Stem Base Pathogens Research Articles

Population dynamics of Rhizoctonia, Oculimacula, and Microdochium species in soil, roots, and stems of English wheat crops

AbstractThis study aimed to elucidate the population dynamics of Rhizoctonia, Oculimacula, and Microdochium species, causing the stem base disease complex of sharp eyespot, eyespot, and brown foot rot in cereals. Pathogen DNA in soil, roots, and stem fractions, and disease expression were quantified in 102 English wheat fields in two seasons. Weather data for each site was collected to determine patterns that correlate with assessed diseases. Oculimacula spp. (66%) and R. solani AG 2‐1 (63%) were most frequently detected in soil, followed by R. cerealis (54%) and Microdochium spp. (33%). Oculimacula spp. (89%) and R. cerealis (56%) predominated on roots and soil but were not associated with root rot symptoms, suggesting that these species used soil and roots for survival and as inoculum source. M. nivale was more frequently detected than M. majus on stems up to GS 21–30 and co‐occurred on plant samples with O. acuformis. O. yallundae had higher DNA concentration than O. acuformis at the lower 5 cm basal region at GS 37–45. R. cerealis predominated in the upper 15 cm above the base beyond stem extension. Brown foot rot by Microdochium spp. was favoured by cool and wet autumns/winters and dominated in English wheat. Eyespot and sharp eyespot disease index by Oculimacula spp. and R. cerealis, respectively, correlated with wet/humid springs and summers. Results suggested that stem base pathogens generally coexisted; however, their abundance in time and space was influenced by favourable weather patterns and host development, with niche differentiation after stem extension.

Antagonistic yeasts competes for iron with winter wheat stem base pathogens

Aureobasidium pullulans and Sporobolomyces roseus are a saprotrophic yeasts fungi commonly found on the leaves of winter wheat and on wheat kernels. The objective of this study was to compare the inhibitory effects of two species of yeasts fungi, Aureobasidium pullulans var. pullulans (de Bary) G. Arnaud and Sporobolomyces roseus Kluyver & van Niel, on the causal agents of stem base diseases, Rhizoctonia cerealis v. d. Hoeven, Gaeumannomyces graminis (Sacc.) Arx & D. Olivier, Helgardia herpotrichoides (Fron) Crous & W. Gams, Fusarium oxysporum (Schlecht) Snyd. et Hans.) and Fusarium culmorum (W. G. Smith). A. pullulans showed stronger inhibitory activity than S. roseus. Among the 70 A. pullulans isolates tested in the study, 25 were capable of suppressing the colony growth of R. cerealis under in vitro conditions. This is the first study to show that A. pullulans competes for iron with stem base pathogens, in particular with fast-growing R. cerealis and F. culmorum. Under greenhouse conditions, A. pullulans protected winter wheat seedlings against infection caused by F. culmorum, from two to four times compared with the control, and its protective effect was determined by the infection susceptibility of wheat cultivars and the time interval between the application of A. pullulans and inoculation with F. culmorum.

Use of SCAR-PCR in diagnostics of stem base pathogens of the Rhizoctonia and Oculimacula genus

The aim of the paper is to compare the efficacy of SCAR-PCR assay and conventional diagnostic technique (visual assessment, isolation on PDA medium) in the identification of fungi from the genera <i>Rhizoctonia</i> and <i>Oculimacula</i> from winter triticale, rye, and barley during the shooting stage. The usefulness of molecular diagnosis of fungal pathogens in crop plants has been demonstrated. The application of SCAR- -PCR assay allowed early detection of the following pathogens: <i>O. yallundae</i>,<i> O. acuformis</i>, <i>R. cerealis</i> and <i>R. solani</i>, in plant tissues. This method was particularly effective in detection of <i>R. solani</i>. The research showed the usefulness of PCR markers for early detection of fungal pathogens, even if symptoms were not visible. Using the PCR technique, especially in combination with conventional methods, substantially increases the precision and effectiveness of disease diagnostics.

EYESPOT INFECTION RISKS ON WHEAT WITH RESPECT TO CLIMATIC CONDITIONS AND SOIL MANAGEMENT

SUMMARY Climatic conditions and soil management rank among the most important factors influencing the infection of winter wheat by eyespot and other stem-base diseases. Records of the climatic conditions (temperature, precipitation, and air humidity) over a 14-year period, 1994-2007, were used to develop a prediction model for eyespot infection using multiple regression. Since the most critical factor affecting eyespot is precipitation, graded watering (0, 10 and 20 liters of water) was tested at various timings of application in small-plot experiments in 2005-2007 in order to determine the severity of stem base infection by eyespot. In addition, in 2006 and 2007, the effect of soil management (tillage 22 cm, 15 cm, and no-till) on wheat infection by eyespot and other stem-base pathogens was measured using molecular diagnosis. The climatic model developed is statistically significant, showing that the most important factor affecting eyespot infection in wheat is the number of days with rainfall above 3 mm through October to April. Experimental watering did not significantly affect the severity of infection. Soil management had only a limited effect on the incidence of stem-base diseases. F. avenaceum was more prevalent in the variant with tillage 22 cm in the year 2007.

