Relationship of Developmental Stage of Cantaloupe Fruit to Black Rot Susceptibility and Enzyme Production by Didymella bryoniae

Gummy stem blight: One disease, three pathogens

Gummy stem blight (GSB) is a major disease of cucurbits worldwide. It is caused by three fungal species that are morphologically identical and have overlapping geographic and host ranges. Controlling GSB is challenging due to the lack of resistant cultivars and the pathogens' significant ability to develop resistance to systemic fungicides. The causal agent of GSB is recognized as a complex of three phylogenetically distinct species belonging to domain Eukaryota, kingdom Fungi, phylum Ascomycota, subphylum Pezizomycotina, class Dothideomycetes, subclass Pleosporomycetida, order Pleosporales, family Didymellaceae, genus Stagonosporopsis, species cucurbitacearum, citrulli, and caricae. Pycnidia are tan with dark rings of cells around the ostiole measuring 120-180 μm in diameter. Conidia are 6-13 μm long, hyaline, cylindrical with round ends, and non- or monoseptate. Pseudothecia are black and globose in shape and have a diameter of 125-213 μm. Ascospores are 14-18 × 4-6 μm long, hyaline, ellipsoidal with round ends, and monoseptate with a distinct constriction at the septum. Eight ascospores are found per ascus. The upper end of the apical cell is pointed, whereas the lower end of the bottom cell is blunt. Species-specific PCR primers that can be used in a multiplex conventional PCR assay are available. The GSB species complex is pathogenic to 37 species of cucurbits from 21 different genera. S. cucurbitacearum and S. citrulli are specific to cucurbits, while S. caricae is also pathogenic to papaya and babaco-mirim (Vasconcellea monoica), a related fruit. Under favourable environmental conditions, symptoms can appear 3-12 days after spore germination. Leaf spots often start at the leaf margin or extend to the margins. Spots expand and coalesce, resulting in leaf blighting. Active lesions are typically water-soaked. Cankers are observed on crowns, main stems, and vines. Red to amber gummy exudates are often seen on the stems after cankers develop on cortical tissue.

Intercropping alleviated the phytotoxic effects of cinnamic acid on the root cell wall structural resistance of faba bean and reduced the occurrence of Fusarium wilt

Soilborne Fusarium wilt is a key factor restricting the cultivation of faba bean. Intercropping faba bean and wheat effectively alleviate faba bean Fusarium wilt. This study analyzed the mechanism by which cinnamic acid promotes Fusarium wilt and the mechanism that enables intercropping alleviated Fusarium wilt. Faba beans were inoculated with Fusarium oxysporum f. sp. fabae (FOF), while the controls were not inoculated. Different concentrations of cinnamic acid were added to the inoculated plants to study the occurrence of Fusarium wilt, seedling growth, the activities of cell wall degradation enzyme (CWDESs) produced by FOF in the root, defense enzymes, total phenolics and lignin, levels of expression of the pathogenesis-related genes (PRs) PR1, PR2, and PR10, and changes in the submicroscopic cell wall structure of the roots under monocropping and intercropping systems. Cinnamic acid increased the activities of CWDEs produced by FOF in the roots, increased the activities of phenylalanine ammonia lyase and polyphenol oxidase and the contents of total phenolics and lignin, and upregulated the levels of expression of PRs in the root, but it decreased the activity of peroxidase. Transmission electron microscopy (TEM) observations identified severe damage and disruption of the root cell walls, and numerous FOF mycelia entered the cytoplasm from the cell wall. The combination of these factors increased the occurrence of Fusarium wilt. The activities of CWDEs produced by FOF in the roots decreased by intercropping wheat with faba bean, which increased the resistance of the root cell walls to infection and decreased the Fusarium wilt.

Construction of the PG-deficient mutant of Fusarium equiseti by CRISPR/Cas9 and its pathogenicity of pitaya

Fusarium is an important plant pathogen and many cell wall-degrading enzymes (CWDEs) are produced in Fusarium-infected plant tissues. To investigate the role of CWDEs in the pathogenicity of pitaya pathogen, we isolated a Fusarium equiseti strain from the diseased pitaya fruit and the activities of CWDEs were determined. The higher polygalacturonase (PG) activity was confirmed both in vitro and vivo. Aiming at the PG gene, the CRISPR/Cas9 system of F. equiseti was constructed and optimized for the first time. Through the process of microhomology-mediated end joining, the flanking region containing 30 bp was used to mediate the homologous recombination of Cas9 double-strand breaks, and the PG gene knockout mutants were obtained by protoplast transformation. Through the phenotypic and pathogenicity experiments of the wild-type strain and mutant strain, the results showed that the colony growth rate and spore production of the strain without the PG gene decreased to some extent, and the lesion diameter and the degree of pericarp cell damage decreased, which showed that the CRISPR/Cas9 system could be used in F. equiseti and PG enzyme and can play a significant role in the interaction between F. equiseti and pitaya fruit.

