Richard A. Sikora

Distribution and toxigenicity of Aspergillus species isolated from maize kernels from three agro-ecological zones in Nigeria

Maize samples were collected during a survey in three agro-ecological zones in Nigeria to determine the distribution and aflatoxin-producing potential of members of Aspergillus section Flavi. The three agro-ecological zones were, Derived Savannah (DS) and Southern Guinea Savannah (SGS) in the humid south and North Guinea Savannah (NGS) in the drier north. Across agro-ecological zones, Aspergillus was the most predominant fungal genera identified followed by Fusarium with mean incidences of 70 and 24%, respectively. Among Aspergillus, A. flavus was the most predominant and L-strains constituted >90% of the species identified, while the frequency of the unnamed taxon S(BG) was <3%. The incidence of atoxigenic strains of A. flavus was higher in all the districts surveyed except in the Ogbomosho and Mokwa districts in DS and SGS zones, respectively, where frequency of toxigenic strains were significantly (P<0.05) higher than that of atoxigenic strains. The highest and lowest incidence of aflatoxin positive samples was recorded in the SGS (72%) and NGS (20%), respectively. Aflatoxin contamination in grain also followed a similar trend and the highest mean levels of B-aflatoxins were detected in maize samples obtained from Bida (612 ng g(-1)) and Mokwa (169 ng g(-1)) districts, respectively, in the SGS. Similarly, the highest concentrations of G-aflatoxins were detected in samples from Akwanga district in the SGS with a mean of 193 and 60 ng g(-1), respectively. When agro-ecological zones were compared, B-aflatoxins were significantly (P<0.05) higher in SGS than in NGS, and intermediate in maize samples from the DS agro-ecological zone.

Evaluation of atoxigenic isolates of Aspergillus flavus as potential biocontrol agents for aflatoxin in maize

Aflatoxin contamination resulting from maize infection by Aspergillus flavus is both an economic and a public health concern. Therefore, strategies for controlling aflatoxin contamination in maize are being investigated. The abilities of eleven naturally occurring atoxigenic isolates in Nigeria to reduce aflatoxin contamination in maize were evaluated in grain competition experiments and in field studies during the 2005 and 2006 growing seasons. Treatments consisted of inoculation of either grains in vials or ears at mid-silking stage in field plots, with the toxigenic isolate (La3228) or atoxigenic isolate alone and co-inoculation of each atoxigenic isolate and La3328. Aflatoxin B1 + B2 concentrations were significantly (p < 0.05) lower in the co-inoculation treatments compared with the treatment in which the aflatoxin-producing isolate La3228 was inoculated alone. Relative levels of aflatoxin B1 + B2 reduction ranged from 70.1% to 99.9%. Among the atoxigenics, two isolates from Lafia, La3279 and La3303, were most effective at reducing aflatoxin B1 + B2 concentrations in both laboratory and field trials. These two isolates have potential value as agents for the biocontrol of aflatoxin contamination in maize. Because these isolates are endemic to West Africa, they are both more likely than introduced isolates to be well adapted to West African environments and to meet regulatory concerns over their use throughout that region.

Fusarium oxysporum endophytes induced systemic resistance against Radopholus similis on banana

Four mutualistic endophytic fungal isolates were investigated for their ability to induce systemic resistance in banana toward the burrowing nematode, Radopholus similis in glasshouse experiments. Two isolates of Fusarium oxysporum and one of F. cf. diversisporum isolated from the cortical tissue of banana, and another isolate of F. oxysporum isolated from tomato, were compared. When the root systems of seedling banana plants were pre-inoculated with the four endophytic fungi, R. similis root penetration was reduced by 29-39% and 22-41% 5 and 15 days after nematode inoculation, respectively. Induction of systemic resistance to R. similis in banana roots by the same endophytic fungi was tested in a split-root system. Depending on the isolate, the penetration rates decreased between 30-38.5% and 26.7-45% after 5 and 15 days in the untreated half of the split-root system of plants treated with the endophytic strains when compared to those treated without the fungi. This is the first time that systemic resistance induced by a fungal endophyte has been demonstrated in banana.

