by Marty Reich, Master Rosarian
Excerpted from the July 2018 Rose Leaf
I assume everyone is still battling Japanese beetles and now we are seeing spider mites.
Spider mites can quickly get a foothold in a dry garden so this is the one time it is recommended to spray the foliage. After the sun is low enough that water droplets cannot burn the foliage, I spray the bushes with water and wet the mulch underneath them as well as actually running the watering system. You can also use a water wand which is effective at getting water underneath the leaves where the mites do their dirty work.
For those of you who do not know the signs of spider mite damage, watch for leaves that become dry and brittle and have black specks like pepper on the undersides. The mites cause damage by puncturing the plant cells to feed. Sometimes there are tiny webs, but not always, and you have to really look to see the mites moving. They have a short life cycle so a new crop is coming quickly, but they do not climb well so the water works, and you can also pull off the bottom row of leaves to hinder them further.
There are very expensive miticides available and they work, particularly if you get a product that kills all stages of the mites, but you really should not need to buy them if you watch and wash.
Another reason it is OK to wet the leaves when fending off mites is that mites come when it is really hot. Blackspot likes 65 degrees for spore germination and 75 degrees for the disease to develop rapidly. Temperatures of 85 degrees and above inhibit it. High temps also inhibit this rosarian from spraying so that is a blessing!
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