Growth under visible light increases conidia and mucilage production and tolerance to UV-B radiation in the plant pathogenic fungus Colletotrichum acutatum

Biological Characteristics and Fungicide Screening of Colletotrichum fructicola Causing Mulberry Anthracnose

Mulberry is an important economic crop in China that is widely planted and has important edible and medicinal value. Anthracnose, a critical leaf disease, severely compromises the yield and quality of mulberry trees. However, there are many kinds of pathogens causing mulberry anthracnose and it is difficult to control. This study was undertaken to elucidate the biological characteristics of Colletotrichum fructicola, the pathogen responsible for mulberry leaf spot in Zhejiang Province, and to screen out effective fungicides for its management. The biological characteristics of the pathogen were studied using the cross method and spore counting method, while the sensitivity of the pathogen to seven different fungicides was determined using the growth rate method. The findings indicated that potato dextrose agar (PDA) is the optimal growth medium for the pathogen. The pathogen was capable of growing across a temperature range of 5 to 40 °C, with optimum growth observed at 25 °C. Exposure to a 56 °C water bath for 10 min resulted in the death of the pathogen. It was also found to grow and sporulate within a pH range of 4 to 12, with an optimum pH of 7. Under alternating 12 h light and dark cycles, the colonies grew rapidly and produced abundant spores. Among the fungicides tested, 97% carbendazim WP exhibited the best inhibitory effect, with an EC50 (concentration for 50% of maximal effect) value of 0.0242 μg/mL. This was followed by 35% propiconazole SC, which had an EC50 of 0.4180 μg/mL. The fungicidal effect of 25 g/L fludioxonil SSCC was relatively poor, with an EC50 value of 103.0170 μg/mL. This study clarifies the optimal conditions for the growth and sporulation of the mulberry anthracnose pathogen and identifies fungicides with effective inhibitory properties. These findings will provide valuable guidance for field applications and disease management in controlling mulberry anthracnose.

Light-Driven Tetra- and Octa-β-substituted Cationic Zinc(II) Phthalocyanines for Eradicating Fusarium oxysporum Conidia

Photodynamic inactivation (PDI) is an emerging therapeutic approach that can effectively inactivate diverse microbial forms, including vegetative forms and spores, while preserving host tissues and avoiding the development of resistance to the photosensitization procedure. This study evaluates the antifungal and sporicidal photodynamic activity of two water-soluble amphiphilic tetra- and octa-β-substituted zinc(II) phthalocyanine (ZnPc) dyes with dimethylaminopyridinium groups at the periphery (ZnPcs 1, 2) and their quaternized derivatives (ZnPcs 1a, 2a). Tetra(1, 1a)- and octa(2, 2a)-β-substituted zinc(II) phthalocyanines were prepared and assessed as photosensitizers (PSs) for their effects on Fusarium oxysporum conidia. Antimicrobial photoinactivation experiments were performed with each PS at 0.1, 1, 10, and 20 µM under white light irradiation at an irradiance of 135 mW·cm-2, for 60 min (light dose of 486 J·cm-2). High PDI efficiency was observed for PSs 1a, 2, and 2a (10 µM), corresponding to inactivation until the method's detection limit. PS 1 (20 µM) also achieved a considerable reduction of >5 log10 in the concentration of viable conidia. The quaternized PSs (1a, 2a) showed better PDI performance than the non-quaternized ones (1, 2), even at the low concentration of 1 µM, and a light dose of 486 J·cm-2. These cationic phthalocyanines are potent photodynamic drugs for antifungal applications due to their ability to effectively inactivate resistant forms, like conidia, with low concentrations and reasonable energy doses.

