Chang-Lin Xiao

Cladosporium rot is an emerging postharvest disease of blueberries and table grapes in the Central Valley of California. To determine the causal agent(s), 308 isolates of Cladosporium spp. were collected from decayed blueberries and table grapes exhibiting Cladosporium rot symptoms. Phylogenetic analyses of the internal transcribed spacer, actin gene, and translation elongation factor 1-alpha gene, combined with morphological characterization, identified 12 Cladosporium species: C. anthropophilum, C. asperulatum, C. cladosporioides, C. delicatulum, C. floccosum, C. limoniforme, C. macrocarpum, C. ramotenellum, C. subtilissimum, C. tenellum, C. tenuissimum, and C. xylophilum. Ten species were identified with the three most prevalent species being C. asperulatum, C. cladosporioides and C. macrocarpum, accounting for 23.2, 18.9, and 34.1% of the isolates from blueberries, respectively. Seven species were identified with the three prevalent species being C. cladosporioides, C. limoniforme, and C. ramotenellum, accounting for 55.6, 22.9, and 18.1% of the isolates from table grapes, respectively. The optimal temperature for mycelial growth was 20°C for all isolates, except for C. tenuissimum that grew faster at 25°C. Although the species composition differed between the two fruit crops, pathogenicity tests on the fruit inoculated with representative isolates of each species showed that all 12 Cladosporium species were able to cause Cladosporium rot on both fruits. Delineation of species compositions responsible for Cladosporium rot will help develop control methods targeting specific pathogens.

Blueberries are highly perishable and susceptible to storage spoilage, especially fruit rot diseases caused by fungal pathogens. Alternaria rot caused by Alternaria alternata and gray mold caused by Botrytis cinerea are the primary postharvest diseases of blueberries grown in California. We conducted a 3-year field experiment to evaluate the effects of preharvest application of acibenzolar- S-methyl (ASM) and potassium silicate (PSi) on Alternaria rot and gray mold in blueberry fruit inoculated with these pathogens after harvest as well as naturally occurring fruit rot, fruit-to-fruit spread of gray mold during cold storage, and fruit quality. After 4 weeks of cold storage, all inoculated fruit developed rots regardless of the treatments applied in the field. In noninoculated fruit, ASM significantly reduced the percentage of naturally occurring decayed fruit compared with the nontreated control in 2023 but not in 2021 and 2022. PSi significantly reduced the percentage of decayed fruit compared with the nontreated control in 2021 and 2023 but not in 2022. A significant increase in soluble solids and reduction in titratable acidity were observed in the fruit treated with ASM and PSi for the second and third years of the experiment, indicating that both treatments may have enhanced fruit maturity. Our results suggested that preharvest use of ASM and PSi had limited efficacy in postharvest disease control but may have potential as tools for adjusting fruit maturity of blueberries. [ ] 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, 2025.

Mucor rot caused by Mucor piriformis is an emerging postharvest disease of mandarin fruit in California. Natamycin is a newly registered biofungicide for postharvest use on citrus and some other fruits. In the study, baseline sensitivity to natamycin in 50 isolates of M. piriformis was determined in vitro. The mean EC₅₀ (effective concentration to inhibit sporangiospore germination by 50%) and MIC (minimum inhibitory concentration to inhibit mycelial growth by 100%) values were 0.59 μg/ml and less than 1.0 μg/ml, respectively. Natamycin at the label rate of 920 μg/ml alone or in combination with 3% potassium sorbate (PS) or 3% sodium carbonate (SC) applied at 20 or 50°C was evaluated for control of Mucor rot on inoculated ‘Tango’ mandarin fruit. Natamycin alone reduced Mucor rot incidence on stored mandarin fruit from 100% among nontreated control fruit to approximately 30%, a reduction of more than 70% compared to the nontreated control, while 3% PS and 3% SC had no to little control. When applied at 50°C, natamycin and 3% PS reduced Mucor rot incidence by 65.0 and 31.2%, respectively; while natamycin in combination with 3% PS reduced disease incidence by 92.5% compared to the nontreated control after 2 weeks of storage at 5°C. This combined treatment remained effective even when the application of the treatment was delayed for 6 and 12 h after inoculation. However, the effectiveness of the treatments declined when storage was extended to 3 or 4 weeks. Natamycin can be an effective tool to control Mucor rot on mandarin fruit, and minimizing the period of extended storage could help maintain the control efficacy of natamycin.

