Strawberry Nursery Research Articles

Suppression of Strawberry Anthracnose by Paenibacillus polymyxa TP3 In Situ and from a Distance.

Strawberry is a popular fruit with valuable nutrition and an attractive fragrance, but its production and propagation are limited by various diseases, including anthracnose and gray mold. For disease management, biological control measures are environmentally friendly and good alternatives to fungicides to avoid crop losses, reduce carbon emissions, and improve food safety. In this study, Paenibacillus polymyxa TP3, which originated from the strawberry phyllosphere, was shown to antagonize the anthracnose fungal pathogen Colletotrichum siamense and reduce leaf symptoms on strawberry plants. Several mass spectra corresponding to fusaricidin were detected in the confrontation assay of P. polymyxa TP3 and C. siamense by image mass spectrometry. The transcription of fusA and fusG in the fusaricidin biosynthesis gene cluster increased while P. polymyxa TP3 was cultured in the medium containing the culture filtrate of C. siamense, as detected by reverse-transcription polymerase chain reaction, indicating the involvement of fusaricidins in P. polymyxa TP3 antagonism against the anthracnose pathogen. Further disease control assays demonstrated the time frame and spatial mode of P. polymyxa TP3-induced systemic resistance of strawberry against C. siamense. The transcript level of the marker gene FaPDF1.2 of the jasmonic acid pathway increased in strawberry leaves after drenching treatment with P. polymyxa TP3, and the callose deposition was enhanced by further flg22 treatment. In addition, P. polymyxa TP3 treatments of the strawberry mother plants reduced C. siamense infection in the daughter plants, which would be a potent feature for the application of P. polymyxa TP3 in strawberry nurseries and fields to reduce the impact of diseases, especially anthracnose.

Telecontrolling Microgreen Indoor Nursery Strawberry

Strawberry is a horticultural commodity that produces a high economy, but in 2014 strawberry production decreased by 34.83%. The reason for the decrease in production is the unstable temperature and humidity conditions due to global warming. Another factor that causes strawberries to fail is due to the slowness of farmers in knowing changes in temperature and humidity in strawberry. The method used in this study was to compare the results of special treatment on microgreen strawberry nurseries indoor using natural vegetative methods and outdoor. System telecontrolling strawberry nursery real time with temperature and humidity parameters using a DHT22 sensor, soil moisture using a soil moisture sensor, soil pH using a soil pH sensor, time using RTC and light intensity using a light intensity sensor BH1750. The results of strawberry nurseries with telecontrolling an indoor plant height of 9 cm and a plant width of 12 cm, results of strawberry nurseries without telecontrolling treatment outdoor with a plant height of 7.5 cm and a plant width of 7 cm.

Survival of Xanthomonas fragariae on Common Materials.

Xanthomonas fragariae causes strawberry angular leaf spot (ALS), an important disease for the strawberry nursery industry in North America. To identify potential inoculum sources, the survival of X. fragariae was examined on the surfaces of 11 common materials found in nurseries: corrugated cardboard, cotton balls, cotton cloth (t-shirt), strawberry leaf, sheet metal, plastic, rubber, Tyvek, wood (balsa), glass (microscope slide), and latex (latex glove). Prefabricated rectangular samples (7.62 by 2.54 cm) of each material were immersed in a bacterial suspension for 15 min, after which the samples were stored at approximately 20°C (room temperature) or -4°C (the cold storage temperature for dormant plants in strawberry nurseries) for 1, 3, 7, 14, 30, 60, 90, 180, 270, and 365 days after inoculation (DAI). After the storage period elapsed, bacteria were recovered from the surfaces of each of the samples with phosphate-buffered saline (PBS)-soaked cotton balls. Survival rate was determined with a viability real-time quantitative PCR procedure and in a plant bioassay that involved rub inoculation of strawberry leaflets with the PBS-soaked cotton balls used to recover bacteria from the samples. Results showed that X. fragariae could survive on all surfaces but that survival rate differed among materials and storage temperature. All materials were capable of harboring viable bacteria up to 7 DAI when stored at -4°C based on the formation of lesions on inoculated leaves in the plant bioassay. The longest survival observed was 270 DAI on cardboard stored at -4°C. At room temperature, cardboard, cotton balls, cotton t-shirt, and strawberry leaf tissue supported small bacterial populations up to 14 DAI. The information from this study can be used to improve sanitation practices for ALS management in strawberry nurseries.