The role of organic vs. conventional farming practice, soil management and preceding crop on the incidence of stem-base pathogens on wheat

A stem-base disease complex of winter wheat includes several causal agents, such as eyespot (Oculimacula yallundae and O. acuformis), sharp eyespot (Rhizoctonia cerealis) and brown foot rots (Fusarium graminearum, F. culmorum, F. avenaceum, F. po ae, Microdochium nivale vars. nivale and majus). Farming practices (organic and conventional), soil management (ploughing compared to min- and no-tillage) and the preceding crop (cereal, maize and oilseed rape) were studied as factors affecting the incidence of stem-base pathogens. The stem-base diseases were assessed visually and by molecular methods (PCR) on four winter wheat cultivars at four growth stages. The comparison of organic and conventional farming practices did not result in large differences in the relative incidence of stem-base pest species over the period of this study; nevertheless, a higher frequency of positive detection was recorded for three organisms (O. acuformis, O. yallundae and M. nivale var. nivale) under conventional farming. Other factors had none or only partial effects. Soil management didn’t affect the stem-base pathogens incidence significantly. The preceding crop maize reduced the incidence of M. nivale var. majus.

Effects of fungicides on eyespot, caused predominantly by Oculimacula acuformis, and yield of early-drilled winter wheat

The effectiveness against eyespot of nine fungicide treatments, applied at GS 31, was evaluated in early-drilled winter wheat crops. Visual assessments of stem-base diseases were made before fungicide application and at GS 39/55 and 69/70 of cultivars Consort, Savannah and Claire. Competitive PCR was used to quantify Oculimacula acuformis, O. yallundae, Microdochium nivale, Fusarium spp. and Rhizoctonia cerealis. PCR results indicated that O. acuformis was the predominant pathogen causing eyespot in all three crops. Treatments containing cyprodinil were most successful in reducing disease index and DNA of O. acuformis across cultivars. Treatments containing prochloraz were less consistent in eyespot control, significantly reducing the DNA of O. acuformis only in one instance at GS 70 of cultivar Claire. None of the fungicide treatments had any significant effect on brown foot rot or sharp eyespot, or DNA of M. nivale or R. cerealis. The potential for O. acuformis to cause yield loss was indicated by significant negative relationships between yield and eyespot incidence or DNA of O. acuformis at GS 69/70 for each wheat crop. Relationships between DNA of O. acuformis and eyespot incidence or severity at each growth stage were weak but generally improved at the later growth stages of the crops when disease symptoms were easily distinguished. Both molecular methods and visual assessment failed to predict eyespot incidence or pathogen DNA late in season using incidence or DNA of O. acuformis at GS 31. PCR assays, however, accurately identified the stem base pathogens at early growth stages of the crops when the symptoms of their respective diseases were not readily diagnosed.

Development of stem-base pathogens on different cultivars of winter wheat determined by quantitative PCR

The progress of development of stem-base pathogens in crops of second winter wheat was plotted in nine experiments in three years. The amount of each pathogen present was determined by quantitative PCR. Where Tapesia yallundae was present in quantifiable amounts, it usually developed earlier than the other eyespot pathogen, T. acuformis. Both species were usually present in greater amounts on cultivars which are more susceptible to eyespot. The sharp eyespot pathogen, Rhizoctonia cerealis, developed more erratically than either of the Tapesia spp. and there were no consistent effects on different cultivars. Fusarium spp., the cause of brown foot rot, were rarely present in quantifiable amounts, but Microdochium nivale was usually present as one or both of the varieties nivale and majus. Late-season (after anthesis) decreases in M. nivale suggest that any brown foot rot symptoms attributable to this fungus would have fully developed earlier. Cultivar differences in amounts of M. nivale were most clear in stems during internode extension and when relatively large amounts of DNA were present. Such differences approximately reflected eyespot susceptibility, cv. Soissons containing most and cv. Lynx containing least DNA. The results emphasise the difficulty in relating diagnoses, by quantitative PCR or other means, at early growth stages when decisions to apply fungicides against stem-base disease are made, to later disease severity.