In Vitro Photoinactivation of Fusarium oxysporum Conidia with Light-Activated Ammonium Phthalocyanines

Antimicrobial photodynamic therapy (aPDT) has been explored as an innovative therapeutic approach because it can be used to inactivate a variety of microbial forms (vegetative forms and spores) without causing significant damage to host tissues, and without the development of resistance to the photosensitization process. This study assesses the photodynamic antifungal/sporicidal activity of tetra- and octasubstituted phthalocyanine (Pc) dyes with ammonium groups. Tetra- and octasubstituted zinc(II) phthalocyanines (1 and 2) were prepared and tested as photosensitizers (PSs) on Fusarium oxysporum conidia. Photoinactivation (PDI) tests were conducted with photosensitizer (PS) concentrations of 20, 40, and 60 µM under white-light exposure at an irradiance of 135 mW·cm^-2, applied during 30 and 60 min (light doses of 243 and 486 J·cm^-2). High PDI efficiency corresponding to the inactivation process until the detection limit was observed for both PSs. The tetrasubstituted PS was the most effective, requiring the lowest concentration and the shortest irradiation time for the complete inactivation of conidia (40 µM, 30 min, 243 J·cm^-2). Complete inactivation was also achieved with PS 2, but a longer irradiation time and a higher concentration (60 µM, 60 min, 486 J·cm^-2) were necessary. Because of the low concentrations and moderate energy doses required to inactivate resistant biological forms such as fungal conidia, these phthalocyanines can be considered potent antifungal photodynamic drugs.

UV-B-induced DNA damage and repair pathways in polar Pseudogymnoascus sp. from the Arctic and Antarctic regions and their effects on growth, pigmentation, and coniodiogenesis

Solar radiation regulates most biological activities on Earth. Prolonged exposure to solar UV radiation can cause deleterious effects by inducing two major types of DNA damage, namely cyclobutane pyrimidine dimers (CPDs) and pyrimidine 6-4 pyrimidone photoproducts (6-4PPs). These lesions may be repaired by the photoreactivation (Phr) and nucleotide excision repair (NER) pathways; however, the principal UV-induced DNA repair pathway is not known in the fungal genus Pseudogymnoascus. In this study, we demonstrated that an unweighted UV-B dosage of 1.6 kJ m^-2 d^-1 significantly reduced fungal growth rates (by between 22 and 35%) and inhibited conidia production in a 10 d exposure. The comparison of two DNA repair conditions, light or dark, which respectively induced photoreactivation (Phr) and nucleotide excision repair (NER), showed that the UV-B induced CPDs were repaired significantly more rapidly in light than in dark conditions. The expression levels of two DNA repair genes, RAD2 and PHR1 (encoding a protein in NER and Phr, respectively) demonstrated that NER rather than Phr was primarily activated for repairing UV-B-induced DNA damage in these Pseudogymnoascus strains. In contrast, Phr was inhibited after exposure to UV-B radiation, suggesting that PHR1 may have other functional roles. This article is protected by copyright. All rights reserved.

Available Strategies for the Management of Andean Lupin Anthracnose

The lupin (Lupinus mutabilis Sweet) is a legume domesticated and cultivated for more than 4000 years by the pre-Hispanic cultures of the Andean zone. Due to its good taste and protein content, the lupin seed contributes significantly to the food and nutritional security of the Andean population. However, lupin is susceptible to diseases, and of these, anthracnose is the most devastating as it affects the whole crop, including leaves, stems, pods, and seeds. This review focuses on available strategies for management of lupin anthracnose from sowing to harvest. Seed disinfection is the primary anthracnose management strategy. Seed treatment with fungicides reduces transmission from seed to seedling, but it does not eradicate anthracnose. Attention is given to alternative strategies to limit this seed-borne pathogen as well as to enhance plant resistance and to promote plant growth. For anthracnose management in the field, integrated practices are discussed that encompass control of volunteer plants, lupin ontogenetic resistance, and rotation of biocontrol with chemical fungicides at susceptible phenological stages. This review covers some local experiences on various aspects of anthracnose management that could prove useful to other the groups focusing on the problem.

