Soil properties, seasonality and crop growth stage exert a stronger effect on rhizosphere prokaryotes than the fungal biocontrol agent Fusarium oxysporum f.sp. strigae.

Fusarium oxysporum f.sp. strigae (Fos) is an effective biocontrol agent (BCA) against the parasitic weed Striga hermonthica. It acts in the rhizosphere of several tropical cereals, where it may interfere with indigenous microbial populations. To test this impact, we assessed in a 2-season field experiment at two contrasting tropical agro-ecological sites the response of nitrifying and total indigenous prokaryotic communities in the rhizosphere of maize to the exposure of the Fos-BCA "Foxy-2". At early leaf development (EC30), flowering (EC60) and senescence (EC90) stage of maize, rhizosphere samples were obtained and subjected to community analysis of bacterial and archaeal amoA (ammonia monooxigenase) (AOB, AOA) and 16S rRNA genes. Abundance and community composition of all studied genes were predominantly influenced by soil type, crop growth stage and seasonality. No major effect of "Foxy-2" was found. Notably, total archaeal community relative to bacteria dominated in the clayey soil which was linked to its strong soil organic carbon (SOC) background. Compared to bacterial nitrifiers, domination of nitrifying archaea increased towards senescence stage which was explained by biochemical differences in organic resource availability between the crop growth stages. During the short rain season, the higher archaeal abundance was mainly driven by increased availability of organic substrates, i.e., extractable organic carbon. Our findings suggested that archaea had greater rhizosphere competence than "Foxy-2" in soils with higher clay and SOC contents. We verified that "Foxy-2" in maize rhizospheres is compatible with nitrifying prokaryotes under the given environments, in particular in clayey soils dominated by archaea.

High Potassium, Calcium, and Nitrogen Application Reduce Susceptibility to Banana Xanthomonas Wilt Caused by Xanthomonas campestris pv. musacearum.

The effect of exogenous applications of potassium (K), calcium (Ca), and nitrogen (N) on the susceptibility of four banana cultivars to Banana Xanthomonas wilt (BXW) was studied. Murashige and Skoog (MS) medium with normal concentrations of K at 783 mg/liter, Ca at 121 mg/liter, and N at 841 mg/liter was modified to contain various concentrations of K, Ca, and N. Each nutrient was varied singly, each with three replicate experiments. The concentrations were K at 78, 157, 391, 783, 1,565, and 3,913 mg/liter; Ca at 12, 24, 60, 121, 241, and 603 mg/liter; and N at 84, 168, 420, 841, and 1,682 mg/liter. Plantlets were generated in vitro on normal MS medium and later exposed to the nutrient concentrations for a total of 8 weeks. Thereafter, they were artificially inoculated with Xanthomonas campestris pv. musacearum using an insulin syringe. In each nutrient, plantlets exposed to higher nutrient concentrations significantly (P < 0.0001) accumulated more nutrient in their tissues compared with those exposed to lesser nutrient concentrations. Wilt incidences were significantly reduced, and incubation periods (time from inoculation to appearance of first disease symptoms) increased, with increasing nutrient application. The study lays a background for in vivo studies aimed at management of BXW using nutrients, such as fertilizer application.

The need for culture collections to support plant pathogen diagnostic networks.

Plant-pathogenic microorganisms, by virtue of their size, similarity in disease symptoms and closely related morphologies, are notoriously difficult to diagnose and detect. Diagnosis gives proof as to the causal agent of disease and is important for developing appropriate control measures. Detection shows the presence of a microorganism and is of importance for safeguarding national and international trade. Live reference collections are required to characterize the taxonomy and function of microorganisms as a prerequisite to development of tools for diagnosis and detection. Two case studies will be presented in this paper to demonstrate the importance of microorganism collections for facilitating knowledge sharing and the development of identification methods. Fusarium wilt of banana caused by Fusarium oxysporum f. sp. cubense and sharka disease of stone fruits caused by plum pox virus (PPV) are considered. Both diseases consist of different races/strains with different host specificities, but Fusarium wilt poses a threat to food security, while PPV poses a threat to trade due to its classification as a quarantine pest, since there is no anti-virus treatment available to control sharka disease in orchards. It is only through comprehensive collections of correctly identified and well-maintained strains representing the genetic diversity of a target organism that robust, specific, reliable and efficient diagnostic and detection tools can be developed.

Plant disease diagnostic capabilities and networks.

Emerging, re-emerging and endemic plant pathogens continue to challege our ability to safeguard plant health worldwide. Further, globalization, climate change, increased human mobility, and pathogen and vector evolution have combined to increase the spread of invasive plant pathogens. Early and accurate diagnoses and pathogen surveillance on local, regional, and global scales are necessary to predict outbreaks and allow time for development and application of mitigation strategies. Plant disease diagnostic networks have developed worldwide to address the problems of efficient and effective disease diagnosis and pathogen detection, engendering cooperation of institutions and experts within countries and across national borders. Networking maximizes impact in the face of shrinking government investments in agriculture and diminishing human resource capacity in diagnostics and applied pathology. New technologies promise to improve the speed and accuracy of disease diagnostics and pathogen detection. Widespread adoption of standard operating procedures and diagnostic laboratory accreditation serve to build trust and confidence among institutions. Case studies of national, regional, and international diagnostic networks are presented.

Integrating Fusarium oxysporum f. sp. strigae into cereal cropping systems in Africa.

BACKGROUND: Striga hermonthica (Del.) Benth. (witchweed) poses the greatest biological constraint to food production in sub-Saharan Africa (SSA). Control options for Striga are currently largely ineffective or unavailable to farmers, and other management possibilities are urgently needed. Biological control obviates some of the problems of several of the other techniques and provides a management option that is durable and environmentally responsive. The efficacy of S. hermonthica control using different formulations of three isolates of Fusarium oxysporum Schlecht. emend. Synder & Hans f. sp. strigae was tested on Striga-resistant and Striga-susceptible varieties of sorghum and maize under African field conditions for the first time. RESULTS: Isolates PSM197 and Foxy 2 were effective in witchweed repression, especially when applied as pesta granules. Isolate M12-4A was less effective under the field conditions investigated. Application of the fungi was generally more beneficial in maize than in sorghum for the varieties tested. Application of the biocontrol agent caused significant decreases in the number of flowering Striga plants, and hence deposition of seeds with impact of enhancing future crop yield. CONCLUSIONS: Synergistic effects between the Striga-resistant maize line and Fusarium oxysporum f. sp strigae led to over 90% reduction in Striga emergence. These results will further encourage the distribution of the isolates tested or selection of country-specific relatives as viable and environmentally safe biocontrol agents to be used against Striga. Pesta was the most effective formulation, while seed coating may be more cost effective.