ABSTRACT
The susceptibility of fabaceous (Lupinus and Lathyrus spp.) and solanaceous (Calibrachoa, Browallia, Nicotiana, Nierembergia, and Petunia spp.) ornamental plants compared with straightneck squash (Cucurbita pepo) inoculated with Phytophthora capsici and P. tropicalis was investigated in greenhouse studies. Four P. capsici isolates and one P. tropicalis isolate were evaluated. Flowering tobacco (Nicotiana × sanderae), sweet pea (Lathyrus latifolius), lupine (Lupinus polyphyllus), squash, and million bells (Calibrachoa × hybrida) were susceptible to P. capsici and P. tropicalis. Bush violet (Browallia speciosa) and cup flower (Nierembergia scoparia) were not susceptible to either pathogen. Petunia (Petunia × hybrida) was susceptible to P. capsici but not P. tropicalis. Area under the plant growth curve (AUPGC) was also affected in some susceptible plants. AUPGC was significantly different in inoculated plants compared with the untreated controls of Nicotiana and Calibrachoa. In addition, six Calibrachoa cultivars were evaluated for susceptibility to P. capsici and P. tropicalis in a separate experiment. Although all Calibrachoa cultivars were susceptible to P. capsici and P. tropicalis, 'Celebration Purple Star' displayed limited disease following inoculation with either pathogen. Calibrachoa 'Cabaret' and 'Can-Can' inoculated with P. capsici or P. tropicalis displayed significant differences in AUPGC compared with the uninoculated controls. 'Callie', 'Million Bells Cherry Pink', and 'Superbells' had significantly reduced AUPGC compared with the controls when inoculated with P. tropicalis but not P. capsici. These results expand the host range of P. capsici to include Calibrachoa spp., L. polyphyllus, and Lathyrus latifolius, and P. tropicalis to include L. latifolius, Nicotiana spp., and straightneck squash.
ABSTRACT
In September 2010, celery plants with leaf cupping and petiole twisting were observed in commercial production fields located in Barry, Kent, Newago, and Van Buren Counties in Michigan. Long, elliptical lesions were observed on petioles but signs (mycelia, conidia, or acervuli) were not readily observed. Celery petioles were incubated in humid chambers (acrylic boxes with wet paper towels). After 24 h, conidia corresponding to the genus Colletotrichum were observed. Isolations were performed by excising pieces of celery tissue from the lesion margin and placing them on potato dextrose agar (PDA) amended with 30 ppm of rifampicin and 100 ppm of ampicillin. Plates were incubated at 21 ± 2°C under fluorescent light for 5 days. Fungal colony morphology was gray with salmon-colored masses of spores when viewed from above, and carmine when viewed from below. Isolates were single-spored and placed on 30% glycerol in -20°C, and cryoconservation media (20% glycerol, 0.04% yeast extract, 0.1% malt extract, 0.04% glucose, 0.02% K2HPO4) at -80°C. Conidia were 8.5 to 12.0 × 2.8 to 4.0 µm and straight fusiform in shape. Three isolates were confirmed as C. acutatum sensu lato based on sequences of the internal transcribed spacer (ITS) region of the nuclear ribosomal RNA and the 1-kb intron of the glutamine synthase gene (3), both with 100% similarity with Glomerella acutata sequences. Sequences were submitted to GenBank (Accession Nos. JQ951599 and JQ951600 for ITS and GS, respectively). Additionally, C. acutatum specific primer CaIntg was used in combination with the primer ITS4 on 54 isolates from symptomatic celery plants, obtaining the expected 490-pb fragment (1). Koch's postulates were completed by inoculating 4-week-old celery seedlings of cultivars Sabroso, Green Bay, and Dutchess using three plants per cultivar. Prior to inoculation, seedlings were incubated for 16 h in high relative humidity (≥95%) by enclosing the plants in humid chambers. Seven-day-old C. acutatum s. l. colonies were used to prepare the inoculum. Seedlings were spray-inoculated with a C. acutatum s. l. conidial suspension of 1 × 106 conidia/ml in double-distilled water plus Tween 0.01%. Two control seedlings per cultivar were sprayed with sterile, double-distilled water plus 0.01% Tween. Plants were enclosed in bags for 96 h post inoculation and incubated in a greenhouse at 27°C by day/20°C by night with a 16-h photoperiod. Leaf curling was observed on all inoculated plants of the three cultivars 4 days after inoculation (DAI). Petiole lesions were observed 14 to 21 DAI. Conidia were observed in lesions after incubation in high humidity at 21 ± 2°C for 24 to 72 h. Symptomatic tissue was excised and cultured onto PDA and resulted in C. acutatum colonies. Control plants remained symptomless. C. acutatum (4) and C. orbiculare (2) were reported to cause celery leaf curl in Australia in 1966 (2,4). To our knowledge, this is the first report of C. acutatum s. l. infecting celery in Michigan. References: (1) A. E. Brown et al. Phytopathology 86:523, 1996. (2) D. F. Farr and A. Y. Rossman. Fungal Databases. Syst. Mycol. Microbiol. Lab., USDA-ARS. Retrieved from http://nt.ars-grin.gov/fungaldatabases/ , 10 September 2010. (3) J. C. Guerber et al. Mycologia 95:872, 2003. (4) D. G. Wright and J. B. Heaton. Austral. Plant Pathol. 20:155, 1991.
