Does the Presence of Detached Root Border Cells of Zea mays Alter the Activity of the Pathogenic Nematode Meloidogyne incognita?

ABSTRACT The root-knot nematode Meloidogyne incognita is a major pathogen of a range of important crops. Currently, control is typically achieved by the use of nematicides. However, recent work suggests that manipulating the ability of roots to slough off border cells, which then act as a decoy to the nematode, can significantly decrease damage to the roots. We investigated the attractiveness of border cells to M. incognita and the response of the nematode to border cells in close proximity. We found very limited attraction, in that nematodes did not preferentially alter direction to move toward the border cells, but a large and significant increase in nematode speed was observed once they were in the immediate vicinity of border cells. We discuss the results in the context of physical and biological mechanisms in relation to the control of pathogenic nematodes.     

Phenotypic and genotypic structure of Phytophthora infestans populations on tomato and potato in the North of Thailand in 2000–2002

A total of 80 single–lesion isolates of Phytophthora infestans were collected from tomatoes and potatoes in several locations in Chiang Mai and Tak provinces in 2000–2002. These isolates were analyzed for mating type, metalaxyl sensitivity, mitochondrial DNA (mtDNA) haplotype RFLP pattern as determined by probe RG57, and for microsatellite markers. All isolates were A1 mating type. Isolates from tomato were usually sensitive to metalaxyl, but isolates from potato were usually resistant to metalaxyl. With one exception, all tomato isolates were related to the US-1 clonal lineage. With two exceptions, all potato isolates were related to two European lineages. In these two provinces, the populations of P. infestans on tomatoes are clearly different from those on potatoes.     

Jasmonic acid and abscisic acid play important roles in host–pathogen interaction between Fusarium graminearum and wheat during the early stages of fusarium head blight

The effect of phytohormones on the defense response of wheat against Fusarium graminearum infection was investigated. Infection of heads with F. graminearum induced accumulation of salicylic acid (SA), jasmonic acid (JA), abscisic acid (ABA), and indole acetic acid (IAA). Exogenous phytohormone treatments showed crosstalk between them and a complex effect on expression of the genes ATB2, ExpB6, LEA Td16, PR1, Pdf1.2, PR4. JA treatment reduced F. graminearum growth and fusarium head blight (FHB) symptoms while an increase in FHB was observed with ABA. Transient down-regulation of allene oxide synthase (AOS) supports a complex role for JA in wheat head.     

Does yield loss due to weed competition differ between organic and conventional cropping systems?

High weed abundance in organic crops is thought to be a key factor contributing to the greater yield loss in organic as compared with conventional cropping systems. However, even with greater weed densities than conventional systems, some organic systems have yields comparable to conventional systems, suggesting that cropping systems might differ in yield loss due to weed competition. The diversity in soil nutrient resources due to diversity in crop rotations and variable inputs might enhance crop tolerance to weed competition. We assessed the long‐term effects of contrasting levels of crop rotations (low, medium and high diversity) on weed density, weed biomass and wheat yield loss in organic and no‐till conventional cropping systems using a microplot study within a long‐term cropping systems trial at Scott, Saskatchewan, Canada. Weed density and biomass were found to be four times higher in the organic systems than in the conventional systems. Under standard weed management practices, organic had 44% lower yield than the conventional system. Lower yields in organic, even without weed competition, suggest that the lower yields are due to low soil productivity rather than weed competition. No differences in yield loss were observed among the organic and conventional systems or among the diverse crop rotations. We conclude that the organic management practices and/or increased crop rotation diversity did not enhance yield or reduce yield loss due to weed competition, due to the factors associated with lower soil fertility.     

How does the Emerald Ash Borer (Agrilus planipennis) affect ecosystem services and biodiversity components in invaded areas?

Environmental risk assessment (ERA) is an important component of risk analysis for plant pests and invasive alien species (IAS), and a standardized and consistent methodology has recently been developed for evaluating their impact on ecosystem services and biodiversity. This paper presents the application of this innovative methodology for ERA to Agrilus planipennis, the emerald ash borer, which causes significant mortality to Fraxinus (ash) species in forests and urban areas of North America (here: USA and Canada, excluding Mexico) and Russia. The methodology follows a retrospective analysis and summarizes information and observations in invaded areas in North America and Russia. Uncertainty distributions were elicited to define quantitatively a general pattern of the environmental impact in terms of reduction in ecosystem provisioning, supporting and regulating services, and biodiversity components. The environmental impacts of A. planipennis are time‐ and context‐dependent, therefore two time horizons of 5 and 20 years after introduction and two ecosystems (urban and forest) were considered. This case study shows that the quantitative assessment of environmental impacts for IAS is both possible and helpful for decision‐makers and risk managers who have to balance control costs against potential impacts of IAS.     

Relations between soil heterogeneity and common reed(Phragmites australis Trin.ex Steud.) colonization in the Keriya River Basin,Xinjiang of China

How common reed （Phragmites australis Trin. ex Steud.） colonization correlates to soil heterogeneity and environmental determinants remains unclear in arid areas. We conducted a field investigation and soil sampling in 100 plots along Keriya River Basin to uncover the relationship between common reed and heterogeneous soils. Reed colonization variables and its soil properties were measured and recorded for the analysis of their relationship using Pearson correlation and redundancy analysis methods. The comparison results of common reed characteris- tics among 100 plots showed that common reeds performed strong tolerance and ecophysiological plasticity to edaphic stresses. Common reed colonization was tightly connected to soil heterogeneity according to the correla- tion analysis between its colonization characteristics and soil properties. Common reed colonization got feedbacks on soil properties as well, including the increase of soil organic matter and the alleviation of salt uplifting. The main limiting environmental determinant of common reed colonization was soil salt, followed by pH and soil water content.     

