Disease Management

Plants can be attacked by a variety of parasitic microorganisms, primarily fungi, bacteria, nematodes, and viruses. From 2001 to 2003, an average of 7% to 15% of the major world crops (wheat, rice, potato, maize, and soybean) were lost due to fungi and bacteria (Oerke, 2005). Along with weeds and insects, plant pathogens are the major biotic limitation to crop health and yield. Many of these pathogens are foliar and attack aboveground parts of plants, with inoculum spread by wind and rain. Examples are rust, powdery mildew, and foliar leaf pathogens such as the fungus Septoria. However, some of the most severe, intractable, and difficult to control pathogens are soil-borne pathogens, which live in the soil for part or all of their life cycle and interact with the soil biota and the edaphic environment. These pathogens can survive in the soil and infect the root systems of plants. Fungi, fungus-like Stramenopiles (Oomycetes) and nematodes are probably the most important of the soil-borne pathogens. Fungi are eukaryotic organisms that form threadlike filamentous hyphae that can spread through the soil and form resistant structures such as oospores or sclerotia. These structures allow the fungus to survive in the absence of the host, or during unfavorable environments such as heat, cold, or dry soils. When these resting structures encounter a seed or root in the soil, they are stimulated to germinate, chemotactically grow toward the root, and infect the epidermal cells. Some fungi can also destroy seedlings before they emerge from the soil. Once the root is infected, fungi can spread inside the root, rotting the root by producing enzymes and toxins. Fungi also destroy lateral roots, feeder roots, and root hairs. As a result, the plant loses its ability to absorb water and nutrients. Above ground, plants are stunted and show nutrient deficiencies, and yields are reduced. Some pathogens can also move up the roots to the base of the plant, girdling the base or infecting the lower stem. Finally, another group of fungal pathogens can induce wilt by colonizing the xylem system, restricting the conduction of water to the leaves

Identification of Resistance to Pratylenchus neglectus and Pratylenchus thornei in Iranian Landrace Accessions of Wheat

The pathogenic nematodes Pratylenchus neglectus (Rensch, 1924) Filipjev and Schuurmans Stekhoven, 1941 and Pratylenchus thornei Sher and Allen, 1953 cause severe yield losses in wheat (Triticum aestivum L.). The objectives in this study were to assay a collection of Iranian landrace accessions collected from 12 provinces in Iran to identify novel sources of resistance to both species and to characterize agronomic traits critical for consideration in wheat breeding. Seventy-eight accessions were assayed for dual resistance to parasitic nematodes P. neglectus and P. thornei in controlled environment assays. Field trials conducted in Pullman, WA, and Pendleton, OR, evaluated stripe rust (Puccinia striiformis f. sp. tritici) resistance, days to heading, grain volume weight, plant height, seed protein content, seed kernel characterization, glume tenacity, and pubescence. The accessions were assayed with simple-sequence repeat (SSR), single-nucleotide polymorphism (SNP), and known vernalization markers for hierarchical cluster analysis to identify relatedness among accessions. Thirty-two accessions were identified as resistant or moderately resistant to both Pratylenchus species. Six were identified with moderate adult plant resistance to stripe rust in the field. The range of mean agronomic traits over locations was 53 to 105 cm for plant height, 46 to 84 d for post planting days to heading, and 151 to 728 kg m⁻³ for grain volume weight. The genetic cluster analysis identified three clusters based on the number of rare polymorphisms in the subset. The nematode resistance was distributed over the three clusters. The diversity within this subset could be useful for wheat breeders to integrate genetic variation and resistance to both Pratylenchus spp

Relationship Between Climatic Factors and Distribution of Pratylenchus spp. in the Dryland Wheat-Production Areas of Eastern Washington

Field surveys were conducted by collecting soil samples to estimate nematode densities in soil from winter wheat, spring wheat, spring barley, and spring legumes (lentil, chickpea, and pea) fields during 2010 and 2011. Pratylenchus spp. were observed in 60% of sampled fields. However, nematodes were detected in nearly all of the survey fields in high numbers where crops were grown every year. To identify climatic variables associated with density of Pratylenchus spp. in soil, correlation and regression analyses were performed using climate data of survey sites from 1979 to 2010. Fifty-seven climate variables were significantly correlated with densities of Pratylenchus spp. All precipitation variables were significantly positively correlated with nematode abundance. Summer maximum air temperature was negatively correlated and winter minimum air temperature was positively correlated with nematode densities. In addition, both years’ nematode densities were significantly correlated with cropping intensity. Five multivariate regression models for 2010 and seven models for 2011 nematode abundance levels were developed. The majority of the climate variables selected in the models were related to precipitation. Knowledge of root-lesion nematode distribution in the dryland region of eastern Washington and associated climate variables may be helpful to determine risk and apply management practices to minimize crop damage

An Improved Method for Establishing Accurate Water Potential Levels at Different Temperatures in Growth Media