Host susceptibility factors render ripe tomato fruit vulnerable to fungal disease despite active immune responses

The increased susceptibility of ripe fruit to fungal pathogens poses a substantial threat to crop production and marketability. Here, we coupled transcriptomic analyses with mutant studies to uncover critical processes associated with defense and susceptibility in tomato (Solanum lycopersicum) fruit. Using unripe and ripe fruit inoculated with three fungal pathogens, we identified common pathogen responses reliant on chitinases, WRKY transcription factors, and reactive oxygen species detoxification. We established that the magnitude and diversity of defense responses do not significantly impact the interaction outcome, as susceptible ripe fruit mounted a strong immune response to pathogen infection. Then, to distinguish features of ripening that may be responsible for susceptibility, we utilized non-ripening tomato mutants that displayed different susceptibility patterns to fungal infection. Based on transcriptional and hormone profiling, susceptible tomato genotypes had losses in the maintenance of cellular redox homeostasis, while jasmonic acid accumulation and signaling coincided with defense activation in resistant fruit. We identified and validated a susceptibility factor, pectate lyase (PL). CRISPR-based knockouts of PL, but not polygalacturonase (PG2a), reduced susceptibility of ripe fruit by >50%. This study suggests that targeting specific genes that promote susceptibility is a viable strategy to improve the resistance of tomato fruit against fungal disease.

Cell-wall-degrading enzymes produced in vitro and in vivo by Rhizoctonia solani, the causative fungus of peanut sheath blight

Rhizoctonia solani causes the disease peanut sheath blight, involving symptoms of maceration and necrosis of infected tissue, mainly caused by cell-wall-degrading enzymes (CWDEs). This study investigated the production of CWDEs including polygalacturonase (PG), polymethyl-galacturonase (PMG), cellulase (Cx) and β-glucosidase by R. solani in vitro (in liquid culture) and in vivo (in peanut plants). Significant PG, PMG, Cx and β-glucosidase activities were detected in infected tissues including stalk and leaves of Baisha and Silihong peanut cultivars. Extracts of healthy tissue showed little or no such activities. In shaken liquid cultures of R. solani in medium containing pectin or pectin plus carboxymethyl cellulose (CMC) as the carbon source(s), PG and PMG were notably active. Significant Cx activity was detected in cultures with CMC or pectin plus CMC as the carbon source(s). However, only a very low level of β-glucosidase activity was observed in cultures with any of the tested carbon sources. An increase of pH was recorded in decayed peanut tissues and liquid culture filtrates; the filtrate pH and fungal growth positively correlated. The fungal growth and/or pH were important factors for the production of PG, PMG and Cx in culture with pectin plus CMC as the carbon source. A single active PG isozyme with isoelectric point around 9.2 was detected in culture filtrates and in infected peanut tissues by the method of isoelectric focusing electrophoresis. The crude enzymes extracted from liquid culture of R. solani induced decay of healthy peanut leaves.

Susceptibility of Fourteen New Cucurbit Species to Gummy Stem Blight Caused by Stagonosporopsis citrulli Under Field Conditions