Phenalenone-type phytoalexins mediate resistance of banana plants (Musa spp.) to the burrowing nematode Radopholus similis

Significance The ongoing decline of banana yields caused by pathogens and the use of toxic chemicals to manage them has attracted considerable attention because of the importance of bananas as a major staple food for more than 400 million people. We demonstrate that secondary metabolites (phenylphenalenones) of Musa are the reason for differences in cultivar resistance, and detected the phenylphenalenone anigorufone in greater concentrations in lesions in roots of a nematode-resistant cultivar than in those of a susceptible one. An in vitro bioassay identified anigorufone as the most active nematostatic and nematocidal compound. We discovered that large lipid–anigorufone complex droplets are formed in the bodies of Radopholus similis exposed to anigorufone, resulting in the nematode being killed. The global yield of bananas—one of the most important food crops—is severely hampered by parasites, such as nematodes, which cause yield losses up to 75%. Plant–nematode interactions of two banana cultivars differing in susceptibility to Radopholus similis were investigated by combining the conventional and spatially resolved analytical techniques 1H NMR spectroscopy, matrix-free UV-laser desorption/ionization mass spectrometric imaging, and Raman microspectroscopy. This innovative combination of analytical techniques was applied to isolate, identify, and locate the banana-specific type of phytoalexins, phenylphenalenones, in the R. similis-caused lesions of the plants. The striking antinematode activity of the phenylphenalenone anigorufone, its ingestion by the nematode, and its subsequent localization in lipid droplets within the nematode is reported. The importance of varying local concentrations of these specialized metabolites in infected plant tissues, their involvement in the plant’s defense system, and derived strategies for improving banana resistance are highlighted.

Precision Crop Protection - the Challenge and Use of Heterogeneity

Precision farming is an agricultural management system using global navigation satellite systems, geographic information systems, remote sensing, and data management systems for optimizing the use of nutrients, water, seed, pesticides and energy in heterogeneous field situations. This book provides extensive information on the state-of-the-art of research on precision crop protection and recent developments in site-specific application technologies for the management of weeds, arthropod pests, pathogens and nematodes. It gives the reader an up-to-date and in-depth review of both basic and applied research developments. The chapters discuss I) biology and epidemiology of pests, II) new sensor technologies, III) applications of multi-scale sensor systems, IV) sensor detection of pests in growing crops, V) spatial and non-spatial data management, VI) impact of pest heterogeneity and VII) precise mechanical and chemical pest control.

Modes of action associated with microbially induced in planta suppression of plant-parasitic nematodes

This article discusses the extensive research that has been conducted on the use of mutualistic bacterial and fungal endophytes for the biological control of plant-parasitic nematodes. This review deals in particular with the modes of action of multitrophic interactions involving endophytic bacteria or fungi that have biological control activity towards the root-knot nematode, Meloidogyne incognita, and the potato cyst nematode, Globodera pallida, on tomato or potato. The bacterial and fungal endophytes discussed here are those that: (1) have the ability to colonise the endorhiza at some point in their life-cycle; (2) can grow saprophytically in the soil or in the rhizosphere; and (3) have plant health promoting activity and antagonistic activity towards sedentary plant-parasitic nematodes.

Molecular characterization of atoxigenic strains for biological control of aflatoxins in Nigeria

