小麦叶锈菌侵染过程的显微和超微结构

采用光学显微技术和电子显微技术对小麦叶锈菌的侵染过程进行了研究。发现叶锈菌从气孔侵入后在气孔腔内形成气孔下泡囊,然后分化出圆形的膨大体,由膨大体产生1—2初生菌丝,初生菌丝在寄主细胞间隙延伸扩展,与叶肉细胞壁接触后分化形成吸器母细胞,吸器母细胞进入寄主细胞后形成吸器。初生菌丝在吸器母细胞处产生分枝,形成次生菌丝在叶肉细胞间蔓延。在病原菌侵染早期(接种后8—24h),寄主细胞的超微结构变化并不明显。侵染中、后期(接种48—72h),被侵染叶肉细胞发生严重质壁分离,叶绿体膨胀变形,基粒片层排列疏松。线粒体嵴突退化。     

Effects of fluquinconazole and silthiofam, applied as seed treatments to single or consecutive crops of wheat, on take-all epidemic development and grain yields

Seed treatments containing fluquinconazole, silthiofam or a standard fungicide mixture with no activity against take‐all were compared in all combinations of sequences in successive second and third winter wheat crops in five field experiments and second to fourth crops in a sixth experiment. Compared with the standard treatment, silthiofam decreased take‐all more effectively than fluquinconazole when crops were sampled at tillering. In samples taken in summer, during grain filling, silthiofam often decreased the incidence of take‐all (percentage of plants with root symptoms) more than fluquinconazole, but fluquinconazole more effectively decreased the incidence of severe take‐all (percentage of plants with more than 75% of their root systems blackened). It is suggested that these differences are a consequence of more effective control of primary infection of roots by silthiofam and of secondary, root‐to‐root, infection by fluquinconazole. Silthiofam usually increased yield more than did fluquinconazole, perhaps as a consequence of better early protection during tiller and/or spikelet formation. Treatment with either of the fungicides affected epidemic development in the treated crop and in crops grown subsequently. In particular, decreased take‐all had the effect of delaying the year‐to‐year epidemic, so that nontreatment of a subsequent crop resulted in an upsurge in disease. Treatment with either take‐all fungicide of a crop grown after a treated crop was relatively effective if the epidemic in the comparable nontreated crop sequence was continuing to increase. It was, however, detrimental if the disease was approaching its peak in the first treated crop, particularly if a treated (fourth wheat) crop was being compared with a similar crop in a nontreated sequence in which take‐all decline had developed. These results provide a basis for recommendations for the use of seed treatment fungicides in sequences of wheat crops.     

Rare symptoms and conidial variation in Septoria lycopersici in Zambia

Surveys during the rainy season of 1996 showed that Septoria lycopersici developed two different types of leaf spots on tomatoes grown in kitchen gardens at the University of Zambia Campus and in nearby gardens. The two types of spots could be easily distinguished on the basis of their external morphology. One type, designated as T1, began as dark brown spots of less than 1 mm diameter. Upon increase in size, the spots differentiated into a dark brown outer ring and a grey centre, reaching a maximum diameter of 5 mm. Spots of this type are common and have been described in reports on Septoria leaf spot disease. A second type of spot found in our survey was designated as T2. This Septoria spot was greyish brown with several concentric rings of shrunken leaf tissue. The type T2 spots were larger and did not differentiate into two parts as in T1. The T2 spot diameter was 4–12 mm. The conidia showed differences in curvature, and significantly length, between T1 and T2. The study has shown that S. lycopersici in Zambia is variable. This revised version was published online in June 2006 with corrections to the Cover Date.     

