariae var. fragariae and P. fragariae var. rubi are major concerns in strawberry and raspberry production in the Pacific Northwest. Of lesser importance is black root rot of strawberry, caused by a complex of fungi and nematodes. Soil solarization was evaluated in 1997 in a strawberry planting and in 1998 in a raspberry planting for: (i) enhancing plant health and growth, and (ii) reducing population densities of root-destroying pathogens. Plots were solarized from mid-July to mid-September. Maximum and mean soil temperatures in solarized plots recorded at 10 cm depth were 48 and 33°C in the strawberry plots and 46 and 29°C in the raspberry plots. These temperatures were 7 to 17°C higher than temperatures recorded in nonsolarized plots. Soil collected after solarization was assayed by growing bait plants, cv. Totem strawberry or cv. Qualicum raspberry, at 15°C for 6 weeks in saturated soil to promote infections. Root health and plant growth were evaluated after 6 weeks. Solarization significantly reduced (P < 0.05) root necrosis and increased root weight of bait plants compared to plants grown in soil from nonsolarized plots. Infection of strawberry roots by P. fragariae, Pythium, Rhizoctonia, and Cylindrocarpon spp. was reduced (P < 0.05) by solarization in sampled soil. Disease was reduced in cv. Hood strawberries and Qualicum and Skeena red raspberries planted in solarized field plots. In the second growing season, total number and number of healthy primocanes of Qualicum plants were greater (P < 0.05) in solarized plots compared to nonsolarized plots. Solarization combined with applications of mefenoxam was no more effective in controlling diseases than solarization alone, but better than mefenoxam alone. Skeena plants responded similarly, but the differences were not significant. Red raspberry plants growing in solarized soil yielded more fruit than plants growing in nonsolarized soil in the third year after solarization. Solarization has potential as a component in an integrated pest management program of root diseases in raspberry and strawberry production, particularly within the first 2 years following planting.

Soil solarization reduces arbuscular mycorrhizal fungi as a consequence of weed suppression.

Soil solarization, the process of heating soil by covering fields with clear plastic, is a promising method to reduce populations of soilborne pests and weeds without the use of pesticides. However, the destruction of beneficial organisms such as arbuscular mycorrhizal (AM) fungi also may occur, thereby reducing positive effects of solarization. We compared the effects of solarization and chemical fumigants on the survival of indigenous AM fungi in 1995 and 1996. The infectivity of AM fungi was monitored before and after solarization using a greenhouse bioassay with Sorghum bicolor L. for both years. AM colonization of roots was also monitored in the field 8 months after solarization in 1995. Weed densities were measured 8 months after treatment in 1996. Solarization increased the average daily soil temperature 6-10°C and the maximum soil temperature reached by 10-16°C (5-20 cm depth). Solarization did not reduce the infectivity of AM fungi immediately after the solarization period in either year, as determined by the greenhouse bioassay. Infectivity was greatly reduced in solarized plots 8 months after solarization (over winter) in both years as assessed in the field (1995) or with the greenhouse bioassay (1996). Fumigation with metam sodium at 930 l ha(-1) (350 kg active ingredient ha(-1)) reduced the infectivity of AM fungi in both years, and fumigation with methyl bromide at 800 kg ha(-1) eliminated infection by AM fungi. Solarization was as effective as methyl bromide and metam sodium at 930 l ha(-1) in controlling winter annual weeds measured 8 months after treatment. Solarization apparently reduced AM fungi in soil indirectly by reducing weed populations that maintained infective propagules over the winter. Fumigation with metam sodium or methyl bromide directly reduced AM fungi in soil.

Effect of Soil Solarization and Cover Crops on Populations of Selected Soilborne Plant Pathogens in Western Oregon.

Field experiments were conducted in silty-clay loam in Corvallis, OR during the summers of 1995 and 1996 to study the effects of green manure cover crops (Sudan grass, rape, and barley), soil solarization, soil fumigation, and combinations of those treatments on population densities of soil pathogens Verticillium dahliae, Phytophthora cinnamomi, Pratylenchus penetrans, and Agrobacterium rhizogenes. Nylon mesh bags containing soil infested with V. dahliae and Phytophthora cinnamomiwere buried 5, 10, 20, and 30 cm deep. Soil solarization was performed over a 54- to 59-day period using a 0.6-mil clear polyethylene film. Maximum soil temperatures recorded at depths of 5, 10, 20, and 30 cm were 53, 48, 39, and 34°C in solarized soil, respectively; these temperatures were 8 to 16°C higher than in corresponding nonsolarized plots. Soil samples were collected before, during, and after solarization to quantify pathogen populations at those four depths. Pot or field studies were conducted subsequent to treatments to determine the effects of treatments on susceptible plants. Soil solarization, cover crops plus solarization, or fumigation with metam sodium resulted in a significant decrease (P< 0.05) in density of P. cinnamomi populations at all four depths and reduced (P< 0.05) V. dahliae at 5 and 10 cm. In greenhouse assays of solarized soils, disease severity was reduced (P< 0.05) for Verticillium spp. on eggplant and Phytophthora spp. on snapdragons. Cover crops alone were not effective in reducing P. cinnamomi and V. dahliae populations. Agrobacterium spp. population densities declined within solarized plots and incidence of crown gall on 'Mazzard' cherry rootstock planted in solarized plots was reduced significantly. Population densities of Pratylenchus penetranswere reduced in the upper 30-cm soil profile by solarization.Solarization for an 8-week period during the warmest months of summer could provide an additional management alternative for several important soilborne pathogens in western Oregon.