Are we analysing knockdown in the right way? how independence of the knockdown-recovery process from mortality may affect measures for behavioural effects in pesticide bioassays

In pesticide bioassays, especially those with neurotoxic agents, effects on animals are typically grouped into classes according to behaviour, such as normal and affected behaviour, which may range from unstable walking behaviour, to unable to move, to mortality. Generally, recovery is observed in all these effect classes, except the last. Mortality, however, disturbs the analysis of the recovery processes because it decreases the number of animals that otherwise could have shown a reversible effect. We consider that this interaction between mortality and other, reversible, effects is a conceptual problem, and give arguments in favour of analysing changes in behaviour and mortality as two independent, simultaneously occurring neurotoxic syndromes. As an illustration, two data sets are analysed in both ways and these show that marked differences may exist between conclusions reached by the two viewpoints. The consequences thereof are discussed in relation to toxico-kinetic explanations for neurotoxicant effects on behaviour and mortality. © 1999 Society of Chemical Industry     

Does agricultural use of azole fungicides contribute to resistance in the human pathogen Aspergillus fumigatus?

Azole resistance in human fungal pathogens has increased over the past twenty years, especially in immunocompromised patients. Similarities between medical and agricultural azoles, and extensive azole (14α‐demethylase inhibitor, DMI) use in crop protection, prompted speculation that resistance in patients with aspergillosis originated in the environment. Aspergillus species, and especially Aspergillus fumigatus, are the largest cause of patient deaths from fungi. Azole levels in soils following crop spraying, and differences in sensitivity between medical and agricultural azoles (DMIs), indicate weaker selection in cropping systems than in patients receiving azole therapy. Most fungi have just one CYP51 paralogue (isozyme CYP51B), but in Aspergillus sp. mutations conferring azole resistance are largely confined to a second paralogue, CYP51A. Binding within the active centre is similar for medical and agricultural azoles but differences elsewhere between the two paralogues may ensure selection depends on the DMI used on crops. Two imidazoles, imazalil and prochloraz, have been widely used since the early 1970s, yet unlike triazoles they have not been linked to resistance in patients. Evidence that DMIs are the origin, or increase the frequency, of azole resistance in human fungal pathogens is lacking. Limiting DMI use would have serious impacts on disease control in many crops, and remove key tools in anti‐resistance strategies. © 2017 Society of Chemical Industry     

Long-term effects of gravel―sand mulch on soil organic carbon and nitrogen in the Loess Plateau of northwestern China

Gravel–sand mulch has been used for centuries to conserve water in the Loess Plateau of northwestern China. In this study, we assessed the influence of long-term(1996–2012) gravel–sand mulching of cultivated soils on total organic carbon(TOC), light fraction organic carbon(LFOC), microbial biomass carbon(MBC), total organic nitrogen(TON), particulate organic carbon(POC), mineral-associated organic carbon(MOC), permanganate-oxidizable carbon(KMn O4-C), and non-KMn O4-C at 0–60 cm depths. Mulching durations were 7, 11 and 16 years, with a non-mulched control. Compared to the control, there was no significant and consistently positive effect of the mulch on TOC, POC, MOC, KMn O4-C and non-KMn O4-C before 11 years of mulching, and these organic C fractions generally decreased significantly by 16 years. LFOC, TON and MBC to at a 0–20 cm depth increased with increasing mulching duration until 11 years, and then these fractions decreased significantly between 11 and 16 years, reaching values comparable to or lower than those in the control. KMn O4-C was most strongly correlated with the labile soil C fractions. Our findings suggest that although gravel–sand mulch may conserve soil moisture, it may also lead to long-term decreases in labile soil organic C fractions and total organic N in the study area. The addition of manure or composted manure would be a good choice to reverse the soil deterioration that occurs after 11 years by increasing the inputs of organic matter.     

Effect of the disodium salt of cyclopentene β,β′-triketone on the development of infections induced by TMV in the leaves of hypersensitive and susceptible tobacco plants

Journal of Plant Diseases and Protection - We show here that the disodium salt of 2-acetyl-5-chloro-4- hydroxycarbonylmethylthiocyclopent-4-ene-1,3-dione (salt 2) inhibits tobacco mosaic virus...     

Annual dynamics of Zymoseptoria tritici populations in wheat cultivar mixtures: A compromise between the efficacy and durability of a recently broken-down resistance gene?

Cultivar mixtures slow polycyclic epidemics but may also affect the evolution of pathogen populations by diversifying the selection pressures exerted by their plant hosts at field scale. We compared the dynamics of natural populations of the fungal pathogen Zymoseptoria tritici in pure stands and in three binary mixtures of wheat cultivars (one susceptible cultivar and one cultivar carrying the recently broken-down Stb16q gene) over two annual field epidemics. We combined analyses of population “size” based on disease severity, and of population “composition” based on changes in the frequency of virulence against Stb16q in seedling assays with more than 3000 strains. Disease reductions were observed in mixtures late in the epidemic, at the whole-canopy scale and on both cultivars, suggesting the existence of a reciprocal protective effect. The three cultivar proportions in the mixtures (0.25, 0.5, and 0.75) modulated the decrease in (a) the size of the pathogen population relative to the two pure stands, (b) the size of the virulent subpopulation, and (c) the frequency of virulence relative to the pure stand of the cultivar carrying Stb16q. Our findings suggest that optimal proportions may differ slightly between the three indicators considered. We argue potential trade-offs that should be taken into account when deploying a resistance gene in cultivar mixtures: between the dual objectives “efficacy” and “durability,” and between the “size” and “frequency” of the virulent subpopulation. Based on current knowledge, it remains unclear whether virulent subpopulation size or frequency has the largest influence on interepidemic virulence transmission.