ATP prevents Woronin bodies from sealing septal pores in unwounded cells of the fungus Zymoseptoria tritici

Septa of filamentous ascomycetes are perforated by septal pores that allow communication between individual hyphal compartments. Upon injury, septal pores are plugged rapidly by Woronin bodies (WBs), thereby preventing extensive cytoplasmic bleeding. The mechanism by which WBs translocate into the pore is not known, but it has been suggested that wound‐induced cytoplasmic bleeding “flushes” WBs into the septal opening. Alternatively, contraction of septum‐associated tethering proteins may pull WBs into the septal pore. Here, we investigate WB dynamics in the wheat pathogen Zymoseptoria tritici. Ultrastructural studies showed that 3.4 ± 0.2 WBs reside on each side of a septum and that single WBs of 128.5 ± 3.6 nm in diameter seal the septal pore (41 ± 1.5 nm). Live cell imaging of green fluorescent ZtHex1, a major protein in WBs, and the integral plasma membrane protein ZtSso1 confirms WB translocation into the septal pore. This was associated with the occasional formation of a plasma membrane “balloon,” extruding into the dead cell, suggesting that the plasma membrane rapidly seals the wounded septal pore wound. Minor amounts of fluorescent ZtHex1‐enhanced green fluorescent protein (eGFP) appeared associated with the “ballooning” plasma membrane, indicating that cytoplasmic ZtHex1‐eGFP is recruited to the extending plasma membrane. Surprisingly, in ~15% of all cases, WBs moved from the ruptured cell into the septal pore. This translocation against the cytoplasmic flow suggests that an active mechanism drives WB plugging. Indeed, treatment of unwounded and intact cells with the respiration inhibitor carbonyl cyanide m‐chlorophenyl hydrazone induced WB translocation into the pores. Moreover, carbonyl cyanide m‐chlorophenyl hydrazone treatment recruited cytoplasmic ZtHex1‐eGFP to the lateral plasma membrane of the cells. Thus, keeping the WBs out of the septal pores, in Z. tritici, is an ATP‐dependent process.     

Stability of Adult‐plant Resistance to Septoria tritici blotch in 24 European Winter Wheat Varieties Across Nine Field Environments

Septoria tritici blotch (STB) is one of the most important leaf diseases in wheat worldwide. Objectives of this study were (i) to compare inoculation and natural infection; (ii) to evaluate the level of adult‐plant resistance to STB using four isolates; and (iii) to analyse environmental stability of 24 winter wheat (Triticum aestivum L.) varieties in inoculated vs. non‐inoculated field trials across 3 years including nine environments (location × year combinations). Field trials were sown in split‐plot design inoculated with four aggressive isolates of S. tritici plus one non‐inoculated variant as main factor and 24 wheat varieties as subfactor. Septoria tritici blotch severity was visually scored as percentage flag leaves covered with lesions bearing pycnidia. Overall STB rating ranged from 8% (Solitär) to 63% (Rubens) flag leaf area affected, resulting in significant (P < 0.01) genotypic variance. Variance of genotype × environment interaction amounted to approximately 50% of the genotypic variance. Genotype × isolate interaction variance was significant too (P < 0.01) but of minor importance. Therefore, environmental stability of varieties should be a major breeding goal. The varieties Solitär, History and Florett were most resistant and stable as revealed by a regression approach, and the susceptible varieties were generally unstable. Hence, STB resistance and stability are correlated (P < 0.01), but there were some exceptions (Tuareg, Ambition). Promising candidates for an environmentally stable, effective adult‐plant resistance have been identified.     

Global migration patterns in the fungal wheat pathogen Phaeosphaeria nodorum

The global migration patterns of the fungal wheat pathogen Phaeosphaeria nodorum were analysed using 12 microsatellite loci. Analysis of 693 isolates from nine populations indicated that the population structure of P. nodorum is characterized by high levels of genetic diversity and a low degree of subdivision between continents. To determine whether genetic similarity of populations was a result of recent divergence or extensive gene flow, the microsatellite data were analysed using an isolation-with-migration model. We found that the continental P. nodorum populations diverged recently, but that enough migration occurred to reduce population differentiation. The migration patterns of the pathogen indicate that immigrants originated mainly from populations in Europe, China and North America.