Conidiation under illumination enhances conidial tolerance of insect-pathogenic fungi to environmental stresses

Light is an important signal for fungi in the environment and induces many genes with roles in stress and virulence responses. Conidia of the entomopathogenic fungi Aschersonia aleyrodis, Beauveria bassiana, Cordyceps fumosorosea, Lecanicillium aphanocladii, Metarhizium anisopliae, Metarhizium brunneum, Metarhizium robertsii, Simplicillium lanosoniveum, Tolypocladium cylindrosporum, and Tolypocladium inflatum were produced on potato dextrose agar (PDA) medium under continuous white light, on PDA medium in the dark, or under nutritional stress (= Czapek medium without sucrose = MM) in the dark. The conidial tolerance of these species produced under these different conditions were evaluated in relation to heat stress, oxidative stress (menadione), osmotic stress (KCl), UV radiation, and genotoxic stress caused by 4-nitroquinoline 1-oxide (4-NQO). Several fungal species demonstrated greater stress tolerance when conidia were produced under white light than in the dark; for instance white light induced higher tolerance of A. aleyrodis to KCl and 4-NQO; B. bassiana to KCl and 4-NQO; C. fumosorosea to UV radiation; M. anisopliae to heat and menadione; M. brunneum to menadione, KCl, UV radiation, and 4-NQO; M. robertsii to heat, menadione, KCl, and UV radiation; and T. cylindrosporum to menadione and KCl. However, conidia of L. aphanocladii, S. lanosoniveum, and T. inflatum produced under white light exhibited similar tolerance as conidia produced in the dark. When conidia were produced on MM, a much stronger stress tolerance was found for B. bassiana to menadione, KCl, UV radiation, and 4-NQO; C. fumosorosea to KCl and 4-NQO; Metarhizium species to heat, menadione, KCl, and UV radiation; T. cylindrosporum to menadione and UV radiation; and T. inflatum to heat and UV radiation. Again, conidia of L. aphanocladii and S. lanosoniveum produced on MM had similar tolerance to conidia produced on PDA medium in the dark. Therefore, white light is an important factor that induces higher stress tolerance in some insect-pathogenic fungi, but growth in nutritional stress always provides in conidia with stronger stress tolerance than conidia produced under white light.

UV-B light and its application potential to reduce disease and pest incidence in crops

Ultraviolet-B radiation (280-315 nm), perceived by the plant photoreceptor UVR8, is a key environmental signal that influences plant growth and development and can reduce disease and pest incidence. The positive effect of UV-B on disease resistance and incidence in various plant species supports the implementation of supplemental UV-B radiation in sustainable crop production. However, despite many studies focusing on UV-B light, there is no consensus on the best mode of application. This review aims to analyze, evaluate, and organize the different application strategies of UV-B radiation in crop production with a focus on disease resistance. We summarize the physiological effects of UV-B light on plants and discuss how plants perceive and transduce UV-B light by the UVR8 photoreceptor as well as how this perception alters plant specialized metabolite production. Next, we bring together conclusions of various studies with respect to different UV-B application methods to improve plant resistance. In general, supplemental UV-B light has a positive effect on disease resistance in many plant-pathogen combinations, mainly through the induction of the production of specialized metabolites. However, many variables (UV-B light source, plant species, dose and intensity, timing during the day, duration, background light, etc.) make it difficult to compare and draw general conclusions. We compiled the information of recent studies on UV-B light applications, including e.g., details on the UV-B light source, experimental set-up, calculated UV-B light dose, intensity, and duration. This review provides practical insights and facilitates future research on UV-B radiation as a promising tool to reduce disease and pest incidence.

Bitter Rot of Apple in the Mid-Atlantic United States: Causal Species and Evaluation of the Impacts of Regional Weather Patterns and Cultivar Susceptibility

