Busy lizzies in a tizzy
A devastating disease threatens popular bedding plants, writes Barbara Smith.
Perfect plants for lining a path or brightening up a municipal roundabout, busy lizzies, Impatiens walleriana, have been easy-care, reliable potted colour stocked in garden centres by the millions. But now they are under threat from fungal disease Plasmopara obducens, known as impatiens downy mildew.
The disease ripped through plants in the United Kingdom in 2012, there are regional outbreaks throughout North America and it has now reached New Zealand.
Plants turn yellow, droop, then drop their leaves, flowers and sometimes even the stems. Plants succumb suddenly, especially in wet weather. There is no cure and the spores remain in the soil so new impatiens plants can’t be put in the same place.
Spores are spread by the wind and in droplets of water or hitchhike on infected plant material or soils. Buying and selling plants potentially spreads the mildew. Healthy plants can be infected and rapidly deteriorate in just a few weeks.
If leaves are light green, yellowing, downward curling or have white furry mould underneath, dig out diseased plants and any others nearby.
Burn diseased plant material or put it in your household rubbish. Don’t compost it.
Many growers have decided not to supply seedlings for sale this year because of the risk of spreading the infection throughout the country.
The news is not all bad. This mildew is specific to Impatiens walleriana and does not affect any other species, including New Guinea impatiens and Sunpatiens hybrids, which are widely available in a range of bright colours and cope with more sun than busy lizzies.
European plant breeders have developed mildew-resistant strains, which are available this year in the UK and marketed as Imara Bizzie Lizzies. Appropriately for plants of East African origin, the name, Imara, comes from a Swahili word meaning strong and resilient. There is no evidence that infection is transmitted by seed, so resistant strains should be available here eventually.
In the meantime, grow something different in the shady spots we usually fill with impatiens. Try coleus (Lynda Hallinan says they are cool again), bedding and tuberous begonias, heucheras or Brunnera ‘Jack Frost’ with speckled leaves and blue forgetme-not-like flowers.
For more information, read the fact sheet from NZPPI nzppi.co.nz/documents/pests/ Idmretailbrochure.pdf.