ISHS XXXI International Horticultural Congress (IHC2022): International Symposium on Advances in Berry Crops Internal bruise damage in machine-harvested blueberries

Alternaria rot, caused by Alternaria alternata and A. arborescens, is one of the common postharvest diseases affecting mandarin fruit in California. Fungicide resistance profilings of A. alternata and A. arborescens to quinone outside inhibitors (QoIs), fludioxonil, pyrimethanil, imazalil and propiconazole were examined in this study. Of the 100 isolates of A. alternata and A. arborescens, 40 were identified as resistant to QoI fungicide according to a PCR-RFLP based on cytochrome b partial gene. Effective concentrations of fludioxonil that caused a 50% reduction in fungal growth relative to the control (EC50) were 0.089 ± 0.020 µg/ml and 0.101 ± 0.032 µg/ml for 43 A. alternata and 19 A. arborescens isolates, respectively. EC50 values of pyrimethanil, imazalil, and propiconazole for the 70 A. alternata isolates tested were 0.373 ± 0.161 µg/ml, 0.492 ± 0.133 µg/ml, and 1.135 ± 0.407 µg/ml, respectively. EC50 values of pyrimethanil, imazalil, and propiconazole for 30 A. arborescens isolates were 0.428 ± 0.190 µg/ml, 0.327 ± 0.180 µg/ml, and 0.669 ± 0.452 µg/ml, respectively. Control tests on mandarin fruit inoculated with representative isolates of both species showed that fludioxonil, pyrimethanil, imazalil and propiconazole significantly reduced disease incidence and severity. Azoxystrobin significantly reduced Alternaria rot severity and incidence on mandarin fruit inoculated with sensitive isolates, but not with resistant isolates regardless of Alternaria species. There were no significant differences in Alternaria rot control effectiveness if treatment with any of the fungicides tested was delayed after inoculation by 6 or 12 hours. These results could help in the development of postharvest fungicide programs to control Alternaria rot on mandarin fruit during storage.

Botrytis is an important genus of plant pathogens causing pre- and postharvest disease on diverse crops worldwide. This study evaluated Botrytis isolates collected from strawberry, blueberry, and table grape berries in California. Isolates were evaluated for resistance to eight different fungicides, and 60 amplicon markers were sequenced (neutral, species identification, and fungicide resistance associated) distributed across 15 of the 18 B. cinerea chromosomes. Fungicide resistance was common among the populations, with resistance to pyraclostrobin and boscalid being most frequent. Isolates from blueberry had resistance to the least number of fungicides, whereas isolates from strawberry had resistance to the highest number. Host and fungicide resistance-specific population structure explained 12 and 7 to 26%, respectively, of the population variability observed. Fungicide resistance was the major driver for population structure, with select fungicides explaining up to 26% and multiple fungicide resistance explaining 17% of the variability observed. Shared and unique significant single-nucleotide polymorphisms (SNPs) associated with host and fungicide (fluopyram, thiabendazole, pyraclostrobin, and fenhexamid) resistance-associated population structures were identified. Although overlap between host and fungicide resistance SNPs were detected, unique SNPs suggest that both host and fungicide resistance play an important role in Botrytis population structure.

