Suppression of Powdery Mildews by UV-B: Application Frequency and Timing, Dose, Reflectance, and Automation.

Disease-suppressive effects of nighttime applications of ultraviolet-B (UV-B) were investigated at two irradiance levels (1.6 or 0.8 W/m2) in strawberry and rosemary plants inoculated with Podosphaera aphanis or Golovinomyces biocellatus, respectively. Plants were exposed to each irradiance level every third night for either 9 or 18 min, every night for either 3 or 6 min, or three times every night for either 1 or 2 min. Thus, over time, all plants received the same cumulative dose of UV-B, and severity of powdery mildew was reduced by 90 to 99% compared with untreated controls in both crops. Use of polished aluminum lamp reflectors and UV-B reflective surfaces on greenhouse benches significantly increased treatment efficacy. An automated apparatus consisting of an adjustable boom with directed airflow was used to move UV-B lamps over greenhouse benches at 25 or 50 cm/min. Directed airflow moved leaves on the subtending plants to better expose upper and lower surfaces to UV-B but directed airflow actually decreased the efficacy of UV-B treatments, possibly by dispersing conidia from lesions before they were exposed to a lethal dose of UV-B. Results indicate broad applicability of nighttime applications of UV-B to suppress powdery mildews, and that cumulative UV-B dose is an overriding factor determining efficacy. Finally, enhanced suppression on shaded or obscured tissues is more likely to be affected by reflective bench surfaces than through attempts to physically manipulate the foliage.

Suppression of Cucumber Powdery Mildew by Supplemental UV-B Radiation in Greenhouses Can be Augmented or Reduced by Background Radiation Quality.

This study demonstrates that the spectral quality of radiation sources applied with ultraviolet-B (UV-B; background radiation) affects the suppression of cucumber powdery mildew (Podosphaera xanthii) by UV-B. Suppression provided by daily UV-B exposure of 1 W/m2 for 10 min was greatest in the presence of red light or by a complete lack of background light, and powdery mildew suppression was least in the presence of ultraviolet-A (UV-A) or blue radiation compared with plants exposed only to 16 h of daily natural light supplemented with high-pressure sodium lamps that supply broad-spectrum radiation with peaks in the yellow-orange region. Exposure of powdery mildew-inoculated plants to supplemental red light without UV-B, beginning at the end of the daylight period, also reduced disease severity; however, supplemental blue light applied in the same fashion had no effect. Daily application of UV-B at 1 W/m2 beginning on the day of inoculation significantly reduced the severity of powdery mildew to 15% compared with 100% severity on control plants. Maximum suppression of powdery mildew was observed following 15 min of exposure to UV-B (1.1% severity compared with 100% severity on control plants) but exposure time had to be limited to 5 to 10 min to reduce phytotoxicity. There was no additional disease suppression when plants were exposed to UV-B beginning 2 days prior to inoculation compared with plants exposed to UV-B beginning on the day of inoculation. UV-B inhibited germination, infection, colony expansion, and sporulation of P. xanthii. The results suggest that efficacy of UV-B treatments, alone or in combination with red light, against P. xanthii can be enhanced by exposure of inoculated plants to these wavelengths of radiation during the night, thereby circumventing the counteracting effects of blue light and UV-A radiation. The effect of UV-B on powdery mildew seemed to be directly upon the pathogen, rather than induced resistance of the host. Night exposure of plants to 5 to 10 min of UV-B at 1 W/m2 and inexpensive, spectral-specific, light-emitting diodes may provide additional tools to suppress powdery mildews of diverse greenhouse crops.

Colletotrichum acutatum, Agent of Anthracnose on the New Host Black Elderberry (Sambucus nigra) in Switzerland.