Aflatoxins are highly toxic carcinogens produced by several species in Aspergillus section Flavi. Strains of A. flavus that do not produce aflatoxins, called atoxigenic strains, have been used commercially in North America as tools for limiting aflatoxin contamination. A similar aflatoxin management strategy is being pursued in Nigeria. In the current study, loci across the 68 kb aflatoxin biosynthesis gene cluster were compared among 18 atoxigenic and two aflatoxin-producing vegetative compatibility groups (VCGs) from Nigeria and an atoxigenic VCG used commercially in North America. Five of the atoxigenic VCGs had large deletions (37–65 kb) extending from the teleomeric side of the aflatoxin biosynthesis cluster. In one VCG (AV0222) the deletion extended through the cluster to the adjacent sugar cluster. The remaining twelve atoxigenic VCGs, including the VCG used for aflatoxin management in North America, contained all the aflatoxin pathway genes, but with defects. Two observations support the long-term persistence of atoxigenicity within A. flavus: first, a comparison of pathway genes revealed more changes in atoxigenic than in aflatoxin-producing isolates relative to the aflatoxin-producing strain NRRL 3357; and second, several non-synonymous changes are unique to atoxigenics. Atoxigenic VCG diversity was assessed with phylogenetic analyses. Although some atoxigenics share relatively recent ancestry, several are more closely related to aflatoxin producers than to other atoxigenics. The current study demonstrates VCGs of A. flavus in West Africa with diverse mechanisms of atoxigenicity and potential value in aflatoxin management programmes.

Isolation, formulation and antagonistic activity of rhizobacteria toward the potato cyst nematode Globodera pallida

Abstract In a screening program, 16 bacterial isolates out of 179 isolated from roots and cysts caused a significant reduction (>25%) in Globodera pallida penetration of potato roots. Six of these isolates caused significant reductions in repeated greenhouse tests. Spray application of the bacteria in suspensions with 0.2% methyl cellulose increased adhesion of the bacteria on seed pieces and promoted antagonistic activity when compared to application in water suspensions. The antagonistic activity was shown to be directly correlated with the number of colony forming units (cfu) present on the tuber. The isolates Agrobacterium radiobacter and Bacillus sphaericus at densities of 9.7 × 10 8 and3.16 × 10 9 cfu ml −1 , respectively, caused significant reductions in root infection of 24–41% in repeated experiments.

In vitro activity of Bacillus firmus against the burrowing nematode Radopholus similis, the root-knot nematode Meloidogyne incognita and the stem nematode Ditylenchus dipsaci

Abstract Antagonistic bacteria have been repeatedly shown to be promising microorganisms for the biological control of sedentary and migratory endoparasitic nematodes. Depending on the bacteria involved, the mechanisms of action include: obligate parasitism, reduction in penetration, growth inhibition due to competition for nutrients and antibiosis associated with bioactive metabolites. In the present studies, the mode of action of the antagonist bacteria Bacillus firmus, isolated from a bionematicide, was evaluated. Significant rates of paralysis and mortality were detected after incubation of three nematode species in low concentrations of the pure culture filtrates following removal of the bacterial cells. The same culture filtrates also significantly reduced hatching of Meloidogyne incognita. Pure bacterial cell suspensions added to sand also reduced survival of R. similis in bioassays by 41% over the controls. The mode-of-action responsible for nematode paralysis and mortality was therefore demonstrated to be closely associated with the production of bioactive compounds secondary metabolites by the bacteria.

Biological control of Radopholus similis in banana by combined application of the mutualistic endophyte Fusarium oxysporum strain 162, the egg pathogen Paecilomyces lilacinus strain 251 and the antagonistic bacteria Bacillus firmus

The biological control efficacy of single or multiple applications of the mutualistic endophyte Fusarium oxysporum strain 162, the egg pathogen Paecilomyces lilacinus strain 251 and the antagonistic bacteria Bacillus firmus toward Radopholus similis was investigated in pot trials with banana under glasshouse conditions. R. similis was controlled substantially in single and combined applications of F. oxysporum with P. lilacinus or B. firmus. The combination of F. oxysporum and P. lilacinus caused a 68.5% reduction in nematode density whereas the individual applications reduced the density by 27.8% and 54.8% over the controls, respectively. Combined application of F. oxysporum and B. firmus was the most effective treatment in controlling R. similis on banana (86.2%), followed by B. firmus alone (63.7%). The compatibility of the biocontrol agents, as well the capacity of F. oxysporum to colonize banana roots in the absence or presence of P. lilacinus was also investigated. P. lilacinus did not adversely affect endophytic colonization by F. oxysporum. Biological control of R. similis in banana can therefore be enhanced via combined applications of antagonists with different modes of action that target different stages in the infection process.