Plant Disease

The effects of incubation temperature, leaf-wetness duration, inoculum concentration, and interaction between leaf-wetness duration and inoculum concentration on the development of Septoria tritici blotch were evaluated at the seedling stage in two bread wheats (Katepwa and 6 Lacos-78) and two durum wheats (AC Melita and Kyle). The study was conducted to assess if bread and durum cultivars widely grown in Manitoba and a resistant cultivar from South America react differently to the disease at temperatures characteristic of Manitoba summers, and to obtain information on conditions that would be used in differentiating resistant and susceptible cultivars under controlled conditions. The experiments were carried out under three temperature regimes. Factors that evaluated included inoculum concentration and duration of leaf wetness. Increasing incubation temperature, duration of leaf wetness, and inoculum concentration resulted in an increase in disease severity. There were significant (P < 0.05) differences for duration of leaf wetness and inoculum concentration within each cultivar. Pycnidia were observed 4 days earlier when incubation temperature increased from 18°C day/15°C night to 22°C day/15°C night or when inoculum concentration increased from 1 × 10⁶ spores/ml to 1 × 10⁷ spores/ml. There were more pycnidia when duration of leaf wetness was 72 h as opposed to 48 h and 60 h. The cultivar that was presumed to be resistant maintained its resistance under environmental conditions that are characteristic of Manitoba summers. We found that the optimal conditions for screening spring wheats for Septoria tritici blotch reaction were incubation temperatures of 18°C day/15°C night, and 22°C day/15°C night. Leaf wetness duration of 48 or 72 h and inoculum concentration of 1 × 10⁷ spores/ml consistently produced a susceptible reaction on Katepwa, AC Melita, and Kyle the three cultivars that were susceptible to Septoria tritici blotch.

A survey to determine the prevalence of potyviruses on yams, Dioscorea alata and D. cayenensis-rotundata, was undertaken in Colombia. Two hundred fifty leaf samples showing mottling symptoms were collected on the Atlantic coast and analyzed by antigen-coated plate enzyme-linked immunosorbent assay with universal potyvirus monoclonal antibodies (Agdia, Elkhart, IN). Potyviruses were detected in 70% (165/235) of the D. alata and in 66% (10/15) of the D. cayenensis-rotundata samples. The presence of Yam mild mosaic virus (YMMV) was indicated in some of these samples by immunocapture reverse-transcriptase polymerase chain reaction performed as previously reported (1). A 600-bp fragment that included the core and C-terminal region of the coat protein gene (CP) and the 3′ untranslated region (3′UTR) was amplified from a D. alata isolate using universal potyvirus primers (1), cloned, and sequenced (EMBL Acc. AJ311725). Comparison with the two previously published YMMV sequences revealed 96.1 and 97.4% identity for the deduced amino acid sequence in the CP region, 74.1 and 83.2% nucleotide identity in the 3′UTR for Papua New Guinea (AB022424 [2]) and Martinique (AJ250336) isolates, respectively. YMMV is known to be widespread on D. alata in Africa and the South Pacific and has been recently identified in the Caribbean (1). To our knowledge, this is the first report of its occurrence in Colombia. A study of its incidence and genetic diversity in South America has been undertaken.

References: (1) M. Bousalem and S. Dallot. Plant Disease 84:200, 2000. (2) S. Fuji et al. Arch Virol. 144:1415, 1999.

Naturally infected Dioscorea alata plants showing mild mosaic were collected in 1998 on the island of Martinique in the Caribbean. Isolates were first screened by double-antibody sandwich enzyme-linked immunosorbent assay (ELISA) with monoclonal antibodies raised against Yam mosaic virus (YMV) and antigen-coated plate ELISA with universal potyvirus monoclonal antibodies (Agdia, Elkhart, IN). A positive reaction was obtained only with the universal potyvirus antiserum. Immunocapture reverse-transcriptase polymerase chain reaction was performed for specific detection of Yam mild mosaic virus (YMMV [3]) and YMV. A product with the predicted size of 249 bp was obtained with YMMV primers. YMMV is a recently recognized distinct potyvirus infecting D. alata in West Africa and the South Pacific (2–4). It was originally described as Yam virus I and is synonymous with Dioscorea alata virus (4). To characterize the YMMV Martinique isolate, total RNA was extracted, and universal potyvirus degenerate primers (1) were used to amplify a 700-bp fragment that included the core and C-terminal region of the coat protein (CP) and 3′ untranslated region (3′UTR). Sequence information generated (EMBL AJ250336) from the cloned fragment was compared with sequences of other yam potyviruses. Sequence comparisons of the partial CP (453 nt) showed a similarity of 94.6% (amino acids [aa]) with the YMMV isolate from Papua New Guinea (EMBL AB022424 [2]); 72.2% (aa) with the Japanese yam mosaic virus (JYMV) isolate (EMBL AB016500); and 67 to 73% (aa) with 27 YMV isolates. These sequences are most diverse in the 3′UTR, which showed a similarity of 72.8% with the YMMV Papua New Guinea isolate, 30% with the JYMV isolate, and 26% with the YMV isolates. These results confirm, as previously shown by S. Fuji et al. (2), that YMMV should be classified as a new potyvirus of yam. This is the first report of the natural occurrence of YMMV in the Caribbean.

