Plant Pathology

Effects of inoculum concentration, wetness duration and plant age on the development of tomato early blight were evaluated in relation to host susceptibility under controlled environmental conditions. The main effect of early blight was premature defoliation, which was linearly related to the percentage of leaf area showing symptoms. As ln(inoculum concentration, conidia mL⁻¹) increased from 6·2 to 11·5, the percentages of leaf area affected and of defoliation increased linearly. Four h of leaf wetness after inoculation were sufficient to initiate the disease on plants of hybrid Skala RZ but not on those of cv. Rio Rojo, for which at least 6 h leaf wetness were needed. As wetness duration increased up to 24 h, there was an increase in the percentage leaf area showing symptoms and in the percentage of defoliation, but thereafter there was no significant increase in either parameter. Tomato plants were susceptible to Alternaria solani at all growth stages, but susceptibility increased as plants matured. There were no significant differences in susceptibility between tomato cultivars and hybrids.

Isolates of maize streak virus (MSV) were examined by thin‐section electron microscopy in plants, assessed for characteristic features of infection and compared with other related geminiviruses infecting monocotyledons from Africa, islands in the Indian Ocean, and the Pacific Island of Vanuatu. Arrays of virus particles, often crystalline, were most often seen in the nucleus. The morphology of the nuclear crystalline arrays was characteristic of certain isolates or groups of isolates (strains). Infected nuclei could be seen in cells from the phloem parenchyma, vascular bundle sheath and mesophyll tissue, and also in epidermal guard cells of plants infected with the maize strain of MSV. The particle arrays varied in morphology from regular rows of virions forming distinctive blocks, to randomly arranged aggregates in certain areas of the nucleus. We consistently failed to find viral crystalline arrays associated with infection of panicum streak virus (PSV) and sugar cane streak virus (SSV) isolates either in these hosts or in maize. Occasionally arrays of MSV particles were found outside the nuclear envelope in physiologically active cells. Accumulations or sheets of MSV particles were seen lining the walls of some phloem companion cells. Crystalline aggregates of particles were frequently observed in the cell vacuole, after lysis of the nuclear membrane of dead cells which made up the chlorotic lesions, the typical symptom of virus infection. Virus preparations from all hosts contained typical geminate particles regardless of the morphology of the virion arrays. The effect on chloroplasts appeared to vary between isolates and this is discussed in relation to lesion colour. The arrangement of virions in the nucleus as a taxonomic character is diagnostic for MSV. Inclusions with crystalline structure found in sieve elements of infected plants were not immunogold labelled when thin sections were probed using antiserum to the virus particles.

Mycelial extracts of Pythium irregulare were processed using CF‐11 chromatography and analysed by gel electrophoresis for double‐stranded RNA (dsRNA) content. DsRNA molecules of between 1 and 6 kb were found in 33 of 39 isolates tested. The dsRNA profiles of individual isolates remained unchanged after repeated subculturing of hyphal tips, after storage under water for 2 years, or when single zoospores were used to generate subcultures. However, dsRNA profiles varied between isolates, and even within individual populations; in the one case eight different dsRNA types were recovered upon testing 17 isolates from a single wheatfield. Isometric virus‐like particles (VLPs) of approximately 45 nm were found in isolates containing dsRNA, while no such particles could be found in isolates in which dsRNA was not detected. Electrophoretic profiles of dsRNA extracted from partial purifications of VLPs were identical to those of dsRNA extracted from whole mycelium, suggesting that the dsRNA is at least partially encapsidated. Attempts to transmit dsRNA between isolates by hyphal anastomosis or by coinfection in wheat plants were unsuccessful, with only parental types being recovered. Analysis of 29 isolates representing 14 other Pythium species failed to detect dsRNA, even when co‐isolated with P. irregulare known to contain dsRNA

The pathogenicity of an isolate of a Pythium species from Signy Island in the South Orkney Islands was tested against the Antarctic hairgrass Deschampsia antarctica. The isolate was found to infect plants at 8ºC and to cause foliar and root symptoms similar to those seen in other Pythium infections in grasses. Analysis of ribosomal RNA sequences placed it, together with another isolate from Antarctica, in a clade that included the known snow moulds caused by Pythium spp. Sporangia and oogonia were produced in culture, but the isolate differed from other Pythium spp. in producing chlamydospores in older cultures and plant tissue. This is the first report of a pathogen of an eukaryotic vascular plant in the maritime Antarctic region.