Diversity and Resistance to Thiophanate-Methyl of Colletotrichum spp. in Strawberry Nursery and the Development of Rapid Detection Using LAMP Method

Anthracnose is a devastating fungal disease in strawberry nurseries. Multiple Colletotrichum species are responsible for strawberry anthracnose. In this study, 105 Colletotrichum isolates were obtained from strawberry seedlings with anthracnose symptoms in fifteen nurseries located in Zhejiang province, China, and were classified based on multilocus sequence and morphological characteristic analyses. Analysis of ITS, ACT, CAL, CHS-1, and GAPDH revealed that four species within C. gloeosporioides species complex, including C. siamense (56 isolates, 53.3%), C. fructicola (37 isolates, 35.2%), C. gloeosporioides (7 isolates, 6.7%), and C. aenigma (5 isolates, 4.8%), were detected in diseased seedlings. Thiophanate-methyl is one of the benzimidazole fungicides, and has long been used to control strawberry anthracnose in China. Here, thiophanate-methyl resistance of Colletotrichum isolates was determined by the minimum inhibitory concentration (MIC) method. Our results indicated that the resistance frequency was up to 96.2%, containing 94.3% of highly resistant isolates. Only four sensitive isolates (two C. fructicola, one C. gloeosporioides, and one C. siamense isolates) and two moderately resistant isolates (one C. aenigma isolate and one C. siamense isolate) were detected. Our data indicated that the high resistance was mainly caused by the E198A mutation in the β-tubulin protein. In addition, F200Y (TTC→TAC) in the β-tubulin protein were detected in two moderately resistant isolates. Based on the point mutation at codon 198 (GAG→GCG) in the β-tubulin gene of Colletotrichum isolates, we developed a loop-mediated isothermal amplification (LAMP) assay to rapidly detect the E198A mutants. Collectively, our study indicated that four species within the C. gloeosporioides species complex were associated with anthracnose symptoms in strawberry nurseries in Zhejiang province, and serious resistance was widespread in each Colletotrichum species.

Development of a nested PCR assay for detecting Colletotrichum siamense and Colletotrichum fructicola on symptomless strawberry plants.

Anthracnose is a major disease of strawberry that seriously impacts the strawberry industry. To prevent the spread of anthracnose through symptomless plants, it is important to detect pathogenic Colletotrichum spp. at the latent infection stage in the nursery. Previous PCR-based methods developed for the diagnosis or detection of Colletotrichum acutatum and Colletotrichum gloeosporioides have used primers targeting the internal transcribed spacer region of ribosomal DNA, β-tubulin gene, or mating type gene. In this study, to specifically detect Colletotrichum siamense and Colletotrichum fructicola, the most predominant and virulent Colletotrichum species causing strawberry anthracnose in Taiwan, we conducted a comparative genomics analysis of 29 Colletotrichum spp. and identified a non-conserved 1157-bp intergenic region suitable for designing specific primers for a nested PCR assay. In silico analysis and actual tests suggested that the new nested PCR assay could detect pathogenic C. siamense and C. fructicola, but not other strawberry pathogens (Botrytis sp., Fusarium spp., Neopestalotiopsis rosae, and Phytophthora sp.) or ubiquitous saprophytes (Fusarium spp. and Trichoderma spp.). The inner to outer primer ratio was optimized to 1:10 to eliminate unexpected bands and enhance the signal. The assay could detect as little as 1 pg of C. siamense genomic DNA, which corresponds to ~15 cells. Application of the new detection assay on 747 leaf samples collected from 18 strawberry nurseries in 2019 and 2020 showed that an average of 20% of strawberry mother plants in Taiwan were latently infected by C. siamense or C. fructicola. The newly developed assay is being applied to facilitate the production of healthy strawberry runner plants in Taiwan.

