Scale Insects & Downy Mildew

November-December 2011

In the landscape we have very few options available for pest control under the pesticide ban here in Ontario. One of the main insecticides we use within the ban is horticultural oil. Horticultural oil smothers the insect, interfering with its ability to respire and it dies. A “dormant oil” spray is a common pest control practice and a lot of landscape professionals carry out these applications, but what pests are we trying to target? Most people will tell you they are trying to target overwintering insects, like scale insects, but does it really work on all of them? 

Several scale species overwinter as nymphs on the undersides of branches. If these immature nymphs overwinter exposed on the surface of twigs, indeed they are susceptible to fall and spring applications of horticultural oil. Overwintering scale nymphs are tiny, dark brown, flat discs about 1-2 mm long that can be found on the undersides of twigs or near buds. Autumn and spring are a great time to apply horticultural oil to target overwintering nymphs. Remember to direct the spray to the undersides of twigs to increase the chances of contacting your target. 

It’s important to note that not all scale insects overwinter as exposed, immature nymphs. Only those scales that overwinter as immature nymphs out in the open are susceptible to fall and/or spring applications of horticultural oil. These scale insects include: European Fruit Lecanium scale (Lecanium corni) on Acer, Quercus, Fraxinus; Fletcher scale (Lecanium fletcheri) on Thuja, Taxus, Magnolia scale (Neolecanium cornuparvum) on Magnolia; Cottony maple scale (Pulvinaria innumerabilis) on Acer, Viburnum, Prunus; and Tuliptree scale on Liriodendron

The following scales overwinter as mature females or eggs protected under the dead female scales. These species of scale insects are NOT susceptible to dormant horticultural oil applications: Euonymus scale (Unaspis euonymi) on Euonymus, Pachysandra; Golden Oak Scale (Asterolecanium variolosum) on Quercus; Oystershell scale (Lepidosaphes ulmi) on Fraxinus and others; and Pine Needle scale (Chionaspis pinifoliae) on Pinus. For these scales, the only susceptible life stage is the crawler stage.  

And under those trees….
Did you happen to notice all the rough-looking beds of impatiens this year? Impatiens everywhere developed yellow foliage and flower buds that dropped mid summer, leaving fleshy, naked stems behind. One of the top-selling bedding plants, Impatiens walleriana, was host to widespread plagues of downy mildew in Ontario and much of the Northeastern US (and the UK). This is the common species of impatiens you’ll find at any garden centre or box store. The disease is caused by a fungus-like organism, Plasmopara obducens, and it is a very successful pathogen. There are a lot of theories about why the outbreak was so bad this year. Firstly, it was a long, cold, wet spring and those are perfect conditions for a lot of fungal diseases. It could be that the fungus has learned to out-smart the products and practices being used to manage it in production, and perhaps infected plants were sold and used in mass plantings across urban areas. And, perhaps the fungus has established itself in planting beds outdoors and has built up enough inoculum to cause infection in annual plantings, regardless of the weather conditions.  

When you think about it, there aren’t too many annuals that give so much colour and foliage at a compact height, that tolerate shade and sun so well once established, that are so economical, and are easy to clean up after. It’s not surprising that we use impatiens over and over again in our cityscapes and gardens giving little thought to the principles of crop rotation and soil rehabilitation that we learn in our winter workshops. Infested plants contain fungal resting structures that persist in the soil for several years. Therefore, infested plants should be removed from the bed and disposed of. Removing infested soil is not practical but removing some soil and adding back compost can do a lot to feed the beneficial microbes in that soil that compete with the pathogen. The good news is that this species of downy mildew is specific to impatiens, so rotating in other types of bedding plants is a good strategy for long-term disease management. 

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