First Report of Sclerotinia minor on Brassica rapa subsp. pekinensis in Central China

Biocontrol Potential of Rhizospheric Bacillus Strains Against Sclerotinia minor Jagger Causing Lettuce Drop

Phytopathogenic Sclerotinia minor Jagger causes lettuce drop, a destructive soil-borne disease. As potential biocontrol agents for this disease, 2 of 31 bacterial strains isolated from soil samples from fields containing S. minor Jagger were identified using in vitro antagonistic assays against S. minor Jagger. Bioactivity experiments showed that Bac20 had higher inhibitory activity against S. minor Jagger than Bac45. Based on 16S rRNA sequences and phylogenetic analysis of a combination of sequences from gyrA, rpoB, purH, polC, and groEL, Bac20 and Bac45 were identified as Bacillus velezensis and Bacillus subtilis, respectively. Lipopeptide compounds produced by each strain were identified using matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS) analysis. Both strains produced three types of lipopeptides, namely surfactins, iturins, and fengycins, whereas Bac20 showed the strongest intensity in its production of iturins, more than that of Bac45. Bac20 inhibited oxalic acid formation in early-stage lettuce leaves infected with S. minor Jagger, delaying pathogen infestation. Greenhouse experiments for controlling lettuce drop demonstrated that inoculation with Bac20 controlled lettuce drop by 71.7%. In conclusion, this study revealed that B. velezensis Bac20 has high potential for use as a biocontrol agent for controlling the lettuce drop caused by S. minor Jagger.

Molecular characterization of a novel botoulivirus from the phytopathogenic fungus Sclerotinia minor

In this study, the complete genomic sequence of a novel botoulivirus (Sclerotinia minor botoulivirus 1, SmBV1) from the phytopathogenic fungus Sclerotinia minor strain LC45 was determined. The genome of SmBV1 is 2,882 nucleotides in length and contains a single large open reading frame (ORF) encoding a putative RNA-dependent RNA polymerase (RdRp). Phylogenetic analysis showed that SmBV1 clustered with the botoulivirus clade within the family Botourmiaviridae. This is the first report of a botoulivirus in S. minor.

Sclerotinia minor Endornavirus 1, a Novel Pathogenicity Debilitation-Associated Mycovirus with a Wide Spectrum of Horizontal Transmissibility

Sclerotinia minor is a phytopathogenic fungus causing sclerotinia blight on many economically important crops. Here, we have characterized the biological and molecular properties of a novel endornavirus, Sclerotinia minor endornavirus 1 (SmEV1), isolated from the hypovirulent strain LC22 of S. minor. The genome of SmEV1 is 12,626 bp long with a single, large open reading frame (ORF), coding for a putative protein of 4020 amino acids. The putative protein contains cysteine-rich region (CRR), viral methyltransferase (MTR), putative DEXDc, viral helicase (Hel), and RNA-dependent RNA polymerase (RdRp) domains. The putative protein and the conserved domains are phylogenetically related to endornaviruses. SmEV1 does not contain a site-specific nick characteristic of most previously described endornaviruses. Hypovirulence and associated traits of strain LC22 and SmEV1 were readily cotransmitted horizontally via hyphal contact to isolates of different vegetative compatibility groups of S. minor. Additionally, SmEV1 in strain LC22 was found capable of being transmitted vertically through sclerotia. Furthermore, mycelium fragments of hypovirulent strain LC22 have a protective activity against attack by S. minor. Taken together, we concluded that SmEV1 is a novel hypovirulence-associated mycovirus with a wide spectrum of transmissibility, and has potential for biological control (virocontrol) of diseases caused by S. minor.

Characterization of the Mycelial Compatibility Groups and Mating Type Alleles in Populations of Sclerotinia minor in Central China

Ninety-five single-sclerotium isolates were obtained from lettuce and weeds in three counties in central China. They were identified belonging to Sclerotinia minor based on colony morphology and the S. minor-specific DNA marker. Mycelial compatibility groups (MCGs) and the mating type (MAT) alleles in these isolates were determined using the methods of paired cultures and specific PCR, respectively, and the MCG data were used to calculate Shannon's H index (H) and Simpson index (S), thereby evaluating diversity of S. minor. Eight MCGs (MCG1 to MCG8) and two MAT alleles (Inv+, Inv-) were identified in these isolates. Low diversity was detected for the total 95 isolates (H = 1.748, S = 0.786). Isolates of different MCGs or with different MAT alleles did not significantly differ (P > 0.05) in mycelial growth rate on potato dextrose agar (PDA, 20°C) or lesion diameter on lettuce leaves (20°C), but slightly differed in the number of sclerotia produced on PDA (20°C). Furthermore, this study reported five new host plants of S. minor in China, including Capsella bursa-pastoris, Oenanthe javanica, Fragaria gracilis, Ranunculus ternatus, and Salvia plebeia, and identified three hypovirulent isolates. These results broaden our understanding about the population biology of S. minor.