Under more intensified cropping conditions agriculture will face increasing incidences of soil-borne plant pests and pathogens, leading to increasingly higher yield losses world-wide. Soil-borne disease complexes, in particular, are especially difficult to control. In order to better understand soil-borne Meloidogyne -based disease complexes, we studied the volatile-based control mechanism of associated bacteria as well as the rhizospheric microbiome on Ugandan tomato plants presenting different levels of root-galling damage, using a multiphasic approach. The experimental design was based on representative samplings of healthy and infected tomato plants from two field locations in Uganda, to establish species collections and DNA libraries. Root galling symptoms on tomato resulted from a multispecies infection of root-knot nematodes ( Meloidogyne spp.). Results revealed that 16.5% of the bacterial strain collection produced nematicidal volatile organic compounds (nVOC) active against Meloidogyne . Using SPME GC-MS, diverse VOC were identified, including sulfuric compounds, alkenes and one pyrazine. Around 28% of the bacterial strains were also antagonistic toward at least one fungal pathogen of the disease complex. However, antagonistic interactions appear highly specific. Nematicidal antagonists included Pseudomonas , Comamonas , and Variovorax and fungicidal antagonists belonged to Bacillus , which interestingly, were primarily recovered from healthy roots, while nematode antagonists were prominent in the rhizosphere and roots of diseased roots. In summary, all antagonists comprised up to 6.4% of the tomato root microbiota. In general, the microbiota of healthy and diseased root endospheres differed significantly in alpha and quantitative beta diversity indices. Bacteria-derived volatiles appear to provide a remarkable, yet wholly unexploited, potential to control Meloidogyne -based soil-borne disease complexes. The highly specific observed antagonism indicates that a combination of volatiles or VOC-producing bacteria are necessary to counter the range of pathogens involved in such complexes.

No abstract

Fungicides in maize production under tropical conditions reduce losses from foliar diseases, but only a few reduce ear rot incidence or mycotoxin contamination in kernels. Biocontrol agents (BCAs) may reduce postharvest losses but their efficacy has not been demonstrated in field conditions. Here, we evaluated the use of bacterial isolates in tandem with fungicides on Fusarium verticillioides incidence and fumonisin content. After an early screening, Bacillus subtilis and Streptomyces araujoniae isolates were used in field trials. Maize plants were sprayed twice: at the end of the vegetative stage (V9) and at the beginning of the reproductive stage (R1). Sprays were made by applying water, B. subtilis strain BIOUFLA2, S. araujoniae strain ASBV‐1T, or fungicide (cyproconazole + azoxystrobin) in different combinations, totalling nine treatments. Ten days later, all maize ears were inoculated with F. verticillioides. Plants were assessed for foliar diseases, grain yield, F. verticillioides incidence and fumonisin content in kernels. The treatment with two fungicide sprays reduced most of the foliar diseases but not F. verticillioides incidence in kernels. Twice‐sprayed B. subtilis and S. araujoniae reduced F. verticillioides, but did not protect leaves against other pathogens. All treatments encompassing a fungicide followed by one of the BCAs reduced F. verticillioides incidence compared to control. Twice‐sprayed fungicide increased fumonisin by 50% compared to water control, while fungicide followed by B. subtilis decreased the fumonisin content by 40%. Replacing the second chemical spray with S. araujoniae did not reduce the fumonisin content but provided a higher yield than a twice‐sprayed fungicide. Exclusive use of chemical fungicides may not ensure higher grain quality and yield, but the integration with B. subtilis BIOUFLA2 can accomplish both.