References: (1) Colinet et al. Phytopathology 84:65, 1994. (2) S. Fuji et al. Arch Virol. 144:1415, 1999. (3) R. A. Munford and S. E. Seal. J. Virol. Methods 69:73, 1997. (4) B. O. Odu et al. Ann. Appl. Biol. 134:65, 1999.

Variability of 45 isolates of Rhizoctonia solani (teleomorph Thanatephorus cucumeris) causing web blight (WB) of common bean, Phaseolus vulgaris, was examined by polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) analysis of the internal transcribed spacer regions (ITS1 and ITS2) and the 5.8S subunit (5.8S) of the nuclear ribosomal DNA repeat (ITS-5.8S-rDNA). Isolates were collected from diseased bean leaves from Argentina, Costa Rica, Cuba, Dominican Republic, Honduras, Panama, and Puerto Rico. These WB isolates belong to AG-1 and AG-2 based on anastomosis reaction. Isolates of AG-1 that cause WB were separated into three distinct groups of RFLP patterns from enzymatic digestion of a 740-bp PCR fragment. Microsclerotia-producing isolates (<1 mm) were differentiated from macrosclerotia-producing isolates (5 to 20 mm) based on PCR-RFLP patterns even though they are placed in the same AG1-1B subgroup by anastomosis reaction. WB isolates of AG-2 were separated into two distinct PCR-RFLP groups as previously reported. AG-1 macrosclerotial-producing isolates were the most virulent, whereas isolates of AG-2 were the least virulent. Genetic variability of the WB pathogen may have influenced the failure or success of management practices implemented in the past in Latin America.

Lúa mì và lúa mạch là những cây lương thực và làm thức ăn gia súc quan trọng trên khắp thế giới. Lúa mì được trồng trên diện tích lớn hơn bất kỳ cây trồng nào khác trên toàn cầu. Tại Hoa Kỳ, sản xuất lúa mì và lúa mạch đóng góp vào nhu cầu lương thực và thức ăn gia súc trong nước, cũng như góp phần vào thị trường xuất khẩu và cán cân thương mại. Mười lăm năm trước, tạp chí Plant Disease đã xuất bản một bài viết nổi bật mang tiêu đề “Bệnh Thối Đầu Con Gié Lúa Mì và Lúa Mạch: Một căn bệnh tái xuất với tác động tàn phá”. Bài viết đó mô tả loạt các đại dịch bệnh thối đầu con gié (Fusarium head blight - FHB) nghiêm trọng xảy ra tại Hoa Kỳ và Canada, chủ yếu từ năm 1991 đến năm 1996, với nhấn mạnh vào những tác động kinh tế và xã hội chưa từng có gây ra bởi đại dịch bệnh FHB năm 1993 trên các loại hạt mùa xuân tại vùng Northern Great Plains. Các ấn phẩm trước đó đã xử lý phạm vi và thiệt hại do bệnh này tại Hoa Kỳ, Canada, châu Âu và Trung Quốc. Các đánh giá được công bố sau năm 1997 đã mô tả thêm về căn bệnh này và ảnh hưởng của nó đối với sản xuất ngũ cốc ở Bắc Mỹ trong thập niên 1990. Bài báo này đánh giá lại căn bệnh và tài liệu về các đại dịch bệnh FHB ở Hoa Kỳ kể từ năm 1997. Mục tiêu chính của bài báo này là tóm tắt một chương trình nghiên cứu phối hợp và hợp tác bền vững được triển khai ngắn sau đại dịch năm 1993, một chương trình nhằm nhanh chóng đưa đến các chiến lược quản lý cải tiến và triển khai việc tiếp cận cộng đồng. Chương trình này đóng vai trò như một mô hình để xử lý các mối đe dọa bệnh cây trồng mới nổi khác.