Black elderberry (Sambucus nigra L.) is grown in Switzerland for flower and fruit production. Flowers are used for candy and syrup production, whereas the fruits are directly consumed as berries. In autumn 2008, the diagnostic laboratory of Agroscope ACW received a sample of strongly shriveled elderberry fruits from the extension office of the canton of St. Gallen. The sample originated from an experimental plot at Flawil, where 80% of the berries exhibited these symptoms. In years with high rainfall, infections of 100% of the berries can be observed in the production areas of Switzerland. Symptoms of anthracnose are only visible on the fruits, but not on the other plant organs. Berries start to shrivel when turning from green to black, and sporulation can be observed on ripe fruits under humid weather conditions. The sample was incubated in a moist chamber at room temperature, where it formed abundant acervuli producing salmon-colored spores at the fruit surface. Isolation from the acervuli on potato dextrose agar (PDA) containing an antibiotic (chlortetracycline) resulted in the growth of white to grey mycelium with salmon-colored spore masses. The reverse side of the PDA was red to violet. One-celled conidiospores were primarily fusiform, with an average size of 16.5 × 4 µm. Based on these morphological traits, the pathogen was previously identified as Colletotrichum acutatum J. H. Simmonds (2). A PCR using the primers CaInt2 and ITS4 (1) was run on a pure culture of the isolate from elderberry and confirmed this identification. A pathogenicity test was conducted from May to August 2010. The isolate from black elderberry and an isolate of C. acutatum from highbush blueberry (Vaccinium corymbosum) were multiplied separately on PDA on the laboratory bench (23 ± 2°C) for one week. Conidiospore suspensions of each isolate were prepared with 0.9% sterile NaCl solution and were adjusted to 1.2 × 106 spores/ml. Flower clusters of a single black elderberry tree at the Agroscope Research Center were inoculated at full flowering stage on May 26, 2010. Two sets of three healthy clusters were sprayed separately with the two spore suspensions until run-off. Spraying three healthy clusters with a sterile 0.9% NaCl solution served as control treatment. Immediately after inoculation, flower clusters were enclosed individually in transparent polyethylene bags for 2 days. To avoid excessive temperature inside the bags caused by solar radiation, the bagged flower clusters were placed below the leaves of the elderberry tree. During the 2 days, the average air temperature measured at the research center was 17 ± 2.5°C. Bags were removed and fruits of the treated clusters were harvested on July 27, 2010. Each cluster was incubated individually in a moist chamber on the laboratory bench (23 ± 2°C) for 10 days. Abundant formation of acervuli producing salmon-colored spores occurred on the fruits inoculated with either strain of C. acutatum. No such symptoms were produced on incubated fruits of the control treatment. From acervuli of the inoculated fruits, C. acutatum was reisolated on PDA. To our knowledge, this is the first report of C. acutatum on black elderberry. In Switzerland, a fungicide containing the active ingredient trifloxystrobine is registered to control C. acutatum on black elderberry. References: (1) S. K. Sreenivasaprasad et al. Plant Pathol. 45:650, 1996. (2) B. C. Sutton. The Coelomycetes. CAB International, Wallingford, UK, 1980.

Suppression of Powdery Mildew (Podosphaera pannosa) in Greenhouse Roses by Brief Exposure to Supplemental UV-B radiation.

Ultraviolet (UV)-B (280 to 315 nm) irradiance from 0.1 to 1.2 W m-2 and exposure times from 2 min to 2 h significantly suppressed powdery mildew (Podosphaera pannosa) in pot rose (Rosa × hybrida 'Toril') via reduced spore germination, infection efficiency, disease severity, and sporulation of surviving colonies. Brief daily exposure to UV-B suppressed disease severity by more than 90% compared with unexposed controls, and severity was held at low levels as long as daily brief exposures continued. Selective removal of wavelengths below 290 nm from the UV lamp sources by cellulose diacetate filters resulted in significant reduction of treatment efficacy. Exposure of plants to 2 h of UV-B during night for 1 week followed by inoculation with P. pannosa did not affect subsequent pathogen development, indicating that the treatment effect was directly upon the exposed pathogen and not operated through the host. Following 20 to 30 days of exposure, chlorophyll and flavonoid content was slightly higher in plants exposed to the highest UV-B levels. Brief daily exposure to UV-B for 5 min at 1.2 W m-2 or 1 h at 0.1 W m-2 substantially reduced mildew severity without significant phytotoxicity, and may represent a useful nonchemical option for suppression of powdery mildew in greenhouse roses and, possibly, other crops.

Yellow Dryad, a New Host Plant of Colletotrichum acutatum in Switzerland.

