Plant Pathology

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.

Seed potato degeneration, the reduction in yield or quality caused by an accumulation of pathogens and pests in planting material due to successive cycles of vegetative propagation, has been a long‐standing production challenge for potato growers around the world. In developed countries this problem has been overcome by general access to and frequent use of seed, produced by specialized growers, that has been certified to have pathogen and pest incidence below established thresholds, often referred to as certified seed. The success of certified seed in developed countries has concentrated the research and development agenda on the establishment of similar systems in developing countries. Despite these efforts, certified seed has had little penetration into the informal seed systems currently in place in most developing countries. Small‐scale farmers in these countries continue to plant seed tubers acquired through the informal seed system, i.e. produced on‐farm or acquired from neighbours or local markets. Informal seed tubers frequently have poor health status, leading to significant reductions in yield and/or market value. This review emphasizes the need to refocus management efforts in developing countries on improving the health status of seed tubers in the informal system by integrating disease resistance and on‐farm management tools with strategic seed replacement. This ‘integrated seed health strategy’ can also prolong the good health status of plants derived from certified seed, which would otherwise be diminished due to potential rapid infection from neighbouring fields. Knowledge gaps, development challenges and impacts of this integrated seed health strategy are discussed.

This study reports an efficient inoculation protocol that allowed cytological analysis of the infection process of the rice false smut pathogen Ustilaginoidea virens. Examination of serial semithin and ultrathin sections of infected spikelets showed that the primary infection sites for the pathogen were the upper parts of the three stamen filaments located between the ovary and the lodicules. The stigma and lodicules were also occasionally infected to a limited extent. The pathogen infected the filaments intercellularly and extended intercellularly along the filament base. The host cells were degraded gradually. The pathogen did not penetrate host cell walls directly and did not form haustoria. In the balls the ovary remained alive and was never infected. This suggests that the pathogen is a biotrophic parasite that grows intercellularly in vivo.

The diagnosis of novel unidentified viral plant diseases can be problematic, as the conventional methods such as real‐time PCR or ELISA may be too specific to a particular species or even strain of a virus, whilst alternatives such as electron microscopy (EM) or sap inoculation of indicator species do not usually give species level diagnosis. Next‐generation sequencing (NGS) offers an alternative solution where sequence is generated in a non‐specific fashion and identification is based on similarity searching against GenBank. The conventional and NGS techniques were applied to a damaging and apparently new disease of maize, which was first identified in Kenya in 2011. ELISA and TEM provided negative results, whilst inoculation of other cereal species identified the presence of an unidentified sap transmissible virus. RNA was purified from material showing symptoms and sequenced using a Roche 454 GS‐FLX+. Database searching of the resulting sequence identified the presence of Maize chlorotic mottle virus and Sugarcane mosaic virus, a combination previously reported to cause maize lethal necrosis disease. Over 90% of both viral genome sequences were obtained, allowing strain characterization and the development of specific real‐time PCR assays which were used to confirm the presence of the virus in material with symptoms from six different fields in two different regions of Kenya. The availability of these assays should aid the assessment of the disease and may be used for routine diagnosis. The work shows that next‐generation sequencing is a valuable investigational technique for rapidly identifying potential disease‐causing agents such as viruses.

Conventional PCR and two real‐time PCR (RTi‐PCR) methods were developed and compared using the primer pairs CQULA03F/CQULA03R and CQULA04F/CQULA04R, and TaqMan probe CQULAP1 designed from a species‐specific sequence of the rplJ/rplL ribosomal protein gene, for diagnosis of citrus huanglongbing (HLB) disease in southern China. The specificity and sensitivity of the three protocols for detecting ‘Candidatus Liberibacter asiaticus’ in total DNA extracts of midribs collected from infected citrus leaves with symptoms in Guangxi municipality, Jiangxi Province and Zhejiang Province, were tested. Sensitivities using extracted total DNA (measured as copy number, CN per µL of recombinant plasmid solution) were 439·0 (1·30 × 10⁵ CN µL⁻¹), 4·39 (1·30 × 10³ CN µL⁻¹) and 0·44 fg µL⁻¹ (1·30 × 10² CN µL⁻¹) for conventional PCR, TaqMan and SYBR Green I (SGI) RTi‐PCR, respectively. SGI RTi‐PCR was the most sensitive, but its specificity needed to be confirmed by running a melt‐curve assay. The TaqMan RTi‐PCR assay was rapid and had the greatest specificity. Concerning the correlation of PCR detection results with the various HLB symptoms, uneven mottling of leaves had the highest positive rate (96·50%), indicating that leaf mottling was the most reliable symptom for field surveys. Dynamic analysis results from the TaqMan assays showed that the titre (CN) g⁻¹ citrus tissue of ‘Ca. L. asiaticus’ was highest between October and December (threshold cycle (C_t) average = 29·3, CN = 3·35 × 10⁷) and lowest between March and May (C_t average = 32·0, CN = 5·10 × 10⁶) in 2004 and 2005. The optimized molecular‐based assays should prove useful for presymptom diagnosis of HLB disease, monitoring and identification of ‘Ca. L. asiaticus’, and field epidemic regulation.

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.

Isolates of Penicillium expansum recovered from stored pears were scored for resistance to the fungicide thiabendazole (TBZ) and for pathogenic fitness. Out of 50 isolates, nine were sensitive (S) and 41 resistant (R or RR). Seven of these resistant isolates (RR) germinated with a higher percentage on TBZ‐amended medium than on unamended medium. Six S isolates and six RR isolates were chosen at random for further analysis. S and RR isolates had similar in vitro growth fitness, although RR isolates were characterized by higher infection severity on fruits. Laboratory‐induced resistant isolates were generated by UV‐irradiating S strains, and a similar correlation between the induced TBZ resistance and pathogenic fitness was observed. The β‐tubulin gene of RR and S isolates was amplified and sequenced; mutations correlating with TBZ resistance were identified at residues Phe 167 and Glu 198. Analogous mutations were detected in the laboratory‐induced resistant isolates.

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.

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.