No abstract

The integrated management of rice leaf blast (Magnaporthe oryzae) is carried out mainly with the adoption of chemical control. However, the search for alternative practices has grown in recent years. Thus, the variability of 28 Sarocladium oryzae isolates was evaluated for the cerulenin production, as well as its potential for reducing the severity of rice leaf blast, quantifying the activity of enzymes linked to the plant defense mechanisms. More than 55 % of the S. oryzae isolates were antagonistic to the pathogens M. oryzae, Cochliobolus miyabeanus, Thanatephorus cucumeris and Monographella albescens, and 60 % of the isolates produced cerulenin at detectable levels. Both BRM 6461 (296.0 µg mL-1) and BRM 6493 (undetectable cerulenin) inhibited the formation of M. oryzae appressoria in 89.5 % and 85 %, respectively. The BRM 6461 isolate, applied as conidial suspension and filtered, reduced the severity of rice leaf blast in 68.8 % and 75.5 %, respectively. The enzymatic activity in the presence of M. oryzae was higher for lipoxygenase at 5 h (filtered) and at 24 h and 72 h (conidial suspension) after the pathogen inoculation. For phenylalanine ammonia lyase, the highest expression was at 5 h (filtered) and 72 h (conidial suspension). The enzymes chitinase, β-1,3-glucanase and peroxidase and the salicylic acid phytohormone presented no differences, in relation to the controls (water and M. oryzae). The filtered from the BRM 6461 isolate, basically constituted by cerulenin, reduced the severity of rice leaf blast and possibly activated the defense mechanisms of the rice plants against M. oryzae.

No abstract

BACKGROUND The combination of biocontrol agents is a desirable strategy to improve control efficacy against the root‐knot nematode (RKN) Meloidogyne incognita under field conditions. However, strains compatibility is generally tested in vitro and incompatible combinations are normally not further examined in experiments in planta. Therefore, there is virtually no information on the performance of incompatible strains. In this study, we evaluated two Bacillus strains previously described as incompatible in vitro for effects on plant growth and suppression of M. incognita, pathogenic fungi and nematophagous fungi. RESULTS Strains BMH and INV were shown to be closely related to Bacillus velezensis. These strains, when applied individually, reduced the number of galls and eggs of M. incognita by more than 90% in tomato roots. When BMH and INV were combined (BMH + INV), RKN suppression and tomato shoot weight were lower compared to single‐strain applications. Additionally, metabolites in cell‐free supernatants and volatile organic compounds (VOCs) from strains BMH and INV had strong effects against the plant pathogens M. incognita, Fusarium oxysporum, Rhizoctonia solani and Sclerotium rolfsiii, but not against three species of nematophagous fungi. Although strain INV and the combination BMH + INV emitted fewer VOCs than strain BMH, they were still capable of killing second‐stage juveniles of M. incognita. CONCLUSIONS Bacillus strains BMH and INV inhibited M. incognita and fungal pathogens, and promoted tomato growth. However, strain INV emitted fewer VOCs and the combination BMH + INV did not enhance the activity of the biocontrol strains against the RKN or their capacity to promote plant growth. © 2021 Society of Chemical Industry.

The incorporation of organic matter to the soil not only improves nutrient content, but also reduces the survival of Stenocarpella, the causal agent of stalk rot, Diplodia ear rot, and grey leaf spot, in maize stubble. We evaluated the effect of organic waste incorporation on Stenocarpella survival in maize stalks, the activity of suppressiveness-related enzymes, and nutrient contents. We conducted the assays in the municipalities of Lavras and Sete Lagoas, Minas Gerais State, Brazil. Maize stalks infested with Stenocarpella were kept in field conditions for three months after poultry litter, swine manure, fish hydrolysate, compost sewage sludge, and urea application. Infested stalks, without residue amendment, were kept on surface or incorporated into the soil, representing negative and positive controls. Stenocarpella concentration in stalks was assessed using qPCR expressed as cycle threshold number. Sewage sludge, buried stalks, and stalks retained on the surface showed reduction of pathogen inoculum. Swine manure and urea did not reduce the quantity of DNA. In the experiment conducted in Lavras, poultry litter stimulated β-glucosidase, urease and hydrolysis of fluorescein diacetate activities when compared to the negative control. Sewage sludge, poultry litter, and swine manure increased Ca in the soil by 44 %, 38 % and 36 %, respectively, in the experiment conducted in Lavras. Poultry litter increased N total three months after application. The results indicate that organic wastes are promising in improving nutrient content, activity of hydrolysis-related enzymes, but Stenocarpella inoculum dynamics should be taken into consideration when deciding on the specific organic amendment.