Tổng quan này về bệnh Fusarium trên hạt nhỏ ngũ cốc cho thấy có tới 17 loại tác nhân gây bệnh đã được xác định có liên quan đến căn bệnh đang phổ biến ở hầu hết các khu vực trồng ngũ cốc trên thế giới. Các loài phổ biến nhất là Fusarium graminearum (Gibberella zeae), F. culmorum, F. avenaceum (G. avenacea), F. poae và Microdochium nivale (Monographella nivalis). Bệnh được ghi nhận phổ biến nhất trong điều kiện khí hậu nóng ẩm, nơi các thiệt hại năng suất đáng kể và sự tích lũy mycotoxin trong hạt đã được báo cáo. Các nguồn bệnh khả thi được xác định bao gồm tàn dư của cây trồng, các vật chủ thay thế và bệnh thối rễ Fusarium ở giai đoạn mầm. Phương thức truyền bệnh đến tai lúa vẫn chưa rõ ràng, nhưng khả năng do côn trùng bị nhiễm, sự phát triển nấm có hệ thống qua cây trồng, và sự phân tán do gió và mưa đã được đề xuất. Sự nhiễm bệnh lên tai lúa mì xảy ra chủ yếu trong giai đoạn thụ phấn, và đã có bằng chứng cho thấy có thể có các chất kích thích nấm trong nhụy hoa. Dù căn bệnh quan trọng này diễn ra đặc biệt nghiêm trọng trong những năm dịch bệnh, các phương pháp kiểm soát vẫn còn hạn chế. Nhiều nỗ lực đã được tập trung vào việc lai tạo giống lúa mì kháng bệnh và cải thiện hiểu biết về các cơ chế có thể và cơ sở di truyền kháng cự, dù chỉ đạt được thành công ở mức độ vừa phải. Cũng có rất ít báo cáo về sự kiểm soát thành công của bệnh bằng thuốc chống nấm hay thông qua phương pháp sinh học trên thực địa.

Mandipropamid is a new mandelic acid amide fungicide expressing high activity against foliar infecting oomycetes, including the grapevine downy mildew, Plasmopara viticola. Because cross‐resistance with the valinamide fungicides iprovalicarb and benthiavalicarb and the cinnamic acid amide fungicides dimethomorph and flumorph was postulated, all five compounds are classified as carboxylic acid amide (CAA) fungicides. To support this classification, cross‐resistance among these compounds with field isolates and the segregation of resistance in F₁ and F₂ progeny of P. viticola were evaluated. A bimodal distribution of sensitivity in field isolates and cross‐resistance among all CAAs for the vast majority of isolates were detected. Crosses between sensitive (s) and CAA‐resistant (r) isolates of opposite mating types, P1 and P2, yielded abundant oospores. All F₁‐progeny isolates were sensitive to CAAs (s:r segregation 1:0), whereas in F₂ progeny segregation of about 9:1 (s:r) was observed suggesting that resistance to CAA fungicides is controlled by two recessive nuclear genes. Mating type segregated in a ratio P1:P2 of c. 2:1 in F₁ and 1:1 in F₂ progeny. In the same crosses, resistance to the phenylamide fungicide mefenoxam segregated in a ratio of c. 1:3:2 (sensitive:intermediate:resistant), reflecting the monogenic, semidominant nature of resistance. The risk of resistance in P. viticola was classified as high for phenylamide and moderate for CAA fungicides. This is the first report on the inheritance of phenotypic traits in P. viticola.

Calcium chloride (2% w/v) significantly inhibited the growth of the pathogen Rhizopus stolonifer, but did not affect the colony‐forming units (CFU) of yeasts Candida guilliermondii and Pichia membranefaciens in potato dextrose broth. The concentration of yeast suspension influenced spore germination and germ tube growth of R. stolonifer in vitro, as well as disease incidence and lesion development in fruits. There were significant negative relationships between the suspension concentrations of the yeasts and the growth as well as infectivity of the pathogen. The addition of calcium resulted in lower spore germination rates and slower growth of germ tubes in vitro, as well as in lower disease incidences and smaller lesion diameters compared with treatments with yeast antagonists alone. When yeast cell suspensions reached a concentration of 5 × 10⁸ CFU mL⁻¹, growth of the pathogen was completely limited in vitro, and no infection was found in peach and nectarine fruits treated with or without calcium.