In spring 2008, yellow dryad (Dryas drummondii L.), an ornamental plant, was studied at the Research Center Conthey (Switzerland) for its possible use by the cosmetical industry. Plants grown from wild-type seeds were multiplied by transplanting cuttings in pots that were later transplanted in an experimental field plot. Before field planting, partial wilting occurred on several plants. The petioles of affected leaves appeared dry, tan, and covered with black acervuli containing black setae. Isolation from the acervuli on potato dextrose agar (PDA) containing chlortetracycline (25 mg/liter) resulted in the growth of white-to-gray mycelium containing salmon-colored conidial masses but no setae. The underside of the colony was carmine red. Conidia were primarily fusiform with an average size of 13 × 4 µm. On the basis of these morphological traits, the pathogen was identified as Colletotrichum acutatum J.H. Simmonds (2). PCR using the species-specific primer CaInt2 in conjunction with the conserved primer ITS4 (3) was conducted on genomic DNA from a single-spore isolate. An isolate of C. acutatum from strawberry was included as a positive control. This primer pair produced a 490-bp fragment that confirmed the identification based on morphology. A pathogenicity test was conducted at the end of August and beginning of September 2009. The single-spore isolate from yellow dryad and a single-spore isolate of C. acutatum from highbush blueberry (Vaccinium corymbosum located at Dürrenroth, Switzerland) were grown on PDA at 23 ± 2°C for 1 week. Conidial suspensions were prepared with 0.9% sterile NaCl solution and were adjusted to 3 to 5 × 105 spores/ml. For each spore suspension, five 3- to 5-cm tall, healthy, yellow dryad plants in the rosette stage were sprayed until runoff. Spraying five plants with a sterile 0.9% NaCl solution served as control treatment. Immediately after inoculation, plants were covered individually with a polyethylene bag and incubated at 23 ± 2°C for 5 days. Polyethylene bags were then removed and the plants were placed outdoors under a plastic shelter (18 ± 4°C). After 1 week, plants inoculated with either strain of C. acutatum showed the same symptoms. The most prevalent symptoms on leaves were brown necroses surrounded by a dark brown margin; the necrotic lesions were covered with black acervuli without setae. Less frequent were small, brown lesions that turned gray and were covered with acervuli containing setae. Acervuli with setae also occurred frequently on the petioles of the inoculated plants. On the control plants, none of these symptoms were observed. Leaves with lesions were incubated in a humid chamber for 1 day, resulting in abundant salmon-colored sporulation from the acervuli. C. acutatum was reisolated from such spore masses on PDA. To our knowledge, this is the first report of C. acutatum on yellow dryad. Since C. acutatum is a widespread pathogen worldwide (1), it represents a potential threat to yellow dryad crops grown for ornamental and potentially cosmetical use. References: (1) S. Freeman. Page 131 in: Colletotrichum: Host Specificity, Pathology, and Host-Pathogen Interaction. D. Prusky et al., eds. The American Phytopathological Society, St. Paul, MN, 2000. (2) P. S. Gunnell and W. D. Gubler. Mycologia 84:157, 1992. (3) S. K. Sreenivasaprasad et al. Plant Pathol. 45:650, 1996.

Evaluation of Six Models to Estimate Ascospore Maturation in Venturia pyrina.

Estimates of ascospore maturity generated by models developed for Venturia pyrina in Victoria, Australia (NV and SV), Oregon, United States (OR), and Italy (IT) or for V. inaequalis in New Hampshire, United States (NH-1) or modified in Norway (NH-2) were compared with observed field ascospore release of V. pyrina from 21 site-year combinations. The models were also compared with ascospore release data from laboratory assays. In the laboratory assays, the forecasts of the NH-1 and NH-2 models provided the best fit to observed spore release. Under field conditions, the lag phases and slope coefficients of all models differed from those of observed release of ascospores. Identifying the precise time of bud break of pear to initiate degree-day accumulation was problematic at both Australian sites. This resulted in a higher deviance between bud break and first released ascospore compared with the sites in Norway and Belgium. Linear regressions of observed release against forecasted maturity generated similarly high concordance correlation coefficients. However, where differences were noted, they most often favored models that included adjustment for dry periods. The NH-2, IT, and NV models using pooled data also provided the most accurate estimates of 95% ascospore depletion, a key event in many disease management programs.

