New Disease Reports (2016) 33, 18. [http://dx.doi.org/10.5197/j.2044-0588.2016.033.018]
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First report of Verticillium dahliae causing wilt in Impatiens New Guinea Group hybrids

E. Beal, J.O. Denton, G.J. Denton and J. Scrace*

*john_scrace@hotmail.com

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Received: 05 Nov 2015; Published: 25 Apr 2016

Keywords: Impatiens hawkeri

In September 2014, wilting plants were observed in a trial of Impatiens hawkeri New Guinea Group hybrids at RHS Garden Wisley. The stems of affected plants were defoliated (Fig. 1) and black staining was noted in the vascular tissues when cut stems were examined in cross section. Stems with staining were incubated in a humid chamber and after three days abundant verticils and conidia of a Verticillium species were produced, corresponding to the dark, stained areas of the stems (Fig. 2). The fungus usually went on to produce microsclerotia suggestive of Verticillium dahliae when the specimens had been incubated for seven days. Natural infection was thus confirmed on 26 cultivars and pure cultures of V. dahliae from Impatiens SunPatiens Vigorous Magenta ‘Misato Fg3’ and Impatiens ‘Impacwhi’ were isolated onto potato dextrose agar and used for infection assays and molecular analysis. The ITS region was sequenced with ITS1F (Gardes & Bruns, 1993) and ITS4 (White et al., 1990). The sequences (GenBank Accession Nos. KT970068-KT970070) were identical to V. dahliae isolates from, for example, tomato (FJ900167, GU461607 and GU461610), cotton (GU461609) and grapevine (FJ475122). Three isolates were deposited into the RHS Plant Pathology culture collection (Accession Nos. M10A6, M10D5 and M10E5).

Plug plants of Impatiens cvs. SunPatiens Blush Pink ‘Sakimp013’ and SunPatiens Vigorous Magenta were potted into 9 cm pots in a 3:1 (v/v) mix of John Innes No. 2 potting mix:vermiculite. After four weeks’ growth, the roots were washed and placed for 30 minutes in conidial suspensions (c. 107 conidia/ml) of V. dahliae (M10E5) in sterile distilled water, or water alone, before transferring into fresh potting mix as above. Two plants per treatment were placed in a growth room maintained at a temperature of 20°C day/16°C night and a humidity of 55%. Plants showed signs of wilting, defoliation and vascular tissue staining four weeks post inoculation (Fig. 3). Uninoculated plants showed no symptoms of wilt. Verticillium dahliae was recovered from the vascular tissue in stained stems of inoculated plants.

This is the first record of Verticillium wilt on Impatiens New Guinea Group hybrids. The disease has been recorded on other species of Impatiens such as I. balsamina (French, 1989) and I. walleriana (Taylor, 1993). However, this report is of particular interest as New Guinea Group hybrids have been grown as a substitute for I. walleriana (busy Lizzie), which is very susceptible to the downy mildew pathogen Plasmopara obducens (Jennings, 2014). New Guinea Group hybrids are not thought to be susceptible to P. obducens.

Figure1+
Figure 1: Impatiens hawkeri New Guinea Group hybrid plant with severe stem collapse and defoliation.
Figure 1: Impatiens hawkeri New Guinea Group hybrid plant with severe stem collapse and defoliation.
Figure2+
Figure 2: Abundant verticils and conidia of Verticillium dahliae produced on infected Impatiens hawkeri stems. Bar = 200 µm.
Figure 2: Abundant verticils and conidia of Verticillium dahliae produced on infected Impatiens hawkeri stems. Bar = 200 µm.
Figure3+
Figure 3: Healthy control plant (left) and inoculated plant (right) showing symptoms of wilting and defoliation, four weeks post inoculation.
Figure 3: Healthy control plant (left) and inoculated plant (right) showing symptoms of wilting and defoliation, four weeks post inoculation.

References

  1. French AM, 1989. California Plant Disease Host Index. Sacramento, California, USA: California Department of Food and Agriculture, 394. [http://dx.doi.org/10.5941/MYCO.2014.42.2.203]
  2. Gardes M, Bruns TD, 1993. ITS primers with enhanced specificity for basidiomycetes - application to the identification of mycorrhizae and rusts. Molecular Ecology 2, 113-118. [http://dx.doi.org/10.1111/j.1365-294X.1993.tb00005.x ]
  3. Jennings P, 2014. Continued management of Impatiens downy mildew. HDC Factsheet 05/14. Horticultural Development Company, Warwickshire, UK.
  4. Taylor NJ, 1993. First report of Verticillium wilt of Impatiens walleriana caused by Verticillium dahliae in the United States. Plant Disease 77, 429.
  5. White TJ, Bruns T, Lee S, Taylor J, 1990. Amplification and direct sequencing of fungal ribosomal RNA genes for phylogenetics. In: Innis MA, Gelfand DH, Sninsky JJ, White TJ, eds. PCR Protocols - A Guide to Methods and Applications. San Diego, USA: Academic Press, 315-322.

To cite this report: Beal E, Denton JO, Denton GJ, Scrace J, 2016. First report of Verticillium dahliae causing wilt in Impatiens New Guinea Group hybrids. New Disease Reports 33, 18. [http://dx.doi.org/10.5197/j.2044-0588.2016.033.018]

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