Anthracnose, caused by Colletotrichum gloeosporioides, is one of the most important diseases in grape‐growing regions worldwide. In Jiangsu Province of China, quinone‐outside inhibitor fungicides (QoIs) have been extensively sprayed as disease control for more than 10 years. A spore germination assay of 64 isolates obtained from 32 commercial vineyards was used to assess isolate sensitivity to azoxystrobin and 62 were found to be resistant to azoxystrobin. The biological fitness of QoI‐resistant (QoI^R) isolates was significantly lower than the sensitive isolates (QoI^S) in terms of mycelial growth and conidiation. Nucleotide sequence alignment of CgCytb genes from the QoI^R and QoI^S isolates revealed that two point mutations (F129L and G143A) are involved in the QoI resistance. Isolates with the G143A mutation expressed high resistance to azoxystrobin, whereas isolates carrying the F129L mutation exhibited moderate resistance. Positive cross‐resistance was observed between azoxystrobin and kersoxim‐methyl, pyraclostrobin, or benzothiostrobin, but not with fluazinam. This study provides important information for management of QoI^R populations of C. gloeosporioides in the field.

The identity of Colletotrichum acutatum as the causal pathogen of grape ripe‐rot, which causes yield loss and a bitter taint that lowers wine quality in Australian subtropical wine‐grape regions, was confirmed using species‐specific primers. Cultural, morphological and molecular methods (RAPD‐PCR and sequencing of parts of the 5·8S‐ITS regions and the β‐tubulin‐2 gene) were used to determine the phylogenetic relationships of Australian C. acutatum isolates from wine grapes and other horticultural crops. A combination of RAPD‐PCR and β‐tubulin‐2 gene data showed that all wine‐grape ripe‐rot isolates from northern regions of New South Wales (NSW) and Queensland belong to a proposed new C. acutatum group (A9), together with isolates from Australian strawberry, mango, blueberry and olive. The 5·8S‐ITS sequences for these grape pathogens were identical to published sequences for an isolate from Cyclamen (the Netherlands) and differed by 1 bp from isolates from Capsicum (Taiwan) and orange (Costa Rica). The grape ripe‐rot isolates from the Shoalhaven Valley (southern NSW) were clustered within two other C. acutatum groups: A2 and A5. In vitro infection studies showed that Australian C. acutatum isolates from almond, blueberry, chilli, grape, mango, olive, strawberry and tomato were able to infect grape and could also infect blueberry and strawberry, indicating a lack of host specificity. This lack of host specificity, the genetic similarity with non‐grape isolates, and the fact that many of the non‐grape hosts were isolated from wine‐growing regions, suggest the potential for cross‐infection between grape and other horticultural crops.

Anthracnose is an important disease in vineyards in south and southeast Brazil, the main grape‐producing regions in the country. This study aimed to identify the causal agents of grapevine anthracnose in Brazil through multilocus phylogenetic analyses, morphological characterization and pathogenicity tests. Thirty‐nine Elsinoë ampelina and 13 Colletotrichum spp. isolates were obtained from leaves, stems and berries with anthracnose symptoms collected in 38 vineyards in southern and southeastern Brazil. For E. ampelina isolates, the internal transcribed spacer (ITS), histone H3 (HIS3) and elongation factor 1‐α (TEF) sequences were analysed. HIS3 was the most informative region with 55 polymorphic sites including deletions and substitutions of bases, enabling the grouping of isolates into five haplotypes. Colonies of E. ampelina showed slow growth, variable colouration and a wrinkled texture on potato dextrose agar. Conidia were cylindrical to oblong with rounded ends, hyaline, aseptate, (3.57–) 5.64 (−6.95) μm long and (2.03–) 2.65 (−3.40) μm wide. Seven species of Colletotrichum were identified: C. siamense, C. gloeosporioides, C. fructicola, C. viniferum, C. nymphaeae, C. truncatum and C. cliviae, with a wide variation in colony and conidium morphology. Only E. ampelina caused anthracnose symptoms on leaves, tendrils and stems of Vitis vinifera and V. labrusca. High disease severity and a negative correlation between disease severity and shoot dry weight were observed only when relative humidity was above 95%. In this study, only E. ampelina caused anthracnose symptoms on grapevine shoots in Brazil.