Apple growers in the Mid-Atlantic region of the United States have been reporting an increase in losses to bitter rot of apple and are requesting up-to-date management recommendations. Management is complicated by variations in apple cultivar susceptibility, temperature, rainfall, and biology of the Colletotrichum spp. that cause bitter rot. Over 500 apple fruit with bitter rot were obtained from 38 orchards across the Mid-Atlantic and the causal species were identified as Colletotrichum fioriniae and C. nymphaeae of the C. acutatum species complex and C. chrysophilum, C. noveboracense, C. siamense, C. fructicola, C. henanense, and C. gloeosporioides sensu stricto of the C. gloeosporioides species complex, the latter two being first reports. Species with faster in vitro growth rates at higher temperatures were more abundant in warmer regions of the Mid-Atlantic, while those with slower growth rates at higher temperatures were more abundant in cooler regions. Regional bloom dates are earlier and weather data show a gradual warming trend that likely influenced but was not necessarily the main cause of the recent increase in bitter rot in the region. A grower survey of apple cultivar susceptibility showed high variation, with the increase in acres planted to the highly susceptible cultivar Honeycrisp broadly corresponding to the increase in reports of bitter rot. These results form a basis for future studies on the biology and ecology of the Colletotrichum spp. responsible, and suggest that integrated bitter rot management must begin with selection of less-susceptible apple cultivars.

Altitudinal Heterogeneity of UV Adaptation in Phytophthorainfestans Is Associated with the Spatial Distribution of a DNA Repair Gene

Climate change is considered a major threat to society and nature. UV irradiation is the most important environmental genotoxic agent. Thus, how elevated UV irradiation may influence human health and ecosystems has generated wide concern in the scientific community, as well as with policy makers and the public in general. In this study, we investigated patterns and mechanisms of UV adaptation in natural ecosystems by studying a gene-specific variation in the potato late blight pathogen, Phytophthora infestans. We compared the sequence characteristics of radiation sensitive 23 (RAD23), a gene involved in the nucleotide excision repair (NER) pathway and UV tolerance, in P. infestans isolates sampled from various altitudes. We found that lower genetic variation in the RAD23 gene was caused by natural selection. The hypothesis that UV irradiation drives this selection was supported by strong correlations between the genomic characteristics and altitudinal origin (historic UV irradiation) of the RAD23 sequences with UV tolerance of the P. infestans isolates. These results indicate that the RAD23 gene plays an important role in the adaptation of P. infestans to UV stress. We also found that different climatic factors could work synergistically to determine the evolutionary adaptation of species, making the influence of climate change on ecological functions and resilience more difficult to predict. Future attention should aim at understanding the collective impact generated by simultaneous change in several climate factors on species adaptation and ecological sustainability, using state of the art technologies such as experimental evolution, genome-wide scanning, and proteomics.

Outcome of blue, green, red, and white light on Metarhizium robertsii during mycelial growth on conidial stress tolerance and gene expression

Fungi sense light and utilize it as a source of environmental information to prepare against many stressful conditions in nature. In this study, Metarhizium robertsii was grown on: 1) potato dextrose agar medium (PDA) in the dark (control); 2) under nutritive stress in the dark; and 3) PDA under continuous (A) white light; (B) blue light lower irradiance = LI; (C) blue light higher irradiance = HI; (D) green light; and (E) red light. Conidia produced under these treatments were tested against osmotic stress and UV radiation. In addition, a suite of genes usually involved in different stress responses were selected to study their expression patterns. Conidia produced under nutritive stress in the dark were the most tolerant to both osmotic stress and UV radiation, and the majority of their stress- and virulence-related genes were up-regulated. For osmotic stress tolerance, conidia produced under white, blue LI, and blue HI lights were the second most tolerant, followed by conidia produced under green light. Conidia produced under red light were the least tolerant to osmotic stress and less tolerant than conidia produced on PDA medium in the dark. For UV tolerance, conidia produced under blue light LI were the second most tolerant to UV radiation, followed by the UV tolerances of conidia produced under white light. Conidia produced under blue HI, green, and red lights were the least UV tolerant and less tolerant than conidia produced in the dark. The superoxide dismutases (sod1 and sod2), photolyases (6-4phr and CPDphr), trehalose-phosphate synthase (tps), and protease (pr1) genes were highly up-regulated under white light condition, suggesting a potential role of these proteins in stress protection as well as virulence after fungal exposure to visible spectrum components.

