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Pathogenic Fungus and Cucurbitaceous Vegetables’ Gummy Stem Blight: History
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Contributor: Jian Zhang ,
, Tan Guo-Fei , Ping-Hong Meng

Cucurbits are an important vegetable crop of the gourd family. Unfortunately, gummy stem blight (GSB) causes a major fungal disease on Cucurbitaceous vegetable crops. It is also known as black root when affecting fruits, and it is found all over the world. GSB is caused by the fungal pathogen Didymella bryoniae. 

  • gummy stem blight
  • pathogenic fungus
  • resistance breeding

1. Acute Symptoms

The pycnidia and pseudothecia of the pathogen forms readily in leaf spots and lesions on other above-ground plant structures, including the petioles, vines, stems, tendrils, and pedicels of flowers and fruits. These have been exploited as diagnostic signs, so fruiting bodies form first in the center of lesions for D. bryoniae as a necrotrophic pathogen [1][2]. Thepathogenic bacteria have different morphological characteristics in different parts of various Cucurbitaceous vegetable crops (such as color variability and symptoms of conidia) [3], as shown in Table 1. These characteristics of D. bryoniae may be similar to some other bacteria (such as P. exigua), and more attentionshould be givento distinguishing these symptoms [1].
Table 1. Morphological and cultural characters of pathogenic bacteria on Cucurbits.
Variety Signs of Plants Symptom
Cucumber (C. sativus) Irregular dark brown lesions at the leaf margin [4] Aerial mycelium (white becoming gray with age), submerged mycelium (dark olive to black) [5];
Pycnidia (sporulation aparse), perithecia (black bodies) [4].
Muskmelon (C. melo) Dry rot in stems (white) [5], leaf spots and dry rot in petiols (dark brown) [6]. Produced a conidial mass with a white aerial mycelium at The center of colony, few pycnidia were observed [5][6].
Watermelon (C. lanatus) Angular water-soaked lesions, defoliation, dry, stem necrosis, gummy exudates, wilt and eventual death [4];
Leaf spots (dark brown) [6].		 White aerial and olivaceous mycelium, and olive to dark green or black substrate mycelium [4];
The colony surface was rough and undulated, the conidia were round-ended, cylindrical, monoseptate, and hyaline [6].
Pumpkin (Cucurbita maxima) - Aerial mycelium (usually absent, very scanty), submerged mycelium (hyaline to brown), pycnidia (sporulation heavy), perithecia (no sporulation) [3];
Type I to type VII, type IV was the most predominant type (mostly white, a few for brown or yellow) [7].
Gherkin cucumber (C. sativus) Leaf spots (tan to blackish-brown), dry rot in stems (brown gummy exudate), brown lesions in fruits (black rot) [8]. Conidia were hyaline, cylindrical with rounded ends, and non- or mono-septate; Ascospores were hyaline and monoseptate with two cells of differing sizes [8].
Squash (Cucurbita digitata) Irregular dark brown lesions at the leaf margin [4]. -
Mellon (C. melo) Leaf spots (brown), cankers (light brown to off-white) [4]. Angular water-soaked lesions; the fungal showed white aerial and olivaceous mycelium, and olive to dark green or black substrate mycelium; the conidia were round-ended, cylindrical, monoseptate, and hyaline [4].
Cantaloupe (C. melo var cantaloupe) Black rot (black) and fruit decay [9]. A brown discoloration of the net; Pycnidia and pseudothecia were not produced in black rot lesions on cantaloupe fruit [9].

2. Biological Characteristics of Pathogenic Fungus

Gummy stem blight (GSB) is caused by the ascomycete fungus D. bryoniae (Auersw.) Rehm (the oldest name) and its anamorph Phoma cucurbitacearum (Fr.: Fr.) Sacc.arebased on morphological similarities [10]. It has teleomorphic (sexually reproducing) and anamorphic (asexual) states [10][11]. Keinath (2014) determined the suitability of the hosts and various plant parts for the formation of sexual and asexual fruiting bodies of the pathogen forthree years; it was found that fruiting bodies showed on high (86%) or low (28%) levels in different years [12].
It has since been established that GSBis caused by three Stagonosporopsis species: S. cucurbitacearum (syn. D bryoniae) [6][13], S. citrulli [14], and S. caricae [8][15]. The pathogen of GSB in muskmelon was identified as D. bryoniae (Auersw) Rehm., whose anamorph is Ascochyta citrullina Smith [16]. Although three Stagonosporopsis species had a similar morphology, they could be distinguished by using polymerase chain reaction-based microsatellite markers [17]. Jia et al. (2003) reported the naturally formed, perfect pathogen stage pseudoperithecium of GSB on gourd crops for the first time in Xingjiang, China. This was later named the Mycosphaerella melonis (Pass.) by Chiu et J. C. Walker [18]. Zhang et al. (2013) foundthe perfect stage of the pathogen of GSB in melons of Hainan, which was identified as ascomycete fungus D. bryoniae (Auersw.) Rehm. by measuring its pseudothecia, ascus, and ascospore [19]. Li et al. (2017) identified the mating-type loci (MAT1) in the three Stagonosporopsis species (S. citrulli, S. cucurbitacearum, and S. caricae) causing GSB in draft genome sequences. Both MAT1-1-1 and MAT1-2-1 were divergent, but all had the highly conserved andhigh mobility group (MATA-HMG-box) domain [20].