Rapid detection of strawberry mottle virus using reverse transcription recombinase polymerase amplification with lateral flow strip

Strawberry mottle virus (SMoV) is one of the main RNA viruses that profoundly affects the growth of strawberries worldwide. The rapid on-site detection of SMoV described here can be applied to produce virus-free strawberry seedlings. Reverse transcriptase recombinase polymerase amplification (RT-RPA) was combined with lateral flow (LF) strip to rapidly detect SMoV. The detection limit was 500 fg of RNA under optimized conditions. The SMoV-RT-RPA-LF assay was optimal with a combination of 2 μL reverse primer (5 μM) and 0.6 μL probe (10 μM) in a 50 μL RT-RPA reaction mixture for isothermal amplification at 40 ℃ for 15 min. In addition, 100 suspected samples were collected from different regions in the Shanghai suburbs. The SMoV-RT-RPA-LF assay showed that 3 of these 100 samples were positive for SMoV, which was in good concordance with the reverse transcription polymerase chain reaction (RT-PCR) results. The primers and probe had a unique specificity to SMoV because there was no cross-reaction with other strawberry viruses. This study provides an effective technique for the rapid on-site detection of SMoV to ensure a virus-free strawberry nursery.

Evaluation of a Steam Application by a Mobile Applicator for Soil Disinfestation in Strawberry Nurseries

Soil disinfestation with steam has been evaluated in strawberry fruiting fields as a nonchemical method of soil disinfestation; however, little is known about the use of steam for field production of strawberry daughter plants. The objective of this study was to compare daughter plant production in soils previously treated with steam compared to those treated with standard methyl bromide (MB) and chloropicrin (Pic) treatments. A prototype field steam applicator and a self-propelled diesel-fueled steam generator and applicator were tested at two high-elevation nurseries near Macdoel, CA, in Sept. 2018 and Aug. 2020, respectively. The steam application heated the soil above 60 °C for ≈60 minutes to a depth of 25 cm at both nurseries. The pest control efficacy of the steam applications against weeds, Verticillium spp., Tylenchulus semipenetrans, and Pythium ultimum were similar to that of MB:Pic. The stolons and daughter plants densities in fields with steam treatment were similar to those in fields with MB:Pic treatment. Therefore, we suggest that soil disinfestation with steam may be a viable method of producing healthy strawberry plants. However, more research is needed to verify plant sanitation and quality.

Impact of Nitrate and Ammonium Ratios on Flowering and Asexual Reproduction in the Everbearing Strawberry Cultivar Fragaria × ananassa Albion

Ever-bearing (EB) strawberries are long-day cultivars that show perpetual flowering behavior. Compared to June-bearing short-day cultivars, EB cultivars can initiate flowers with less dependency on light and temperature levels. This leads to a more consistent flowering and fruiting pattern, making EB cultivars favorable for areas with long growing seasons. However, this flowering pattern also brings significant challenges to open-field strawberry nurseries. Consistent flower development in EB cultivars frequently leads to increased labor cost for manual flower removal on nursery ground. The alteration of flowering behavior via fertilizer regimes could be a cost-effective tool for strawberry nurseries. However, while it is known that the source of nitrogen (N) impacts strawberry flowering, its effect on strawberry propagation rates needs further investigation. The objective of this study was to investigate the impact of nitrate (NO3−) to ammonium (NH4+) ratio on flower and daughter plant production in the EB strawberry cultivar ‘Albion’ (Fragaria × ananassa c.v. ‘Albion’). Strawberry plants were grown in a completely randomized design under greenhouse conditions (26.6 °C, 16 h photoperiod). Four treatments of NO3−:NH4+ were implemented: (1) 100%:0%; (2) 80%:20%; (3) 60%:40%; (4) 50%:50%. Strawberry plants fertilized with a 60%:40% NO3−:NH4+ ratio produced 17–31% fewer inflorescences than those fertilized with 100%:0% (8.8 ± 1.19) and 80%:20% (10.3 ± 1.85) ratios. The production of daughter plants remained similar in all four treatments. Our results show that increased ratios of ammonium in combination with decreased ratios of nitrate reduce flowering of EB strawberry cultivars, while propagation rates remain consistent. These results could potentially lead to the development of fertilizer regimes for strawberry nurseries to reduce flower production in EB cultivars.

Resistance to Mefenoxam of Phytophthora cactorum and Phytophthora nicotianae Causing Crown and Leather Rot in Florida Strawberry.