Strategies of Phytophthora infestans adaptation to local UV radiation conditions

Expected global changes in environmental conditions underline the need for a better understanding of genetic variation in ecological traits and their strategies of adaptation to the stresses. In this study, evolutionary mechanisms and processes of UV adaptation in plant pathogens were investigated by combining statistical genetics, physiological assays, and common garden experiment approaches in an assessment of the potato late blight pathogen, Phytophthora infestans, sampled from various geographic locations in China. We found spatial divergence caused by diversifying selection in UV tolerance among P. infestans populations. Local UV radiation was the driving force of selection as indicated by a positive correlation between UV tolerance in P. infestans populations and the altitude of collection sites. Plasticity accounted for 68% of population variation while heritability was negligible, suggesting temporary changes in gene expression and/or enzymatic activity play a more important role than permanent modification of gene structure in the evolution of UV adaptation. This adaptation strategy may explain the lack of fitness penalty observed in genotypes with higher UV tolerance.

Effects of UV-B radiation on the infectivity of Magnaporthe oryzae and rice disease-resistant physiology in Yuanyang terraces

The traditional rice variety "Baijiaolaojing" was planted in Yuanyang terraces (1600 m altitude) under field conditions. The effects of enhanced UV-B radiation (0 kJ m^-2, 2.5 kJ m^-2, 5.0 kJ m^-2 and 7.5 kJ m^-2) on the rice-Magnaporthe oryzae system were studied with respect to the Magnaporthe oryzae infection, the disease-resistance physiology of the rice and the rice blast disease condition. The results showed that under enhanced UV-B radiation, the infectivity of Magnaporthe oryzae was decreased, which could significantly inhibit its growth and sporulation. The activities of rice leaf disease-resistance-related enzymes (phenylalanine ammonia-lyase, lipoxygenase, chitinase and β-1,3-glucanase) were significantly increased under enhanced UV-B radiation. Following inoculation with Magnaporthe oryzae, levels of disease-resistance-related substances in the rice leaves were significantly increased. Among the results, it was found that leaves after UV-B radiation had a more significant resistance response. The level of UV-B irradiation showed a parabolic relationship with the rice blast index (r² = 0.85, P < 0.01; in the control group, r² = 0.88, P < 0.01). The disease index decreased with increase in irradiation. The DI was at a minimum with enhanced UV-B irradiance of 4 kJ m^-2; thereafter, it increased with increasing irradiation. The enhanced UV-B radiation had a direct impact on the growth of rice and Magnaporthe oryzae, and it indirectly changed the rice-Magnaporthe oryzae system. UV-B radiation could reduce the harmful impact of rice blast.

Metarhizium robertsii illuminated during mycelial growth produces conidia with increased germination speed and virulence

Light conditions during fungal growth are well known to cause several physiological adaptations in the conidia produced. In this study, conidia of the entomopathogenic fungi Metarhizium robertsii were produced on: 1) potato dextrose agar (PDA) medium in the dark; 2) PDA medium under white light (4.98 W m^-2); 3) PDA medium under blue light (4.8 W m^-2); 4) PDA medium under red light (2.8 W m^-2); and 5) minimum medium (Czapek medium without sucrose) supplemented with 3 % lactose (MML) in the dark. The conidial production, the speed of conidial germination, and the virulence to the insect Tenebrio molitor (Coleoptera: Tenebrionidae) were evaluated. Conidia produced on MML or PDA medium under white or blue light germinated faster than conidia produced on PDA medium in the dark. Conidia produced under red light germinated slower than conidia produced on PDA medium in the dark. Conidia produced on MML were the most virulent, followed by conidia produced on PDA medium under white light. The fungus grown under blue light produced more conidia than the fungus grown in the dark. The quantity of conidia produced for the fungus grown in the dark, under white, and red light was similar. The MML afforded the least conidial production. In conclusion, white light produced conidia that germinated faster and killed the insects faster; in addition, blue light afforded the highest conidial production.