In October of 2006, yellow straightneck and zucchini squash plants (Cucurbita pepo L.) with crumpled, curled, thickened leaves were found in St. Johns and Marion counties in central Florida, respectively. Both locations had high populations of the whitefly, Bemisia tabaci. Incidences of symptomatic plants were greater than 95% in three squash fields (33 ha total) in St. Johns County and 35% in an experimental plot in Marion County. Twenty-three samples were collected from symptomatic plants (two from St. Johns County and 21 from Marion County). DNA was extracted for PCR and tested for the presence of begomoviruses using the following pairs of degenerate primers: AC1048/AV494, which amplifies a conserved region of the coat protein gene (2), PAR1c496/PAL1v1978, which amplifies a region of the begomovirus A component, and PBL1v2040/PCRc154, which amplifies a hypervariable region of the begomovirus B component (1). All squash samples yielded amplicons of sizes expected for a bipartite begomovirus: 1,159 nt with PAR1c496/PAL1v1978, 550 nt with AC1048/AV494, and 493 nt with PBL1v2040/PCRc154. The 1,159- and 493-nt amplicons obtained from two squash plants were cloned and sequenced. The 1,159 nt sequences from both plants shared 98% sequence identity with each other and 97% identity with equivalent regions of the A component of Cucurbit leaf crumple virus (CuLCrV) from Arizona and California (GenBank Accession Nos. AF256200 and AF224760, respectively). The 493-nt sequences amplified with PBL1v2040/PCRc154 were identical and shared a 96% identity with CuLCrV sequence (GenBank Accession No. AF327559) from Arizona and 97% identity with CuLCrV B component sequence (GenBank Accession No. AF224761) from California. Leaves were collected from eight symptomatic squash plants from Citra, FL and used for whitefly transmission assays. Approximately 100 adults of Bemisia tabaci biotype B were released onto each caged leaf and given a 24-h acquisition access period, after which a healthy squash seedling was introduced. Symptoms developed within 10 days on all test plants, and the presence of CuLCrV was confirmed by PCR assays, (primer pairs PAR1c496/PAL1v1978 and PBL1v2040/PCRc154) followed by sequencing. In 2007, similar symptoms were seen in several locations around the state. The same assays confirmed the presence of CuLCrV in watermelon (Citrullus lanatus L.) and squash in the following counties: Collier and Hendry in southwest Florida and Hillsborough, Manatee, and Sarasota in west-central Florida. To our knowledge, this is the first report of CuLCrV, and the first report of any begomovirus in cucurbits in Florida.

References: (1) M. R. Rojas et al. Plant Dis. 77:340, 1993. (2) S. D. Wyatt and J. K. Brown. Phytopathology 86:1288, 1996.

During fall 1998, volunteer watermelons (Citrullus lunatus L. (Thunb.) Matsum. & Nakai) showing leaf curl, crumpling, and yellowing symptoms were found in a commercial honeydew melon (Cucumis melo L. subsp. melo Inodorus group) field in the Imperial Valley of California. The plants were infected with a begomovirus (family Geminiviridae, genus Begomovirus) based on (i) a positive response in squash blots probed with a general begomovirus DNA probe (1) and (ii) amplification of DNA-A (≈1.2 kb) and DNA-B (≈1.4 kb) fragments by polymerase chain reaction (PCR) with degenerate DNA-A (PAL1v1978/PAR1c496) and DNA-B (PBL1v2040/PBR1c970) primers, respectively (3). The DNA-A and -B fragments were cloned and sequenced (GenBank accession nos. AF224760 [DNA-A] and AF224761 [DNA-B]). The DNA-A and -B fragments had a nearly identical (99.5%) common region (CR) of 186 (DNA-A) and 187 (DNA-B) nucleotides, indicating they were from the same begomovirus. Database searches conducted with these sequences revealed no high degree of sequence identity (i.e., >90%) with other begomoviruses, including Squash leaf curl virus (SqLCV [2]) from southern California. The partial AC1 sequence (669 nt) was most identical to Tomato severe leaf curl virus (ToSLCV) from Guatemala (83%) and SqLCV (81%), the partial AV1 sequence (135 nt) was most identical to Tomato golden mosaic virus from Brazil (84%) and SqLCV (81%), and the CR was most identical to Squash yellow mottle virus from Costa Rica (81%), ToSLCV (81%), and SqLCV (77%). The partial BV1 sequence (465 nt) was most identical to Bean calico mosaic virus and SqLCV (72%), and the partial BC1 sequence (158 nt) was most identical to SqLCV (75%). Watermelon seedlings bombarded with a DNA extract from infected watermelon volunteers developed crumpling and distortion symptoms, whereas seedlings bombarded with gold particles alone developed no symptoms. Geminivirus infection in symptomatic seedlings was confirmed by PCR. These results suggest a new begomovirus caused the disease symptoms in the watermelon volunteers. Leaf crumpling and curling symptoms were not observed in spring melons in the Imperial Valley in 1999, but on 2 July and 17 August 1999, cantaloupe (C. melo L. subsp. melo Cantalupensis group), muskmelon (C. melo L. subsp. melo Cantalupensis group), and watermelon plants with leaf crumpling and yellowing were found. These plants were infected with the new begomovirus based on sequence analysis of PCR-amplified DNA-A fragments (97 to 98% identity for CR and partial AC1 sequence). A survey of fall melons, conducted 23 to 24 September 1999, revealed widespread symptoms of leaf curl and crumpling on new growth of muskmelon plants in all seven commercial fields examined (estimated incidence 25 to 50%) and on watermelon volunteers. No such symptoms were observed on leaves of honeydew melons. Symptomatic muskmelon and watermelon leaves, collected from eight locations throughout the Imperial Valley, were infected with the new begomovirus based on sequence analysis of PCR-amplified DNA-A fragments. Thus, a new begomovirus has emerged in the Imperial Valley; the name Cucurbit leaf crumple virus (CuLCrV) is proposed.