Phytophthora cactorum and Phytophthora nicotianae cause leather rot on fruit and crown rot (PhCR) of strawberry plants. Leather rot is not a common disease in Florida; however, up to 50% yield loss has been reported in harvests after intense rainfall events. PhCR is an important disease worldwide and is characterized by a sudden wilting and collapse of plants. Mefenoxam is the most effective and widely used fungicide to control both diseases. P. cactorum and P. nicotianae isolates from leather rot and PhCR have been collected from multiple strawberry fields in Florida since 1997, and the sensitivity of 185 isolates was tested at 0, 0.05, 0.5, 5, and 100 µg/ml. The 50% effective concentration (EC50) values of sensitive isolates ranged from 0.05 to 1 µg/ml. Resistance to mefenoxam (EC50 values >100 µg/ml) was found among P. cactorum isolates collected after 2015, but no resistance was found in P. nicotianae isolates. During the 2015 to 2016, 2016 to 2017, 2017 to 2018, and 2018 to 2019 seasons, resistance was detected on 9, 10, 21, and 23% of the isolates collected, respectively. Mefenoxam-resistant isolates originated from 3 of the 24 strawberry nurseries monitored. This is the first report of the occurrence of P. cactorum resistance to mefenoxam in Florida, suggesting that alternative control strategies are needed to avoid the increase of mefenoxam-resistant populations of P. cactorum in Florida fields.

Timing of Stolon Removal Alters Daughter Plant Production and Quality in the Ever-bearing Strawberry ‘Albion’

Commercial strawberry (Fragaria ×ananassa Duch.) plants propagate through the development of stolons (runners) with attached daughter plants. While it is known that temperature and photoperiod affect strawberry propagation, little knowledge exists on whether cultural methods may influence stolon and daughter plant development. The objective of this study was to characterize the impact of three stolon removal treatments on the development of daughter plants in the ever-bearing strawberry ‘Albion’. Treatments included 1) stolon removal every 7 days, nine times total; 2) stolon removal every 21 days, three times total; and 3) one-time stolon removal after 63 days. Strawberry plants were grown in a controlled environment (26 °C, 507 μmol⋅m–2⋅s–1 photosynthetic photon flux density, 14-hour photoperiod) in soilless media and fertilized with a customized nutrient solution. Mother plants in the 63-day treatment produced more daughter plants (102 per plant), than in the 21-day treatment (33 per plant) and the 7-day treatment (16 per plant). In the 63-day treatment, daughter plants and stolons accumulated to 86.6% of the total biomass, to 42.9% in the 7-day treatment and to 60.6% of total biomass in the 21-day treatment. Mother plant organs (including roots, crown, and leaves) had less dry weight in the 63-day treatment compared with the 7-day treatment and 21-day treatment, respectively. Furthermore, the daughter plants produced at the 63-day treatment had smaller crown diameters (0.65 cm) and less dry weight (0.51 g) and a higher number of fully expanded leaves (2.9) and visible roots (13.4) compared with the 21-day treatment and the 7-day treatment. The results of this study show daughter plant production of strawberry plants declines significantly with shorter stolon removal intervals, indicating the need to adjust stolon removal in strawberry nurseries for optimal daughter plant production.

Development of a multiplex PCR for the identification of Fusarium solani and F. oxysporum in a single step

Fusarium solani and F. oxysporum are plant pathogenic fungi that cause root rot and wilt, respectively, in many economically important crops. Identification of both species by morphological characteristics is complex and time-consuming, especially when a crop is host to several species, as in the case of strawberry and other many crops. The objective of this research was to develop a molecular method for the precise and sensitive identification of Fusarium solani and F. oxysporum by multiplex PCR. Specific PCR conditions were developed for the amplification of about 170-bp PCR product from the internal transcribed spacer of F. oxysporum and a PCR product of about 650 bp from the elongation factor 1-alpha gene of F. solani. The specificity of the multiplex PCR was tested using gDNA extracted from 232 strains of F. solani and F. oxysporum, 12 strains of other Fusarium spp. and 14 strains of other genera. The sensitivity of each primer set was tested individually and simultaneously with one strain of F. oxysporum and F. solani. This protocol was then used to identify both species in natural isolates from diseased plant and soil samples from strawberry nursery. Each primer set was validated using genomic DNA from non-sterile and sterile soils artificially inoculated with both species. Under these conditions, both Fusarium species were detected and distinguished over a range of concentrations (1 × 107 to 1 × 102 conidia g−1 soil) by successful amplification of corresponding PCR products for each species. This multiplex PCR technique is a fast tool to identify natural isolates of both species, F. solani and F. oxysporum, in a single PCR reaction.