3. Genetic Diversity of Pathogenic Fungus

Corlett (1981) provided a detailed description and illustration of 15 species of Didymella and Didymella-like species, in which species of Didymella fall into two small but well-defined subgeneric groups and one large heterogeneous intermediate group [21]. There is less research regardingthe molecular and phylogenetic relationships between D. bryoniae and these Phoma species, but many molecular techniques, such as AFLP: amplified fragment length polymorphism, RAPD: random amplified polymorphic DNA, SCAR: sequence characterized amplified regions, ELISA: enzyme linked immunosorbent assay, LAMP, and loop-mediated amplification have been well established for characterizing D. bryoniae and facilitating genetic fingerprinting of isolates from specific geographical locations [22][23][24][25][26].
Based on the available sequence data, Didymellaceae can be segregated into at least 18 distinct clusters (includingthe taxonomic description of eight species and two varieties); four of these clusters were defined well enough by means of phylogeny and morphology [27]. Many isolates of D. bryoniae were placed into four phylogenetic groups (RG I, RG II, and RG IV) through RAPD analysis. Meanwhile, phoma spp. clustered into a separate group, RG III [6][22][28]. Shim et al. (2006) isolated D. bryoniae clusters and divided them into two major genotypes, the RG I (I-a, I-b, I-c, and I-d) and RG II (II-a, II-b, and II-c) [24]. The isolates were grouped into cluster DB Ia (RG I group), DB Ib (RG II group), DB II, and DB III [29][30]. Workneh (2014) identified the presence of two isolates (DB-05 and DB-33) based on their biological and molecular diversity, whichhad a higher similarity to D. bryoniae isolated from China, with internal transcribed spacer (ITS) region analysis [31].

4. Differentiation of Physiological Race

The understanding of the pathogenesis and virulence factors of D. Bryoniae may provide new information to develop effective methods of controlling D. bryoniae on Cucurbit crops [32]. Isolates from the RG I group were the most predominant and highly virulent, while RG III was slightly virulent [28]. Virulence of the RG I isolates was stronger than RG IV in cucumber [24]. Hu et al. (2012) found that the pathogenicity of 19 strains of D. bryoniae was significantly different with disease indexes of 85.11–4.58 on watermelon and melon [33].
Fungal isolates produced polygalacturonase (PG) activity, while PG played an important role in the pathogenesis of D. bryoniae in Cucurbitaceous decayed tissue [9]. Furthermore, the virulence factors of D. bryoniae have been studied regarding fungal growth and the production of cell wall-degrading enzymes, pectate lyase (PL), polygalacturonase (PG), β-galactosidase (β-Gal), pectin lyase (PNL), and cellulase (Cx); the results suggest that these enzymesappeared to be virulence factors of D. bryoniae in cantaloupe decay with PG and β-Gal as the most predominant fruit decay enzymes [34]. Three kinds of defense enzymes (PAL: phenylalnineammonialyse, PPO: polyphenol oxidase, and POD: peroxidase) were closely related to the resistance of GSB on melons [35]. At the same time, the activities POD, SOD (superoxide dismutase), CAT (catalase), and PPO were also positively correlated with resistance of GSB in melons [36][37].

5. Genomic Characteristics

To better understand the pathogenicity of the fungus GSB, research on the genomic characteristics of D. bryoniae on selected Cucurbitaceous vegetables, including Cucurbits, muskmelons, watermelon, pumpkin, and melon, has been performed, and the results are summarized in Table 2.
Table 2. Genetic characteristics of the fungus of GSB in various Cucurbitaceous vegetables.
Variety Genetic Characteristics References
Cucubits Two to four amplified fragments were unique to all 27 isolated bacterium, 13 additional fragments were present in all D. bryoniae; [1]
RG I group with a single band of 650 bp fragment while RG II group with about a 1.4 kb fragment. [24]
Muskmelon (C. melo) The similarity in sequence identity between the rDNA ITS region was 100% and 95.0%; Nucleotide sequences of the rDNA ITS reform BLAST search from pure culture ranged from 98.2% to 99.8%; [5]
Two isolates possessed a single nucleotide substitution of A to G at position 131 of the ITS 1 region. [6]
Watermelon (C. lanatus) The isolates produced fragment sizes of approximately 120, 780, and 560 bp; [4]
Two isolates possessed a single nucleotide substitution of A to G at position 131 of the ITS 1 region. [6][30]
Pumpkin (C. maxima) Two motifs contained sequence variations unique to two groups: Type A (exhibited high similarity with one another, a typical dominant physiological Cucurbita GSB fungal group) and Type B (variant genotypic offshoots with the farthest genetic distance). [7]
Melon (C. melo) Ace 2 of Sphaerorheeafuliginea in C. melo PI 12411l conferred by an incompletely dominant gene. [38]

This entry is adapted from the peer-reviewed paper 10.3390/agronomy12061283

References

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