Plant-Parasitic Nematodes Associated with Grapevines, Vitis vinifera, in Oregon Vineyards.

A survey of vineyards in western Oregon was conducted in 1994 and 1995 to determine the association of plant-parasitic nematodes with vine health. Seventy vineyards in four regions of western Oregon (16 to 21 vineyards per region) were sampled. The regions were the northern, middle, and southern Willamette Valley, and southern Oregon. Vineyards were selected and partitioned into blocks by variety, age of planting, crop history, and soil characteristics. Mesocriconema xenoplax, Xiphinema americanum, Pratylenchus spp., and Paratylenchus spp. were recovered from more than 85% of the vineyards; only 10% of vineyards had detectable populations of Meloidogyne hapla. Mesocriconema xenoplax and X. americanum were found in 20% and 8% of vineyard blocks, respectively, at population densities reported to cause moderate yield loss in California. Mesocriconema xenoplax was found at greatest population densities in vineyards older than 10 years and on former Prunus orchard sites in the northern Willamette Valley. Populations of Mesocriconema xenoplax and X. americanum were associated with both healthy and stunted vines. The long-term impact of M. xenoplax, X. americanum, and other nematodes on Oregon vineyard production has not yet been determined.

Maturation and Seasonal Discharge Pattern of Ascospores of Anisogramma anomala.

ABSTRACT Maturation and release of ascospores of Anisogramma anomala were monitored over a 6-year period (1988 to 1995) in European hazelnut orchards located in western Oregon. Perithecia of A. anomala were dissected from stromata collected monthly from September to May to determine spore maturation. Spore maturation began in late summer; by January, >90% of the spores were morphologically mature. Similarly, both the number of mature ascospores per perithecium and the proportion of ascospores that germinated increased through autumn. After January, the number of spores per perithecium declined until May, when few viable spores remained. Each of the 6 years, rain catch-type spore traps were placed under cankers in diseased trees from 15 September to 30 June. Based on spore collection periods of 1 to 4 weeks, three patterns for the seasonal release of A. anomala ascospores were observed: in the 1988-1989 season, >80% of the seasonal ascospore release occurred between September and January; in the 1989-1990 season, 32 to 42% of the seasonal ascospore release occurred after mid-April; and in the other 4 years, monthly releases of ascospores were relatively uniform over the 9-month seasonal period. Timing and amount of precipitation were the most important variables accounting for the differences among the yearly patterns of ascospore release. Over all years and sites, the cumulative proportion of total ascospores collected in each orchard was highly correlated (R(2) = 0.90) with cumulative precipitation. This relationship was confirmed in mist chamber experiments. A regression model was developed relating cumulative ascospore release to cumulative hours of precipitation. The model provides an estimate of the proportion of ascospores remaining to be released after budbreak, which coincides with the period of highest susceptibility to infection.

Factors Affecting the Release of Ascospores of Anisogramma anomala.

ABSTRACT Relationships between environmental factors and release of ascospores of Anisogramma anomala, the causal agent of eastern filbert blight, were examined in four European hazelnut (Corylus avellana) orchards during a 2-year period. In each orchard, Burkhard volumetric spore traps and automated weather-monitoring equipment were deployed for 12-week periods beginning at budbreak, when hazelnut becomes susceptible to infection. Ascospores of A. anomala were released when stromata on the surface of hazelnut branches were wet from rain but not from dew. Release of ascospores ceased after branch surfaces dried. The duration of free moisture on branch surfaces regulated the initiation and rate of ascospore release, but no significant effects of temperature, relative humidity, wind, or light on ascospore release were apparent. Most (>90%) ascospores were captured during precipitation events that exceeded 20 h in duration, which represented about 10% of the total precipitation events each season. Quantitative relationships between the hourly capture of A. anomala ascospores and hours since the beginning of a precipitation event were developed. With the onset of precipitation, the hourly rate of ascospore capture increased until the fifth hour of rain, remained relatively constant between the fifth and twelfth hours, and then declined gradually. During the 12-week spore-trapping periods, the likelihood and rates of ascospore release associated with precipitation were highest at budbreak and then declined through April and May until early June, when the reserve of ascospores in the perithecia was depleted. Large numbers of ascospores were captured in the volumetric spore traps, indicating that ascospores may be commonly dispersed long distances on air currents as well as locally by splash dispersal within the canopy, as reported previously. The results indicate that monitoring seasonal precipitation patterns may be useful for estimating the quantity and temporal distribution of airborne inoculum during the period that the host is susceptible to infection.