Sustainability of strawberry nurseries and fruit production in relation to fumigation practices in Europe

Sustainability of strawberry nurseries and fruit production in relation to fumigation practices in Europe

The Use of Aerated Steam as a Heat Treatment for Managing Angular Leaf Spot in Strawberry Nursery Production and Its Effect on Plant Yield

Xanthomonas fragariae, the bacterium causing angular leaf spot (ALS) of strawberry, is found routinely on strawberry nursery stock. Although ALS can be a serious disease in production fields, it can be particularly problematic in the sale and trade of commercial nursery stock because of international trade regulations. Heat treatment has been shown to be an effective treatment for managing ALS on nursery stock in small-scale experimental trials. The objective of this research was to design, build, and trial precision thermotherapy units for managing ALS for both research and commercial applications and to test a new thermotherapeutic protocol on strawberry nursery stock that combines a conditioning thermal treatment with an eradicative thermal treatment. Small-plot trials were conducted to evaluate the efficacy of precision thermotherapy on control of ALS using known sources of infected nursery stock. Additionally, trials were conducted in cooperation with commercial nurseries to determine the impact of thermotherapy on plant health and yield and on the natural development of ALS. In the small-plot trials, ALS incidence was significantly lower in plots treated with precision thermotherapy. In the commercial trials, precision thermotherapy had a variable, but negligible, effect on plant growth and yield. ALS, when it occurred, was always lower in thermotherapy-treated plots. Heat is a near-universal biocide. Thus, in addition to managing ALS, the commercial application of precision thermal therapy to strawberry nursery stock may be effective for managing a wide range of pest and disease threats to strawberry while simultaneously reducing pesticide usage. [Formula: see text] The author(s) have dedicated the work to the public domain under the Creative Commons CC0 “No Rights Reserved” license by waiving all of his or her rights to the work worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law. 2021

Pestalotia Leaf Spot and Fruit Rot of Strawberry

Pestalotiopsis-like fungi cause diseases on many different species of plants worldwide, including strawberry. The pathogen is not necessarily new to strawberry and was first reported causing fruit rot in Florida and Israel in the 1970s. However, during the 2018–19 and 2019–20 strawberry seasons, severe and unprecedented outbreaks were reported in Florida, characterized by symptoms on nearly all plant parts including roots, crowns, petioles, fruit, and leaves. This new 4-page publication of the UF/IFAS Plant Pathology Department describes the disease and its causal agent and briefly covers what is known about disease development and control practices. This document will provide valuable information to strawberry nursery and fruit production growers. Written by Juliana S. Baggio and Natalia A. Peres.https://edis.ifas.ufl.edu/pp357

Effect of chemical alternatives to methyl bromide on soil-borne disease incidence and fungal populations in Spanish strawberry nurseries: A long-term study.

Chloropicrin (PIC) mixtures of 1,3-dichloropropene and chloropicrin (DD:PIC), dazomet, and metam sodium (MS) have been applied as chemical alternatives to methyl bromide (MB) in Spanish strawberry nurseries since MB was banned as a soil fumigant in 2005. These chemical alternatives were applied to soil in two Spanish strawberry nurseries between 2003 and 2017 to test their efficacy against the main crown and root disease and soil fungal populations in comparison with the use of MB and PIC (MB:PIC). These chemicals were applied at several doses with different application methods under plastic films. Crown and root disease incidence was calculated as the percentage of plants with symptoms caused by soil-borne pathogens. Soil fungal populations were estimated as colony forming units per gram of dry soil. All chemicals significantly reduced soil-borne fungal disease incidence and fungal population in both nurseries over the years. Phytophthora cactorum and Fusarium spp. were the main pathogens causing soil-borne diseases, followed by Verticillium spp. MB:PIC remained the treatment that best controlled P. cactorum. MS and DD:PIC controlled Fusarium disease to a lesser extent than MB:PIC and dazomet in both nurseries. MB:PIC and PIC were the two treatments that most reduced Verticillium spp. The population of Verticillium spp. declined and the presence of other species such as Colletotrichum spp. and Rhizoctonia spp. was minimal during the study. Chemicals are necessary to obtain healthy strawberry plants. The use of chemical alternatives to MB has resulted in changes in the incidence of soil-borne diseases and soil fungal populations in strawberry nurseries. Dazomet was an effective alternative to MB as a soil-borne disease control, except against Verticillium spp. MB alternatives in strawberry nursery soils have caused Fusarium spp. to displace Verticillium spp.