ABSTRACT
Cucumber (Cucumis sativus) and squash (Cucurbita spp.) production in Michigan is limited by the oomycete pathogen Phytophthora capsici. Cucumber, summer squash, and winter squash fruit were evaluated for susceptibility to five isolates of P. capsici. Detached fruit were inoculated with a 5-mm-diameter culture plug of mycelia and sporangia and were incubated in a laboratory or greenhouse. Lesion and pathogen growth diameters were measured and pathogen growth density was visually assessed. All P. capsici isolates incited rot, with significant differences found among fruit type and pathogen isolate. Straightneck squash (Cucurbita pepo), slicing cucumber, and butternut squash (C. moschata) exhibited more severe symptoms than the other fruit tested. Summer and winter squash seedlings were evaluated in greenhouse experiments, in which P. capsici-infested millet seed (approximately 1 g) were placed on the surface of soilless potting media. Disease severity was visually assessed every 2 days for 14 days post inoculation. Crop type, pathogen isolate, or the crop type-pathogen isolate interaction term were significant for symptom appearance and area under the disease progress curve values. Differences in susceptibility of butternut squash and zucchini cultivars were observed following inoculation with solanaceous isolate 13351. Results from this study can refine management programs for Phytophthora rot.
ABSTRACT
In July of 2010, dry, oval lesions, each with a salmon-colored center and bleached overall appearance, were observed on the leaves and neck of onions plants growing in production fields of Newaygo, Ottawa, Kent, and Ionia counties, Michigan. Acervuli and setae that are characteristic of Colletotrichum spp. were observed with a dissecting microscope, and elliptical conidia (8 to 23 × 3 to 12 µm) with attenuated ends were observed with a compound microscope. Symptomatic tissues were excised and cultured onto potato dextrose agar amended with 30 and 100 ppm of rifampicin and ampicillin, respectively. The cultures produced pale salmon-colored sporulation after growing for 5 days at 22 ± 2°C and black microsclerotia after 2 weeks. Six isolates were confirmed as C. coccodes based on sequence analysis of the internal transcribed (ITS) region of the ribosomal DNA and a 1-kb intron of the glutamine synthase gene (GS) (2). Sequences were submitted to GenBank (Accession Nos. JQ682644 and JQ682645 for ITS and GS, respectively). Pathogenicity tests were conducted on two- to three-leaved 'Stanley' and 'Cortland' onion seedlings. Prior to inoculation, seedlings were enclosed in clear plastic bags overnight to provide high relative humidity. The bags were removed, and seedlings were sprayed inoculated with a C. coccodes conidial suspension (5 × 105 conidia/ml and 25 ml/plant) in sterile double-distilled water. Control seedlings were sprayed with sterile double-distilled water. Tween (0.01%) was added to the conidial suspension and the water. Plants were enclosed in bags for 72 h postinoculation and incubated in growth chambers at 28°C day/23°C night with a 12-h photoperiod. Sunken, oval lesions were observed on the foliage of the onion seedlings inoculated with C. coccodes 4 days postinoculation. Lesions coalesced and foliage collapsed 7 days postinoculation. Control plants remained asymptomatic. When five leaf samples per replication were detached and incubated in a moist chamber for 3 days at 21 ± 2°C, abundant acervuli and setae were observed on the symptomatic tissue but not on control tissue. C. coccodes was consistently recovered from the onion seedling lesions. Six different Colletotrichum spp. have been reported to cause diseases on onions worldwide (1,4). C. circinans, which causes smudge, is an occasional onion pathogen in Michigan, while C. gloeosporioides has only been reported to be infecting onions in Georgia (3). To our knowledge, this is the first report of C. coccodes infecting and causing disease in onions plants. References: (1) D. F. Farr and A. Y. Rossman. Fungal Databases. Systematic Mycology and Microbiology Laboratory, ARS, USDA. Retrieved from http://nt.ars-grin.gov/fungaldatabases/ , August 6, 2010. (2) J. C. Guerber et al. Mycologia 95:872. 2003. (3) C. Nischwitz et al. Plant Dis. 92:974. 2008. (4) H. F. Schwartz, and K. S. Mohan. Compendium of Onion and Garlic Diseases and Pests, 2nd ed. The American Phytopathological Society, St. Paul, MN. 1995.