Effects of cultivar and fungicides on stem‐base pathogens, determined by quantitative PCR, and on diseases and yield of wheat

SummaryThe polymerase chain reaction (PCR) was used to identify and quantify all fungal pathogens of wheat (Triticum aestivum) stem bases in nine field experiments at three locations in England. The main aim was to apply quantitative PCR to provide robust data on the efficacy of new fungicides against the individual components of the stem‐base disease complex. Cyprodinil most effectively controlled eyespot by decreasing both pathogens, Tapesia yallundae and T. acuformis (the most widespread species), and sometimes contributed to increased yields. Prochloraz controlled eyespot less consistently, its effectiveness dependent mainly on the presence of T. yallundae or on rainfall events soon after application. Azoxystrobin contributed to yield increases most consistently. Although it decreased sharp eyespot and its pathogen, Rhizoctonia cerealis, these effects were insufficient to account for much of the yield increases. The effects of fungicides on eyespot were sometimes greatest on the most susceptible cultivars. Amounts of Tapesia DNA were usually consistent with cultivar susceptibility ratings. The only pathogens of brown foot rot present in significant amounts were Microdochium nivale vars nivale and majus. They appeared not to affect yield or to respond greatly to fungicides. The susceptibility of cultivars to these pathogens was sometimes similar to their susceptibility to eyespot, suggesting that they may respond to the same host resistance genes or may in some cases be secondary colonisers of eyespot‐infected plants.

Development of a PCR assay to detect Fusarium poae in wheat

Random amplified polymorphic DNA assays were carried out on a range of isolates of F. poae to identify markers common to all isolates. Two fragments were isolated, cloned and used to probe Southern blots of DNA from F. poae and isolates from a range of wheat seed and stem base pathogens. One fragment, which hybridized preferentially to DNA of F. poae was partially sequenced and two pairs of primers (Fp8 F/R and Fp82 F/R) were generated for use in the polymerase chain reaction (PCR). Amplification of target DNA occurred following PCR of all isolates of F. poae but not from any of a range of other fungal species associated with diseases of cereal ears and seed. The primer pair Fp82 F/R was used to detect F. poae in extracts from wheat seed samples contaminated with Fusarium species. This system offers the potential to determine the presence of F. poae in wheat and avoid problems commonly associated with conventional diagnosis of the disease and isolation of the pathogen.

Strain Variation in Tolerance of Water Stress by Idriella(Microdochium)bolleyi, a Biocontrol Agent of Cereal Root and Stem Base Pathogens

Three randomly chosen isolates of Idriella bolleyi differed markedly in tolerance of water stress down to—5 MPa in vitro. The differences were seen with respect to osmotic potential of media adjusted with KCl or matric potential of media adjusted with polyethylene glycol 8000. They were consistent when assessed by linear extension of colonies, spore germination, biomass production in liquid culture and sporulation in liquid culture. In comparative tests, one strain of I. bolleyi showed similar stress tolerance to that oF Fusarium culmorum, noted as a pathogen of cereals in dry conditions, whereas another strain of I. bolleyi showed low tolerance equivalent to that of the take‐all fungus Gaeumannomyces graminis and a Phialophora sp. from maize. The extreme variation in water stress tolerance of I. bolleyi might be used to select strains for biocontrol of different cereal root‐ and foot‐rot pathogens or strains might be combined in seed‐applied inocula for consistency of biocontrol in different site or seasonal conditions.

Diversity of cutinases from plant pathogenic fungi: Purification and characterization of two cutinases from Alternaria brassicicola

Previous evidence indicated a relationship between the pH optima of fungal cutinases and the tissue specificities of respective pathogens. Cutinases with slightly acidic pH optima were associated with leaf pathogens, whereas stem base pathogens produced cutinases most active under alkaline conditions. Alternaria brassicicola secreted both types of cutinases and caused disease on all above-ground host tissues. Two cutinases, A c and B a, which were expressed after growth of the pathogen on polymer cutin and in the presence of cutin monomers as cutinase inducers, were purified to apparent homogeneity. Cutinase A c had a cutinolytic pH optimum of 6·5 and cutinase B a had a cutinolytic pH optimum of 8·5. Their molecular weights were 23·0 kDa and 21·0 kDa, respectively. Other enzymatic and structural parameters were similar for both enzymes. The V- and K m-values for p-nitrophenyl esters with acyl chains ranging from C 4 to C 16 decreased for both enzymes with increasing chain length. The esterase inhibitors ebelactones A and B were equally active competitive inhibitors of both cutinases. Amino acid compositions were similar but not identical, and the separation pattern of tryptic peptides indicated structural differences between the two isozymes. Although both cutinases could be distinguished by their pH dependency of cutinolytic activities and their size, the similarities indicate a close evolutionary relationship.