Inactivation of plant-pathogenic fungus Colletotrichum acutatum with natural plant-produced photosensitizers under solar radiation

The increasing tolerance to currently used fungicides and the need for environmentally friendly antimicrobial approaches have stimulated the development of novel strategies to control plant-pathogenic fungi such as antimicrobial phototreatment (APT). We investigated the in vitro APT of the plant-pathogenic fungus Colletotrichum acutatum with furocoumarins and coumarins and solar radiation. The compounds used were: furocoumarins 8-methoxypsoralen (8-MOP) and 5,8-dimethoxypsoralen (isopimpinellin), coumarins 2H-chromen-2-one (coumarin), 7-hydroxycoumarin, 5,7-dimethoxycoumarin (citropten) and a mixture (3:1) of 7-methoxycoumarin and 5,7-dimethoxycoumarin. APT of conidia with crude extracts from 'Tahiti' acid lime, red and white grapefruit were also performed. Pure compounds were tested at 50μM concentration and mixtures and extracts at 12.5mgL(-1). The C. acutatum conidia suspension with or without the compounds was exposed to solar radiation for 1h. In addition, the effects of APT on the leaves of the plant host Citrus sinensis were determined. APT with 8-MOP was the most effective treatment, killing 100% of the conidia followed by the mixture of two coumarins and isopimpinellin that killed 99% and 64% of the conidia, respectively. APT with the extracts killed from 20% to 70% of the conidia, and the extract from 'Tahiti' lime was the most effective. No damage to sweet orange leaves was observed after APT with any of the compounds or extracts.

Molecular and physiological effects of environmental UV radiation on fungal conidia

Conidia are specialized structures produced at the end of the asexual life cycle of most filamentous fungi. They are responsible for fungal dispersal and environmental persistence. In pathogenic species, they are also involved in host recognition and infection. Conidial production, survival, dispersal, germination, pathogenicity and virulence can be strongly influenced by exposure to solar radiation, although its effects are diverse and often species dependent. UV radiation is the most harmful and mutagenic waveband of the solar spectrum. Direct exposure to solar radiation for a few hours can kill conidia of most fungal species. Conidia are killed both by solar UV-A and UV-B radiation. In addition to killing conidia, which limits the size of the fungal population and its dispersion, exposures to sublethal doses of UV radiation can reduce conidial germination speed and virulence. The focus of this review is to provide an overview of the effects of solar radiation on conidia and on the major systems involved in protection from and repair of damage induced by solar UV radiation. The efforts that have been made to obtain strains of fungi of interest such as entomopathogens more tolerant to solar radiation will also be reviewed.

Stress tolerance and virulence of insect-pathogenic fungi are determined by environmental conditions during conidial formation

The virulence to insects and tolerance to heat and UV-B radiation of conidia of entomopathogenic fungi are greatly influenced by physical, chemical, and nutritional conditions during mycelial growth. This is evidenced, for example, by the stress phenotypes of Metarhizium robertsii produced on various substrates. Conidia from minimal medium (Czapek's medium without sucrose), complex medium, and insect (Lepidoptera and Coleoptera) cadavers had high, moderate, and poor tolerance to UV-B radiation, respectively. Furthermore, conidia from minimal medium germinated faster and had increased heat tolerance and were more virulent to insects than those from complex medium. Low water-activity or alkaline culture conditions also resulted in production of conidia with high tolerance to heat or UV-B radiation. Conidia produced on complex media exhibited lower stress tolerance, whereas those from complex media supplemented with NaCl or KCl (to reduce water activity) were more tolerant to heat and UV-B than those from the unmodified complex medium. Osmotic and nutritive stresses resulted in production of conidia with a robust stress phenotype, but also were associated with low conidial yield. Physical conditions such as growth under illumination, hypoxic conditions, and heat shock before conidial production also induced both higher UV-B and heat tolerance; but conidial production was not decreased. In conclusion, physical and chemical parameters, as well as nutrition source, can induce great variability in conidial tolerance to stress for entomopathogenic fungi. Implications are discussed in relation to the ecology of entomopathogenic fungi in the field, and to their use for biological control. This review will cover recent technologies on improving stress tolerance of entomopathogenic fungi for biological control of insects.