Specific Light-Emitting Diodes Can Suppress Sporulation of Podosphaera pannosa on Greenhouse Roses.

When rose plants bearing colonies of Podosphaera pannosa were placed in a wind tunnel, the number of conidia trapped was directly proportional to intensity of daylight-balanced (white) light from 5 to 150 µmol m-2 s-1. Illumination of samples using blue (420 to 520 nm) light-emitting diodes (LEDs) increased the number of conidia trapped by a factor of approximately 2.7 over white light but germination of conidia under blue light was reduced by approximately 16.5% compared with conidia germination under white light. The number of conidia trapped under far-red (>685 nm) LEDs was approximately 4.7 times higher than in white light, and 13.3 times higher than under red (575 to 675 nm) LEDs, and germination was not induced compared with white light. When mildewed plants were exposed to cycles of 18 h of white light followed by 6 h of blue, red, far-red light, or darkness, light from the red LEDs reduced the number of conidia trapped by approximately 88% compared with darkness or far-red light. Interrupting the above dark period with 1 h of light from red LEDs also reduced the number of conidia trapped, while a 1-h period of light from far-red following the 1 h of light from red LEDs nullified the suppressive effect of red light. Our results indicate that brief exposure to red light during the dark interval may be as effective as continuous illumination in suppressing powdery mildew in greenhouse rose plant (Rosa × hybrida).

Chalara fraxinea Isolated from Diseased Ash in Norway.

European ash (Fraxinus excelsior), also known as common ash, occurs naturally inland in lower areas of southeastern Norway and along the southern coast of the country. It is important both as a forest and ornamental tree. During the last decade, dieback has become a disastrous disease on F. excelsior in many European countries. The anamorphic fungus Chalara fraxinea T. Kowalski (1), described for the first time from dying ash trees in Poland, is now considered the cause of ash dieback (2). In May of 2008, C. fraxinea was isolated from 1.5 m high diseased F. excelsior in a nursery in Østfold County in southeastern Norway. Symptoms included wilting, necrotic lesions around leaf scars and side branches, and discoloration of the wood. From symptomatic branches, small pieces (approximately 1 cm3) were excised in the transition area between healthy and discolored wood. After surface sterilization (10 s in 70% ethanol + 90 s in NaOCl), the pieces were air dried for 1 min in a safety cabinet, cut into smaller pieces, and placed on media. The fungus was isolated on potato dextrose agar (PDA) and water agar (WA). On PDA, the cultures were tomentose, light orange, and grew slowly (21 mm mean colony diameter after 2 weeks at room temperature). Typical morphological features of C. fraxinea developed in culture. Brownish phialides (14.8 to 30.0 [19.5] × 2.5 to 5.0 [4.1] µm, n = 50) first appeared in the center of the colonies on the agar plugs that had been transferred. The agar plugs were 21 days old when phialides were observed. Abundant sporulation occurred 3 days later. Conidia (phialospores) extruded apically from the phialides and formed droplets. Conidia measured 2.1 to 4.0 (3.0) × 1.4 to 1.9 (1.7) µm (n = 50). The first-formed conidia from each phialide were different in size and shape from the rest by being longer (6 µm, n = 10) and more narrow in the end that first appeared at the opening of the phialide. Internal transcribed spacer sequencing confirmed that the morphological identification was correct (Accession No. EU848544 in GenBank). A pathogenicity test was carried out in June of 2008 by carefully removing one leaf per plant on 10 to 25 cm high F. excelsior trees (18 trees) and placing agar plugs from a 31-day-old C. fraxinea culture (isolate number 10636) on the leaf scars and covering with Parafilm. After 46 days, isolations were carried out as described above from discolored wood that had developed underneath necrotic lesions in the bark and subsequently caused wilting of leaves. All the inoculated plants showed symptoms, and C. fraxinea was successfully reisolated. No symptoms were seen on uninoculated control plants (eight trees) that had received the same treatment except that sterile PDA agar plugs had been used. References: (1) T. Kowalski. For. Pathol. 36:264, 2006. (2) T. Kowalski and O. Holdenrieder, For. Pathol. Online publication, doi: 10.1111/j.1439-0329.2008.00565.x, 2008.

First Report of Phytophthora ramorum in Ornamental Plants in Norway.