At least 24 species of cucurbits from 13 genera are known to be susceptible to gummy stem blight, caused by three species of Stagonosporopsis. Cankers that are formed on crowns and stems play an important role in the disease cycle and the survival of the pathogen. Fourteen cucurbit species of unknown susceptibility representing 12 genera, four taxonomic tribes, and four geographic origins were inoculated with Stagonosporopsis citrulli in Charleston, SC, in spring 2015, spring 2016, and fall 2016 to evaluate their level of susceptibility to gummy stem blight and the ability of the pathogen to reproduce on crown cankers. An additional species, Cucumis melo, was included as a reference due to its known high susceptibility. Data sets of area under the disease progress curve (AUDPC) for foliar severity and crown cankers, final percentage of diseased leaf area, final percentage of plants with cankers, final percentage of plants with fruiting bodies, and rates of increase in canker incidence were analyzed to evaluate susceptibility. Results were similar for datasets of AUDPC and final ratings but there were more differences for AUDPC. In all experiments, Apodanthera sagittifolia, Ecballium elaterium, and Kedrostis leloja were at least as susceptible to foliar blight as the reference C. melo. K. leloja was as susceptible to crown cankers as C. melo in all experiments and A. sagittifolia and E. elaterium were among the species most susceptible to crown cankers in two experiments. Coccinia grandis was highly resistant to gummy stem blight and had a few cankers only in fall 2016. Sicana odorifera and Zehneria pallidinervia also consistently grouped with the most resistant species. Incidence of crown cankers on Cucumis melo and K. leloja increased at the fastest rate of all species in all experiments and had, along with E. elaterium, the highest incidence of crowns with fruiting bodies. In general, the most susceptible species also were most suitable for reproduction of the pathogen and had the fastest disease progression. The tribes Benincaseae and Cucurbiteae had consistently lower levels of foliar blight than Bryonieae and Coniandreae. The tribe Benincaseae had a consistently lower AUDPC for canker incidence than Bryonieae and Coniandreae. The species originating from Europe (E. elaterium) was consistently most susceptible to both symptoms, while African species grouped with the least susceptible species in all experiments. To the best of our knowledge, this is the first report of susceptibility to gummy stem blight of 14 species and the first report of susceptibility of the cucurbit tribes Coniandreae and Gomphogyneae. This expands the host range of Stagonosporopsis citrulli to 37 species representing 21 genera and seven tribes in the family Cucurbitaceae. This study demonstrates the importance of crown cankers as reproductive sites for S. citrulli.

Reproduction of Didymella bryoniae on Nine Species of Cucurbits Under Field Conditions

Eighteen cucurbit cultivars representing 5 genera, 9 species, and 11 taxa susceptible to gummy stem blight were inoculated with Didymella bryoniae in field plots in Charleston, SC, in autumn 2008, autumn 2009, and spring 2011 to determine the suitability of the hosts and various plant parts for formation of sexual and asexual fruiting bodies of the pathogen. In 1, 2, or 3 years, D. bryoniae reproduced on all 18 cultivars, on leaves, and on all plant parts examined-pedicles, peduncles, petioles, tendrils, and vines. Watermelon and citron (both Citrullus lanatus) and melon (Cucumis melo) had significantly more leaves with fruiting bodies than cucumber (Cucumis sativus), bottle gourd (Lagenaria siceraria), and eight cultivars of squash and pumpkin (Cucurbita maxima, C. moschata, and C. pepo). On plant parts other than leaves, melon had the greatest proportions of tissue pieces with fruiting bodies, and citron had the least. Fruiting bodies were observed on 86% of plant parts examined in autumn 2009 but on only 28% in spring 2011, when environmental conditions were hot and dry. In 2009, pseudothecia and pycnidia were found in equal proportions on leaves, but pseudothecia were found more frequently than pycnidia on leaves in 2011 and on other plant parts in 2009 and 2011.

Cell wall-degrading enzymes of Didymella bryoniae in relation to fungal growth and virulence in cantaloupe fruit

Didymella bryoniae is an important pathogen of cucurbits worldwide. Virulence factors of D. bryoniae were investigated in regard to fungal growth and the production of cell wall-degrading enzymes, polygalacturonase (PG), pectate lyase (PL), pectin lyase (PNL), β-galactosidase (β-Gal) and cellulase (Cx). Virulence levels of five D. bryoniae isolates were determined by the severity of inoculated cantaloupe fruit decay. The highly virulent isolates had more mycelial growth than the moderately virulent isolates in different media. PG activities produced by the highly virulent isolates in shake cultures and in decayed fruit were greater than those of the moderately virulent isolates. PNL, but not PL, in decayed fruit was higher with the highly virulent isolates compared to the moderately virulent ones. The highly virulent isolates showed higher Cx activity than the moderately virulent ones in decayed fruit and in fruit tissue shake culture. β-Gal activities of the highly virulent isolates in pectin shake culture and in decayed fruit were greater than those of the two moderately virulent isolates although fruit also produced β-Gal. Protein analysis showed two fungal β-Gal isozymes in decayed fruit compared to those of healthy fruit. Correlation analysis indicated that the activities of PG, PNL, β-Gal and Cx in cultures and in decayed fruit positively correlated with fungal growth and fruit decay severity. The results of this study suggest that PG, PNL, β-Gal, and Cx appear to be virulence factors of D. bryoniae in cantaloupe decay with PG and β-Gal as the most predominant fruit decay enzymes.