NaCl, KCl, or PEG (polyethylene glycol)-amended potato dextrose broth (PDB), and potato dextrose agar (PDA) are essential for pure culture studies of water stress on fungi. Direct information on the actual water potential (WP) of this salt-amended PDB and PDA is lacking. Much fungal research in the past calculated WP of these salt-amended growth media by adding the WP of their constituents taken from individual salt dilution studies. But the WP of any complex solution will be modified by the level of synergism between its solutes. This study presents evidence of change in NaCl concentration due to synergism for attaining the same level of WP in NaCl solution, and NaCl amended PDB and PDA. The relation between WP and temperature and WP and salt concentration is also modified depending on the number of solutes in a growth medium. The WP of PEG-amended PDB increases with rising temperature, while that of NaCl/KCl amended PDB and PDA decreases with the increase of temperature. These results can be useful for doing pure culture studies on the biology and modeling the growth of air, water, and soil-borne fungi important in the food and agriculture industry and in terrestrial and aquatic ecosystems

Diseases Which Challenge Global Wheat Production: Root, Crown, and Culm Rots

Most wheat ('Triticum' spp.) diseases caused by root-, crown- and lower culm-infecting fungi are not yet effectively managed by genetic resistance or by application of a fungicide or biological control agent. The best management strategy for many of these diseases continues to depend upon changing the soil environment in ways that either influence the survival of the pathogen between susceptible host crops or the pathogen's virulence during the infective stage (Cook and Veseth 1991). The soilborne plant-pathogenic fungi that cause the seven root, crown, and culm rots summarized in this chapter are heavily influenced by soil physical and chemical properties, by interactions with associated microbes and microfauna in soil and on plant surfaces, and by the capacity of plants to serve as hosts for growth and multiplication. The complexity of factors affecting these pathogens before and during pathogenesis is immense

The rhizosphere: a playground and battlefield for soilborne pathogens and beneficial microorganisms

International audienceThe rhizosphere is a hot spot of microbial interactions as exudates released by plant roots are a main food source for microorganisms and a driving force of their population density and activities. The rhizosphere harbors many organisms that have a neutral effect on the plant, but also attracts organisms that exert deleterious or beneficial effects on the plant. Microorganisms that adversely affect plant growth and health are the pathogenic fungi, oomycetes,bacteria and nematodes. Most of the soilborne pathogens are adapted to grow and survive in the bulk soil, but the rhizosphere is the playground and infection court where the pathogen establishes a parasitic relationship with the plant. The rhizosphere is also a battlefield where the complex rhizosphere community, both microflora and microfauna, interact with pathogens and influence the outcome of pathogen infection. A wide range of microorganisms are beneficial to the plant and include nitrogen-fixing bacteria, endo- and ectomycorrhizal fungi, and plant growth-promoting bacteria and fungi. This review focuses on the population dynamics and activity of soilborne pathogens and beneficial microorganisms. Specific attention is given to mechanisms involved in the tripartite interactions between beneficial micro-organisms, pathogens and the plant. We also discuss how agricultural practices affect pathogen and antagonist populations and how these practices can be adopted to promote plant growth and health

Characterization of sporulation of Alternaria alternata f. sp. sphenocleae

Studies were conducted on agar media to characterize the factors for the optimization of sporulation of Alternaria alternata f. sp. sphenocleae, a fungal pathogen being evaluated as a biological control agent for Sphenoclea zeylanica (gooseweed). A. alternata f. sp. sphenocleae conidiation was affected by nutrition, temperature, light conditions, and moisture. On all agar media tested, except for half-strength potato dextrose agar ( 1/2 PDA) and V-8 juice agar (VJA), exposure to different light conditions did not have any significant effect on conidia production. However, when comparing 1/2 PDA and VJA, sporulation under constant near-ultraviolet (NUV) light at 28°C increased markedly on VJA, but decreased substantially on 1/2 PDA. This trend, however, was opposite under dark conditions since 1/2 PDA produced the greatest number of conidia whereas a 75% reduction in conidia production occurred on VJA in the dark. On all the standard agar media evaluated, the most virulent conidia were obtained on 1/2 PDA at 28°C under constant NUV incubated for 4 weeks. Sporulation of A. alternata f. sp. sphenocleae using the sporulation medium (S-medium) technique was rapid. Conidia were produced within 24 h and continuous sporulation was still observed until 120 h. The best primary agar media for conidia production were PDA, 1/2 PDA and VJA, while water agar was the poorest. Conidia production was optimized with the addition of 20 g l of calcium carbonate (CaCO) and the addition of 2 ml of sterile distilled water on the medium. The most virulent conidia were produced when the primary agar was 1/2 PDA, the CaCO concentration was 20 g l, and the cultures were incubated at 18°C in the dark. Conidiophore induction occurred on nutrient rich media and was stimulated by NUV, while formation of conidia proceeded in darkness after nutrients were depleted under warm dry or cool moist conditions. Culture media, growth conditions, and CaCO affected the inoculum potential of A. alternata f. sp. sphenocleae conidia