In November 2002, Phytophthora ramorum was isolated from Rhododendron catawbiense with wilted branches in a nursery in Bergen. The isolate was identified by characteristic deciduous, semipapillate sporangia, abundance of large chlamydospores, and slow growth (2). The identification was confirmed by ITS rDNA sequencing. After the first detection, the Norwegian Food Safety Authority (NFSA) started a survey of different ornamental plants during 2003. Of 21 samples from 10 locations, two rhododendron samples were positive. The rhododendron plants containing positive samples in 2002 and 2003 had been imported that same year as the disease was detected on them. In 2003, NFSA made regulations similar to those in the EU for P. ramorum, including the destruction of all infected plants and all plants susceptible to P. ramorum within a 2-m distance of the infected ones. The production of rhododendron in Norwegian nurseries is limited, and most rhododendrons marketed in the country are imported from March to May from other European countries. The main sale of rhododendron occurs in May and June, often before symptoms of P. ramorum are easy to observe. In 2004, 133 samples from 53 locations were analyzed. P. ramorum was found in 29 new locations. It was detected in 57 samples of rhododendron, in one sample of Pieris japonica, and one of Kalmia sp. Symptoms on pieris were similar to those on rhododendron with blighted twigs and leaf spots. In Kalmia sp., P. ramorum was isolated from small foliar spots. In 2005, special efforts were directed to detect P. ramorum before the spring sale. Between January and May, 142 samples were analyzed (including plants from 45 import shipments) and 19 yielded positive (including six samples from five import shipments). In 2005, 370 samples from 74 nurseries and garden centers were analyzed and 97 samples from 43 locations were positive (all were rhododendron). Ten of the 43 locations had been positive in 2004. Some of the samples that yielded positive in the summer and autumn came from import shipments or nurseries controlled earlier and found free from P. ramorum. As suggested previously, the disease is probably moving in trade as symptom-free plants (1) and also likely in batches with few infected plants with mild infections that are difficult to detect when random control is carried out in large shipments. Most nurseries receive new plants every year. It is thus difficult to determine if it is a reintroduction or an eradication failure when a nursery yields positive to P. ramorum in two consecutive years. In 2005, P. ramorum was detected on well-established Viburnum fragrans and rhododendron plants in a private garden in Bergen. The viburnum plants of this garden were heavily infected, with wilting of whole branches from the root collar to the top. The pathogen was also found on established rhododendron shrubs in four public greens in Bergen and two in Stavanger. The two cities are located at the southwestern coast of Norway and have more than 2,000 mm of annual precipitation, cool summers, and mild winters. The pathogenicity of 26 isolates from rhododendron, one from pieris, and one from Kalmia sp. was tested by placing mycelial plugs (18 isolates) or drops of zoospore suspension (7 isolates) on the unwounded abaxial surface of rhododendron leaves of cv. Cunninghams White. After 7 days, all isolates produced lesions larger then 2 cm at each inoculation site. P. ramorum was reisolated from the leaves. References: (1) C. M. Brassier et al. Mycol Res. 108:1107, 2004. (2) S. Werres et al. Mycol. Res. 105:1155, 2001.

First Report of Root Rot and Stem Canker Caused by Phytophthora cambivora on Noble Fir (Abies procera) for Bough Production in Norway.