References: (1) R. L. Gilbertson et al. Plant Dis. 75: 336, 1991. (2) S. G. Lazarowitz and I. B. Lazdins. Virology 180:58, 1991. (3) M. R. Rojas et al. Plant Dis. 77:340, 1993.

In 1998 to 1999, geminivirus-like symptoms were observed in whitefly-infested pumpkin, honeydew melon, and muskmelon in Arizona and Texas and in Coahuilla, Mexico (MX), respectively. Plants exhibited leaf curl and/or mottling, reminiscent of symptoms caused by Squash leaf curl virus (SLCV-WAZ) described from Arizona in 1981 (2). The isolate from Arizona pumpkin fields was experimentally transmitted to pumpkin seedlings by the “B type” of Bemisia tabaci (Genn.), and symptoms were indistinguishable from those observed in infected fields. Samples from AZ, MX, and TX were assessed for begomovirus presence by polymerase chain reaction (PCR) using degenerate primers that amplify a contiguous fragment containing the viral coat protein (Cp) gene and common region (CR) of the A component (CR-A) (~2,100 bp) and a fragment containing the CR of the B component (CR-B) (~1,100 bp). One to four isolates from each location were examined by PCR using both primer pairs, and at least three amplicons per isolate were cloned and their sequences determined. Alignment of viral Cp nucleotide (nt) sequences revealed that AZ [AF256199], MX, and TX field isolates shared 98.7 to 100% sequence identity, but were only 84.5 to 85.6% identical to the Cp gene of SLCV-extended (SLCV-E) [M38183] and SLCV-restricted (SLCV-R) (S. G. Lazarowitz, unpublished), respectively, suggesting a new, previously undescribed begomoviral species (3). Further, the Cp nt sequence of the three field isolates was 6 nt shorter than SLCV-E, SLCV-WAZ [AF256203], and SLCV-R Cp sequences. The CR-A [AF256200] and CR-B [AF256201] sequences (179 nt, each) of field isolates, including the theoretical Rep binding element, GGTGT, were 100% identical. Although the Rep binding site is identical among field isolates, SLCV-E, SLCV-R, and SLCV-WAZ, the field isolate CR sequence shared only 64.2, 67.5, and 66.9% overall identity with CR-A SLCV-E, SLCV-R [M63155], and SLCV-WAZ [AF256202], respectively. Prior to 1998 to 1999, SLCV-WAZ was the only New World begomovirus of cucurbits known to infect both melon (Cucumis) and pumpkin (Cucurbita) (1). Therefore, SLCV was initially suspected as the causal agent. However, here we provide evidence for a new, previously undescribed bipartite begomovirus of cucurbits in AZ, MX, and TX that is herein provisionally designated Cucurbit leaf curl virus (CuLCV). Prediction of its closest begomovirus relatives by Cp nt sequence and Rep binding site comparisons suggest that CuLCV is a new member of the SLCV lineage, also containing Bean calico mosaic virus, Cabbage leaf curl virus, SLCV-E, and Texas pepper virus-TAM.

References: (1) J. K. Brown and M. R. Nelson. Phytopathology 74:1136, 1984. (2) J. K. Brown and M. R. Nelson. Ann. Appl. Biol. 115:243, 1986. (3) M. A. Mayo and C. R. Pringle. J. Gen. Virol. 97:649, 1998.