Susceptibility of some new strawberry genotypes to infestation by western flower thrips, Frankliniella occidentalis (Pergande) (Thysanoptera: Thripidae) in the nursery

Strawberries are infested with Frankliniella occidentalis (Pergande) (Thysanoptera: Thripidae) in Egypt. This study evaluated the effects of the biochemical components and leaf surface characteristics of ten new strawberry cultivars (Fortuna, Festival, Sahary, Forintaris, Red Merlin, Winter Star, Winter Dawn, Eliana, Montary and Florida) on the levels of infestation by F. occidentalis. There were significant differences in the susceptibility of strawberry cultivars to infestation by F. occidentalis. Fortuna, Sahary, Festival and Forntaris were the most susceptible cultivars. Red Merlin, Winter Star, Winter Dawn and Eliana had intermediate infestations. Montary and Florida were the most resistant cultivars. The relationship between mean number of F. occidentalis and leaf nitrogen was not significant. There was a significant relationship between mean numbers of F. occidentalis and phosphorus, potassium and total phenols content of leaves. The highest mean numbers of F. occidentalis were found on leaves of the Fortuna cultivar with high levels of phosphorus and low levels of potassium and total phenol. The lowest mean numbers of F. occidentalis were found on the leaves of the Montary and Florida cultivars with low levels of phosphorus and high levels of total phenols and potassium. Cultivars with higher density of non-glandular trichomes had significantly fewer F. occidentalis than cultivars with lower densities of non-glandular trichomes on the upper leaf surface. Similarly, cultivars with thicker leaves had significantly fewer thrips than cultivars with thinner leaves. The proper choice of strawberry cultivar is an important component of integrated pest management programs for controlling thrips in strawberry nurseries.

Acceleration of Flower Bud Differentiation of Runner Plants in “Maehyang” Strawberries Using Nutrient Solution Resupply during the Nursery Period

The forced cultivation of strawberries (Fragaria × ananassa Duch.) requires fast flower bud differentiation. Using temporary nutrient-withholding periods is a common management practice for inducing flower bud differentiation at strawberry nurseries in the Republic of Korea. After the temporary nutrient-withholding period, nutrient solution resupply can advance both flower bud growth and fruit yield. This study aims to determine the optimal nutrient solution resupply period with anatomical analysis in order to find a method for fast flower bud differentiation in the early harvest period. Here, the runner plants were divided into 5 groups, each receiving a treatment period of watering (W) and nutrient solution (N) (W40 + N0 (control), W30 + N10, W20 + N20, W10 + N30, and W0 + N40; each number represents the days of the treatment period). The nutrient solution treatments were supplied using a strawberry nutrient solution developed by Gyeongsangnam-do Agricultural Research and Extension. Rapid flower bud differentiation was found for W20 + N20 via anatomical analysis. When the temporary nutrient-withholding period was decreased, the T-N (total nitrogen), P, K, and S concentrations showed a tendency to increase. The C/N ratio showed a tendency to decrease when the nutrient solution resupply period was increased. The W20 + N20 group showed faster flower bud development than the other groups at 10 days before transplanting and on the day of transplanting (2.2 and 5.5), 6 days in a primary cluster budding ratio, and 16 days in flowering plants. No differences in fruit characteristics were observed for the different treatments. In conclusion, the W20 + N20 treatment, which maintains fast flowering, seems to be appropriate for nutrient solution resupply treatment for “Maehyang” strawberries during the nursery period.

Strawberries at the Crossroads: Management of Soilborne Diseases in California Without Methyl Bromide.