In 2004, damages resembling those caused by Phytophthora spp. were observed in a 15-year-old bough plantation of noble fir (Abies procera). When removing bark upward from the roots and base of a diseased tree, a reddish brown discoloration with distinct borders to surrounding wood appeared. The discoloration extended approximately 1.5 m above ground, but only on one side of the stem. This resulted in dead basal branches (flagging) on the cankered side of the tree. Other dying trees in the same field did not show flagging symptoms but turned chlorotic to brown after being girdled by the expanding stem canker. Approximately 25% of the trees were dead or dying. Isolations were carried out from the area between healthy and diseased tissue both from roots and base of the stem of the tree with flagging symptoms. Samples were rinsed in running tap water and plated on the Phytophthora selective medium PARP (17 g of cornmeal agar, 10 mg of pimaricin, 250 mg of ampicillin, 10 mg of rifampicin, and 100 mg of pentachloronitrobenzene (PCNB) in 1 liter of water), with and without hymexazol added (50 mg/l). Morphological characters of the isolated Phytophthora sp. included nonpapillate sporangia (37 to 64 µm), internal proliferation, and characteristic hyphal swellings. The isolate was heterothallic and produced amphigynous antheridia when crossed with tester strains of P. cryptogea. The mating type was A2. The internal transcribed spacer (ITS) rDNA sequences were identical to P. cambivora (GenBank Accession No. AY880985). Thus, both morphological characters and DNA analysis supported the species identification. A pathogenesis test to fulfill Koch's postulate was carried out during 2005. Inoculation was done by placing agar with culture in the growth medium close to the roots of noble fir seedlings. Eleven weeks after inoculation, clearly visible stem canker symptoms were observed. The ITS sequences of the reisolated Phytophthora sp. were determined and found identical to P. cambivora. P. cambivora was reported to cause root rot and stem canker in a noble fir Christmas tree plantation in the United States (1). P. citricola and P. citrophthora are known to cause problems on Lawson Falsecypress/Port-Orford-cedar (Chamaecyparis lawsoniana) in Norway, but damages by Phytophthora spp. have never been reported in Abies spp. plantations or forest stands in Norway. Currently, we are also working on Phytophthora problems discovered in two different Christmas tree plantations (A. lasiocarpa and A. nordmanniana). Reference: (1) G. A. Chastagner et al. Plant Dis. 79:290, 1995.

Induced Resistance as a Possible Means to Control Diseases of Strawberry Caused by Phytophthora spp.

Two putative elicitors of disease resistance (acibenzolar-S-methyl and chitosan) were tested for their effect on crown rot (Phytophthora cactorum) in strawberry. The effect of both compounds was enhanced when the time between treatment and inoculation was prolonged from 2 to 20 days. There were no significant differences between treatments when the concentration of acibenzolar-S-methyl was increased from 10 to 1,000 µg a.i./plant. The lowest tested concentrations of chitosan (10 and 50 µg a.i./plant) resulted in a lower disease score compared with the highest concentrations (250 or 1,000 µg a.i./plant). There were no differences in disease score between treatment with fosetyl-Al, acibenzolar-S-methyl, or chitosan when applied 5 or 15 days before inoculation. The effect of acibenzolar-S-methyl and chitosan also was tested against P. fragariae var. fragariae in alpine strawberry (Fragaria vesca var. alpina cv. Alexandria). Chitosan had no effect, whereas fosetyl-Al and all treatments with acibenzolar-S-methyl (50 or 250 µg a.i./plant; 5, 10, 20, or 40 days before inoculation) reduced the severity of the disease. There were no significant differences between acibenzolar-S-methyl and fosetyl-Al when applied at the same time. Acibenzolar-S-methyl and chitosan at concentrations of 0.5, 5, 50, and 500 µg a.i. ml-1 in V8 juice agar were tested for possible effects on P. cactorum and P. fragariae var. fragariae in vitro. Only chitosan at concentrations of 50 and 500 µg a.i. ml-1 had a growth-retarding effect on P. cactorum. Both acibenzolar-S-methyl and chitosan at a concentration of 500 µg a.i. ml-1 reduced the growth rate of P. fragariae var. fragariae.

Antagonism of Nutrient-Activated Conidia of Trichoderma harzianum (atroviride) P1 Against Botrytis cinerea.

ABSTRACT The effect of preliminary nutrient activation on the ability of conidia of the antagonist Trichoderma harzianum (atroviride) P1 to suppress Botrytis cinerea was investigated in laboratory, greenhouse, and field trials. Preliminary nutrient activation at 21 degrees C accelerated subsequent germination of the antagonist at temperatures from 9 to 21 degrees C; at >/=18 degrees C, the germination time of preactivated T. harzianum P1 conidia did not differ significantly from that of B. cinerea. When coinoculated with B. cinerea, concentrated inocula of preactivated but ungerminated T. harzianum P1 conidia reduced in vitro germination of the pathogen by >/=87% at 12 to 25 degrees C; initially quiescent conidia achieved this level of suppression only at 25 degrees C. Application of quiescent T. harzianum P1 conidia to detached strawberry flowers in moist chambers reduced infection by B. cinerea by >/=85% at 24 degrees C, but only by 35% at 12 degrees C. Preactivated conidia reduced infection by >/=60% at 12 degrees C. Both quiescent and preactivated conidia significantly reduced latent infection in greenhouse-grown strawberries at a mean temperature of 19 degrees C, whereas only preactivated conidia were effective in the field at a mean temperature of 14 degrees C on the day of treatment application. An antagonistic mechanism based on initiation of germination in sufficiently concentrated inocula suggests that at suboptimal temperatures the efficacy of Trichoderma antagonists might be improved by conidia activation prior to application.

