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Downy Mildew in Impatiens (Bizzy Lizzies)

Impatiens downy mildew (Plasmopara obducens) is a foliar disease which is specific to Impatiens (Bizzy Lizzies). It was found affecting Impatiens walleriana for the first time in the UK in 2003 and thereafter remained absent or at low levels before re-appearing in 2007 and 2008 and again in 2011.

Usually, the first symptom of downy mildew infection on impatiens is a pale green to slight yellow discolouration on the upper leaf surface, often with a downward curling of the leaf. A few days after (or even overnight given the correct conditions) the same leaves can exhibit a white 'downy' growth largely on the lower leaf surface. Following infection, symptoms can remain unseen (latent) for anything from a few days to several weeks depending upon the environmental conditions. Warm termperatures and high humidity or leaf wetness favour sporulation. Severely affected plants are often defoliated and stunted.

The disease moves from plant to plant via spores spread by water splash or air currents. Leaf wetness is required for successful spore germination and infection. Resting spores, first discovered in 2008, are also produced which can survive and over-winter in decaying host plant tissue and in the soil, potentially allowing carryover of new infections from year to year.

Action for gardeners

Remove dying Impatiens plants from beds and dispose of them to land fill. Do not dig in plant material into beds, put in your green waste bin or include in compost. Avoid planting Impatiens into beds where plants have died unexpectedly. Use other bedding plant species in areas where the disease is suspected to have occurred previously. This disease is specific to Impatiens and hence does not present a risk to other bedding plant species.

 

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