Strawberry production has historically been affected by soilborne diseases such as Verticillium wilt. This disease was a major limiting factor in strawberry production in California in the 1950s and was the main reason that preplant soil fumigation with methyl bromide (MB) was developed in the late 1950s. MB fumigation was so successful that over 90% of the commercial strawberry fruit production in California utilized this technique. However, MB was subsequently linked to ozone depletion, and its use was phased out in 2005. The California strawberry industry was awarded exemption to the full phase-out until 2016, when all MB use in strawberry fruit production was prohibited. MB use continues in strawberry nurseries under an exemption to prevent spread of nematodes and diseases on planting stock. This review examines the impact of the MB phase-out on the California strawberry industry and evaluates the outlook for the industry in the absence of one of the most effective tools for managing soilborne diseases. New soilborne diseases have emerged, and historically important soilborne diseases have reemerged. Registration of new fumigants has been difficult and replacement of MB with a new and effective alternative is unlikely in the foreseeable future. Thus, crop losses due to soilborne diseases are likely to increase. Host plant resistance to soilborne diseases has become a top priority for strawberry breeding programs, and cultivars are increasingly selected for their resistance to soilborne diseases. The intelligent integration of a variety of management tactics is necessary to sustain strawberry production in California.

Baseline Sensitivity of Botrytis cinerea Isolates from Strawberry to Isofetamid Compared to other SDHIs.

Succinate dehydrogenase inhibitors (SDHIs) are the fungicides most commonly used to control Botrytis fruit rot on commercial strawberry in Florida. The medium-to-high risk of selection of resistance in the causal agent Botrytis cinerea is a threat to the efficacy of this fungicide group. In this study, we characterized the sensitivity of B. cinerea to the SDHI isofetamid, evaluated the SdhB gene mutation associated with resistance, and monitored resistance frequencies to five SDHI fungicides for two consecutive seasons. EC50 values of 70 isolates were obtained using the spiral gradient dilution (SGD) method and averaged 0.098 µg/ml of isofetamid. EC50 averages of 3.04 and >500.00 µg/ml were obtained for isolates with the N230I and P225F mutations indicating moderate and high resistance to isofetamid, respectively. A total of 565 B. cinerea isolates collected during 2015-2016 and 2016-2017 seasons from strawberry nurseries and Florida production fields were evaluated using conidial germination assays. Results for the first season showed resistance frequencies of 95, 33, 21, 25, and 0% to boscalid, penthiopyrad, fluopyram, benzovindiflupyr, and isofetamid, respectively. The respective resistance frequencies for the following season were 91, 95, 44, 27, and 1.3%. Only three isolates were found to be moderately resistant to isofetamid during the second season, and the mutation N230I was identified after sequence analysis. These isolates were confirmed to be resistant to isofetamid in fruit assays with disease incidence of 55.6 to 77.0%; however, the conidial production of the isolates was inhibited by an average of 83.9%. In general, isofetamid efficacy was higher than the other evaluated SDHIs, but a slight increase in resistance frequencies was observed in our study.

Anthracnose Fruit and Root Necrosis of Strawberry Are Caused by a Dominant Species Within the Colletotrichum acutatum Species Complex in the United States.

Strawberry anthracnose fruit rot and root necrosis, caused by Colletotrichum acutatum, are primary limiting factors in fruit production fields in the United States. Recent research focusing on the phenotypic and genetic characteristics of this species has shed light on the diversity of the C. acutatum species complex. In this study, we performed multilocus sequence analysis of four genetic loci to characterize 217 C. acutatum isolates collected over a 23-year period from symptomatic plant tissues of strawberry from six different states. The results revealed two Colletotrichum spp. (C. nymphaeae and C. fioriniae), with 97.7% of the isolate collection (212 of 217) belonging to C. nymphaeae as a dominant clonal linage, regardless of the isolation source. No correlation between species groups and geographical origins of the isolates was observed. Further sequence comparison between historical and contemporary isolates showed the same populations being widely distributed throughout the strawberry nurseries and production fields in the United States and Canada. Subsequently, a subset of 12 isolates representing different quinone-outside inhibitor fungicide resistance profiles from root or fruit tissue of strawberry was selected for comparison of pathogenicity on strawberry. In this test, isolates of different resistance groups or different isolation sources exhibited a similar degree of aggressiveness and caused indistinguishable symptoms on strawberry crowns (P = 0.9555 and 0.7873, respectively) and fruit (P = 0.1638 and 0.1141, respectively), although a significant difference among individual isolates was observed in detached-fruit assays (P = 0.0123). Separate pathogenicity tests using isolates of the two species revealed C. nymphaeae being more aggressive than C. fioriniae in infecting strawberry roots and crowns (P = 0.0073). Therefore, given the occurrence and pathogenicity of C. nymphaeae, this species is likely the sole cause responsible for strawberry anthracnose in the United States.