Alternaria rot caused by Alternaria alternata is one of the major postharvest diseases affecting blueberries in California. The sensitivity profiles of A. alternata from blueberry field to quinone outside inhibitors (QoIs), boscalid, fluopyram, fludioxonil, cyprodinil, and polyoxin D in California were examined in this study. EC₅₀ values of 51 A. alternata isolates for boscalid varied greatly among the isolates, ranging from 0.265 to >100 μg/ml. EC₅₀ values of 51 A. alternata isolates to fluopyram, fludioxonil, cyprodinil, and polyoxin D were 5.188 ± 7.118, 0.078 ± 0.021, 0.465 ± 0.302, and 6.238 ± 7.352 μg/ml, respectively. In total, 143 isolates were screened for resistance at 5 and 10 μg/ml for fludioxonil, cyprodinil, and fluopyram, 10 μg/ml for polyoxin D, and 10 and 50 μg/ml for boscalid. Based on the published discriminatory concentrations for phenotyping resistance, of the 143 isolates, all were considered resistant to boscalid; 32, 69, and 42 were sensitive, low resistant, and resistant to fluopyram, respectively; and all were sensitive to fludioxonil and cyprodinil. In a PCR-restriction fragment length polymorphism method for phenotyping, 60 out of the 143 isolates were classified as resistant to QoIs. Control tests on detached blueberry fruit inoculated with different Alternaria isolates showed that fludioxonil and cyprodinil significantly reduced disease incidence and severity; however, pyraclostrobin, boscalid, fluopyram, and polyoxin D significantly reduced only disease severity. The obtained results will be helpful in making decisions on fungicide programs to control A. alternata isolates with resistance or reduced sensitivities to multiple fungicides.

Alternaria rot caused by Alternaria alternata and A. arborescens is one of the major postharvest diseases on mandarin fruit in California. In this study, natamycin, a newly registered biofungicide, was evaluated for its potential as a postharvest treatment to control Alternaria rot on mandarin fruit. The baseline sensitivities of A. alternata and A. arborescens to natamycin were determined. Effective concentration inhibiting 50% of fungal growth (EC₅₀) values of natamycin for 70 A. alternata isolates ranged from 0.694 to 1.275 µg/ml (mean = 0.921 µg/ml) in a conidial germination assay and from 2.001 to 3.788 µg/ml (mean = 2.797 µg/ml) for 40 A. alternata isolates in a mycelial growth assay. EC₅₀ values of natamycin for 30 A. arborescens isolates ranged from 0.698 to 1.203 µg/ml (mean = 0.923 µg/ml) in a conidial germination assay and from 2.035 to 3.368 µg/ml (mean = 2.658 µg/ml) for 20 A. arborescens isolates in a mycelial growth assay. Control tests on detached mandarin fruit showed that natamycin at both low (460 µg/ml) and high (920 µg/ml) recommended rates significantly reduced disease incidence and severity on mandarin fruit inoculated with Alternaria isolates, regardless of species. High rate of natamycin significantly reduced disease incidence and severity compared with the nontreated control even when natamycin treatment was delayed for 6, 12, and 18 h after inoculation. Our results suggested that natamycin can be an effective postharvest fungicide for control of Alternaria rot on mandarin fruit.

Postharvest diseases are a limiting factor in the storage of fresh blueberries. Gray mold caused by Botrytis cinerea and Alternaria rot caused by Alternaria spp. are important postharvest diseases in blueberries grown in California. Control of these fungal pathogens is generally dependent on preharvest sprays of synthetic fungicides, but in California multiple fungicide resistance has already developed in those pathogens, leading to the failure of disease control. Therefore, alternatives to synthetic fungicides are needed for the control of postharvest diseases. Peroxyacetic acid (PAA) is a disinfectant agent that poses low risk to human health. In this study, we evaluated the effects of postharvest use of PAA at 24 µl liter⁻¹and 85 µl liter⁻¹on fruit decay caused by fungal pathogens and quality of stored blueberry fruit. PAA treatment was applied to four cultivars over three seasons using two methods, dipping or spraying. Dipping blueberries compared with spraying them with PAA and its application at 85 µl liter⁻¹were the most effective treatments. For example, when applied to ‘Snowchaser’ blueberries, this combination reduced naturally occurring decay after 4 weeks of storage at 0 to 1°C from 14.3% among water-treated controls to 2.7% in 2018, and from 25.7% among water-treated controls to 8.6% in 2020. In general, PAA did not adversely affect fruit quality or sensory quality of blueberries. Postharvest use of PAA appears to be a promising means to reduce postharvest decay of blueberries. To reliably obtain an acceptable level of disease control, the best use of PAA may be in combination with other practices rather than using it alone.