Differential Susceptibility of Nine Cucurbit Species to the Foliar Blight and Crown Canker Phases of Gummy Stem Blight

Eighteen cucurbit cultivars representing five genera, nine species, and 17 horticultural types were inoculated with Didymella bryoniae in field plots in Charleston, SC, in autumn 2008, autumn 2009, and spring 2011 to determine susceptibility to gummy stem blight. In each year, gummy stem blight cankers occurred more frequently on crowns or main stems of 'Athena' muskmelon (Cucumis melo) and 'Green Striped Cushaw' squash (Cucurbita argyrosperma) than on all other cultivars except 'Rocio' honeydew (Cucumis melo) in 2009, and bottle gourd (Lagenaria siceraria) and Ojakkyo citron (Citrullus lanatus var. citroides) in 2011. Cucurbita moschata was highly resistant to stem cankers. Severity of gummy stem blight on foliage was moderate to severe in autumn 2009, mild to severe in autumn 2008, and very mild in spring 2011 (due to unusually dry weather). Watermelon (Citrullus lanatus var. lanatus) and melon were the most susceptible to foliar blight in 2008 and 2009. In all 3 years, 'Cheese' and 'Bugle' squash (both Cucurbita moschata), and 'Judgment III' and 'Lioness' summer squash (C. pepo) were among the cultivars with the least diseased leaf area. C. moschata, C. pepo, C. maxima, and Luffa cylindrica (smooth loofah) were significantly less susceptible to foliar blight than Cucumis melo and three Citrullus lanatus cultivars in 2008 and 2009. This study also is the first documentation of susceptibility of Cucurbita argyrosperma to gummy stem blight.

The purification, physical/chemical characterization, and cDNA sequence of cantaloupe fruit polygalacturonase-inhibiting protein

ABSTRACT Polygalacturonase-inhibiting proteins (PGIPs) are believed to aid in plant defense against fungal pathogens by inhibiting polygalacturonases (PGs) secreted by the invading organism. In an effort to better understand this type of plant-pathogen interaction in cucurbits, we have isolated a cantaloupe PGIP (CmPGIP) from 5 to 15 day postanthesis cantaloupe fruit. CmPGIP inhibited crude extracts of PG from two of four fungal pathogens of cantaloupe that were tested. Results from assays for PG activity that utilized rate of substrate viscosity reduction or rate of reducing group formation were consistent with CmPGIP inhibition of endo-PG activity. The M(r) of CmPGIP by sedimentation equilibrium or MALDITOF MS was 38,500. The pI of CmPGIP was approximately 8.2, and its absorptivity at 280 nm was 0.93 ml/mg. The circular dichroism spectrum of native CmPGIP exhibited strong negative ellipticity in the near UV and possessed a far UV spectrum indicative of beta-sheet periodic structure. Amino acid sequences of the N terminus and a cyanogen bromide peptide were used to construct oligonucleotide primers for polymerase chain reaction sequencing. The sequenced open reading frame predicts a mature protein of 307 amino acids with up to 68% identity to other PGIP molecules. Northern blot analysis revealed differential expression during fruit development. The isolation and structural information obtained for CmPGIP by this investigation provide a foundation for the development of molecular strategies for pre- and postharvest crop protection.

Effect of Fungicide Applications Scheduled to Control Gummy Stem Blight on Yield and Quality of Watermelon Fruit

Yield and quality reductions in watermelon infected with Didymella bryoniae may be attributed to reduced number or weight of fruit, sunburned fruit, fruit rot, or low sugar content due to gummy stem blight on foliage and black rot on fruit. Number, weight, soluble solids content, and external appearance of fruit were determined in four experiments conducted in fall 1996 and 1997 and spring 1997 and 1998. Severity of gummy stem blight was varied by applying no fungicide, mancozeb, or chlorothalonil according to different schedules. In the fall, when disease severity was high, total fruit weight, percent marketable fruit, and soluble solids content were lower and percent fruit with black rot was higher in nonsprayed than in sprayed treatments. Fungicide applications did not affect total fruit weight, soluble solids content, or black rot in the spring, when disease severity was moderate to low. Percent sunburned fruit was greater in treatments sprayed every 14 days than in those sprayed weekly. In fall experiments, the number of healthy, unblemished fruit increased linearly as the number of fungicide applications was increased from zero to nine per season. Yield losses in watermelon to gummy stem blight and black rot resulted primarily from a reduction in total fruit weight and an increase in number of diseased and sunburned fruit.