Nanoparticles and Chemical Inducers: A Sustainable Shield against Onion White Rot

This study investigated the effectiveness of nanoparticles and chemical inducers in managing onion white rot caused by Sclerotium cepivorum. The pathogen severely threatens onion cultivation, resulting in significant yield losses and economic setbacks. Traditional fungicides, though effective, raise environmental concerns, prompting a shift toward eco-friendly alternatives. In this study, four S. cepivorum isolates were utilized, each exhibiting varying degrees of pathogenicity, with the third isolate from Abu-Hamad demonstrating the highest potency. During the in vitro studies, three nanoparticles (NPs) were investigated, including Fe3O4 NPs, Cu NPs, and ZnO NPs, which demonstrated the potential to inhibit mycelial growth, with salicylic acid and Fe3O4 NPs exhibiting synergistic effects. In vivo, these nanoparticles reduced the disease incidence and severity, with Fe3O4 NPs at 1000-1400 ppm resulting in 65.0-80.0% incidence and 80.0-90.0% severity. ZnO NPs had the most positive impact on the chlorophyll content, while Cu NPs had minimal effects. At 1000 ppm, Fe3O4 NPs had variable effects on the phenolic compounds (total: 6.28, free: 4.81, related: 2.59), while ZnO NPs caused minor fluctuations (total: 3.60, free: 1.82, related: 1.73). For the chemical inducers, salicylic acid reduced the disease (10.0% incidence, 25.0% to 10.0% severity) and promoted growth, and it elevated the chlorophyll values and enhanced the phenolic compounds in infected onions. Potassium phosphate dibasic (PDP) had mixed effects, and ascorbic acid showed limited efficacy toward disease reduction. However, PDP at 1400 ppm and ascorbic acid at 1000 ppm elevated the chlorophyll values and enhanced the phenolic compounds. Furthermore, this study extended to traditional fungicides, highlighting their inhibitory effects on S. cepivorum. This research provides a comprehensive comparative analysis of these approaches, emphasizing their potential in eco-friendly onion white rot management.

Evaluation of Sulfur-Based Biostimulants for the Germination of Sclerotium cepivorum Sclerotia and Their Interaction with Soil

White rot is an economically significant disease of Allium crops. The pathogen Sclerotium cepivorum produces long-lived sclerotia that germinate in response to sulfur-containing compounds released from Allium roots. Diallyl disulfide (DADS) was the primary organic sulfur compound detected in the rhizosphere soil of two garlic cultivars, "California Early and Late", growing in greenhouse conditions. DADS, dimethyl trisulfide (DMTS), dimethyl disulfide (DMDS), isopropyl disulfide (IPDS), dipropyl disulfide (DPDS), diethyl disulfide (DEDS), together with garlic oil, garlic juice, garlic powder, raw onion pieces, cabbage pieces, and Chinese cabbage pieces were investigated for their activities toward germinating dormant sclerotia. Results showed that DADS and other volatile sulfur compounds could stimulate sclerotial germination, and a dose-response was observed. In addition, garlic juice, powder, raw onion, and the two cabbages could stimulate sclerotial germination. Furthermore, the laboratory soil incubation experiments demonstrated the strong interaction of organic sulfur compounds with soil.

Visualization of Three Sclerotiniaceae Species Pathogenic on Onion Reveals Distinct Biology and Infection Strategies

Botrytis squamosa, Botrytis aclada, and Sclerotium cepivorum are three fungal species of the family Sclerotiniaceae that are pathogenic on onion. Despite their close relatedness, these fungi cause very distinct diseases, respectively called leaf blight, neck rot, and white rot, which pose serious threats to onion cultivation. The infection biology of neck rot and white rot in particular is poorly understood. In this study, we used GFP-expressing transformants of all three fungi to visualize the early phases of infection. B. squamosa entered onion leaves by growing either through stomata or into anticlinal walls of onion epidermal cells. B. aclada, known to cause post-harvest rot and spoilage of onion bulbs, did not penetrate the leaf surface but instead formed superficial colonies which produced new conidia. S. cepivorum entered onion roots via infection cushions and appressorium-like structures. In the non-host tomato, S. cepivorum also produced appressorium-like structures and infection cushions, but upon prolonged contact with the non-host the infection structures died. With this study, we have gained understanding in the infection biology and strategy of each of these onion pathogens. Moreover, by comparing the infection mechanisms we were able to increase insight into how these closely related fungi can cause such different diseases.

Allium sativum produces terpenes with fungistatic properties in response to infection with Sclerotium cepivorum

This study investigated terpene biosynthesis in different tissues (root, protobulb, leaf sheath and blade) of in vitro-grown garlic plants either infected or not (control) with Sclerotium cepivorum, the causative agent of Allium White Rot disease. The terpenes identified by gas chromatography-electron impact mass spectrometry (GC-EIMS) in infected plants were nerolidol, phytol, squalene, α-pinene, terpinolene, limonene, 1,8-cineole and γ-terpinene, whose levels significantly increased when exposed to the fungus. Consistent with this, an increase in terpene synthase (TPS) activity was measured in infected plants. Among the terpenes identified, nerolidol, α-pinene and terpinolene were the most abundant with antifungal activity against S. cepivorum being assessed in vitro by mycelium growth inhibition. Nerolidol and terpinolene significantly reduced sclerotia production, while α-pinene stimulated it in a concentration-dependent manner. Parallel to fungal growth inhibition, electron microscopy observations established morphological alterations in the hyphae exposed to terpinolene and nerolidol. Differences in hyphal EtBr uptake suggested that one of the antifungal mechanisms of nerolidol and terpinolene might be disruption of fungal membrane integrity.

The Effect of Nano-Silver Liquid against the White Rot of the Green Onion Caused by Sclerotium cepivorum

White rot, which is caused by Sclerotium cepivorum, is a lethal disease affecting green onions. Three different types of nano-silver liquid (WA-CV-WA13B, WA-AT-WB13R, and WA-PR-WB13R) were tested in several different concentrations on three types of media to assess their antifungal activities. Results from in vitro experiments showed that all three of the nano-silver liquids had more than 90% inhibition rates at a concentration of 7 ppm. Greenhouse experiments revealed that all of the nano-silver liquids increased biomass and dry weights, and there were minimal changes in the population of various bacteria and fungi from the soil of greenhouse-cultivated green onions. In addition, a soil chemical analysis showed that there were minimal changes in soil composition.

Apoplastic permeability of sclerotia of Sclerotium rolfsii, Sclerotium cepivorum and Rhizoctonia solani

Intact mature sclerotia of Sclerotium rolfsii sacc, and Sclerotium cepivorum Berk, produced in culture are impermeable to the apoplastic tracer sulphorhodamine G. Both of these species produce sclerotia with rinds. Some movement of sulphorhodamine into sclerotia of Rhizoctonia solani Kühn, which have no rind, occurred but the fluorochrome was arrested after permeation of at most the outer five layers of cells. In all cases, low permeability depended on an intact outer layer, and when sclerotia of each species were bisected to provide direct access of sulphorhodamine to all tissue layers, the fluorochrome permeated the cell walls and extracellular matrix (where present) of many cells within the sclerotium. A marked reduction in permeability of intact sclerotia occurs at maturity in a number of species and might he important in long-term Survival.