First Report of Colletotrichum acutatum in Strawberry in Norway.

Anthracnose caused by Colletotrichum acutatum J. H. Simmonds was detected in strawberry (Fragaria × ananassa Duch.) for the first time in Norway in 1999. Symptoms were found in greenhouse grown strawberries in the cultivar Korona. Symptoms were typical of strawberry anthracnose: sunken, brown, and firm lesions appeared on maturing fruits. Masses of conidia were produced in acervuli in the center of lesions. The fungus was isolated on acidified potato dextrose agar. Colonies grown on potato dextrose agar (PDA) were pale to mouse gray and became dark greenish to blackish in reverse. Conidia were formed in orange to salmon pink masses in the center of the culture. Conidia in cultures were 16.5 (13.8 to 18.8) × 4.5 (3.8 to 5) µm, and were hyaline, cylindrical, with pointed ends, and aseptate. Setae were never observed in culture or on fruits. The fungus did not form an ascigerous stage in culture. Mycelial growth rate at 25 to 26°C on PDA was 8.1 to 8.4 mm per day. Morphological characters and growth rate were in accordance with previous reports on C. acutatum (1,2). The isolated fungus was confirmed to be C. acutatum by both the International Mycological Institute, Egham, England, and Centraalbureau voor Schimmelcultures, Baarn, the Netherlands. Koch's postulates were fulfilled by inoculating ripe and unripe fruits on strawberry plants with the isolated fungus. Fruits were either sprayed with a conidial suspension (106 conidia per ml) or slightly wounded with a needle that had been dipped in a conidial mass from a pure culture of C. acutatum. Symptoms appeared after 4 days at 20°C, and after 5 days, brown, sunken, circular lesions reached a size of 1 cm in diameter on wounded, ripe fruits. In unripe fruits the lesions developed more slowly, and in unwounded fruits sprayed with a conidial suspension, large, irregular spots developed. Leaves were inoculated by placing a small block of agar at the base of petioles on intact strawberry plants. The tissue underneath the agar was either unwounded or slightly wounded with a needle. After 20 days (at 20 to 25°C) some necrosis developed on both unwounded and wounded petioles. No symptoms were observed in the crown tissue where the inoculated petioles were attached. The fungus was readily reisolated from both fruits and petioles, after which typical morphological characters developed in culture as described above. References: (1) P. S. Gunnell and W. D. Gubler. Mycologia 84:157, 1992. (2) B. J. Smith and L. L. Black. Plant Dis. 74:69, 1990.

Investigation on fungicide residues in greenhouse-grown strawberries.

Maximum residue limits (MRL's) for different agricultural food products in Norway are harmonized with EU standards. In field-grown strawberries in Norway, tolylfluanid has a 7 day quarantine from last application to harvest, while other approved fungicides have 14 days quarantine. Greenhouse production of strawberries is newly introduced to the country. Residue levels in strawberries of the cultivar Korona grown in a commercial greenhouse were investigated 4, 7, and 14 days after application of eight different fungicides at rates recommended by the manufacturers and at half rates. Iprodione, tolylfluanid, and vinclozolin were tested in two experiments, while chinomethionat, chlorothalonil, imazalil, penconazole, and triadimefon were tested once. For chinomethionat, imazalil, iprodione, penconazole, and vinclozolin, the residue levels were below MRL 2 weeks after application. Application of triadimefon in normal rate gave residues below MRL 14 days after application. However, its metabolite, triadimenol, was above MRL at the same time. Tolylfluanid gave very high residue levels, and except from half concentration in the second experiment, all other residue levels were above MRL. Seven days after application, residues in both experiments were approximately 3 times higher than MRL when normal rate of tolylfluanid was applied. For chlorothalonil at the recommended rate, the residue level was above MRL at any sampling time, while half rate gave residues below MRL 14 days after treatment. In view of the present results, tolylfluanid, chlorothalonil, and triadimefon will need longer time from last application to harvest and/or reduced application rates in greenhouse-grown compared to field-grown strawberries. In addition or as an alternative, recommended rates could be lowered.

Influence of Light, Relative Humidity, and Maturity of Populations on Discharge of Ascospores of Venturia inaequalis.

ABSTRACT Ascospore release in 20 populations of Venturia inaequalis was generally suppressed in wind tunnel tests during darkness and simulated rain, but the following relieved this suppression: (i) exposure to low relative humidity during simulated rain and (ii) protracted incubation of leaf samples and the consequent senescence of the pathogen population. No counterpart to (i) was observed under orchard conditions. Although V. inaequalis also released a high percentage of ascospores during darkness in field studies under simulated rain late in the season of ascospore release, this phenomenon has not been reported for natural rain events. A threshold value of 0.5 muW/cm(2) at 725 nm was identified as the minimum stimulatory light intensity. Ascospore release increased with increasing light intensity from 0.5 to 5.2 muW/cm(2) at 725 nm. There was also an intrinsic increase in ascospore release as duration of rain increased. In orchards, the combined impact of both processes is probably responsible for a delay in reaching peak ascospore release at several hours after sunrise. Ascospore release during darkness will generally constitute a small proportion of the total available supply of primary inoculum. Significant ascospore release, and therefore infection periods, can be assumed to begin shortly after sunrise, when rain begins at night in orchards with low potential ascospore dose (PAD). A PAD level of 1,000 ascospores per m(2) of orchard floor per season is suggested as a threshold, above which the night-released ascospores should not be ignored.

Ascospore Release and Infection of Apple Leaves by Conidia and Ascospores of Venturia inaequalis at Low Temperatures.

ABSTRACT Mills' infection period table describes the number of hours of continuous leaf wetness required at temperatures from 6 to 25 degrees C for infection of apple leaves by ascospores of Venturia inaequalis and reports that conidia require approximately two-thirds the duration of leaf wetness required by ascospores at any given temperature. Mills' table also provides a general guideline that more than 2 days of wetting is required for leaf infection by ascospores below 6 degrees C. Although the table is widely used, infection times shorter than those in the table have been reported in lab and field studies. In 1989 a published revision of the table eliminated a potential source of error, the delay of ascospore release until dawn when rain begins at night, and shortened the times reported by Mills for ascospore infection by 3 h at all temperatures. Data to support the infection times below 6 degrees C were lacking, however. Our objective was to quantify the effects of low temperatures on ascospore discharge, ascospore infection, and infection by conidia. In two of three experiments at 1 degrees C, the initial release of ascospores occurred after 131 and 153 min. In the third experiment at 1 degrees C, no ascospores were detected during the first 6 h. The mean time required to exceed a cumulative catch of 1% was 143 min at 2 degrees C, 67 min at 4 degrees C, 56 min at 6 degrees C, and 40 min at 8 degrees C. At 4, 6, and 8 degrees C, the mean times required to exceed a cumulative catch of 5% were 103, 84, and 53 min, respectively. Infection of potted apple trees by ascospores at 2, 4, 6, and 8 degrees C required 35, 28, 18, and 13 h, respectively; substantially shorter times than previously were reported. In parallel inoculations of potted apple trees, conidia required approximately the same periods of leaf wetness as ascospores at temperatures from 2 to 8 degrees C, rather than the shorter times reported by Mills or the longer times reported in the revision of the Mills table. We propose the following revisions to infection period tables: (i) shorter minimum infection times for ascospores and conidia at or below 8 degrees C, and (ii) because both ascospores and conidia are often present simultaneously during the season of ascospore production and the required minimum infection times appear to be similar for both spore types, the adoption of a uniform set of criteria for ascosporic and conidial infection based on times required for infection by ascospores to be applied during the period prior to the exhaustion of the ascospore supply. Further revisions of infection times for ascospores may be warranted in view of the delay of ascospore discharge and the reduction of airborne ascospore doses at temperatures at or below 2 degrees C.