Leaf Spot

(Question)

This is an example of something that has invaded my Joe Pye plants and a new Nannyberry (viburnum). The viburnum looks very bad. The Joe Pyes will come back next year.
Is this a fungus, an insect?
I cannot seem to find out what it is. Thanks for any help you can give.

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(Answer)

I don’t think it is an insect, but a large number of fungi and bacteria cause spots on plant leaves, in a variety of colors, shapes and sizes. In some cases the spots may spread to cover entire leaves, stunting plant growth. Other leaf spots have centers that die and fall out of the leaf, giving a ‘shothole’ effect.

Fortunately the same controls are effective against many leaf spot diseases. Pick off the infected leaves of the viburnum and cut down the Joe Pye weed and dispose of these items in the city yard waste program, not in your own compost pile if you have one (unlikely to be hot enough to destroy fungus or bacteria). Clear the ground of plant debris around the infected plants to eliminate overwintering spores. To prevent the spread of mild infections, spray leaves with bordeaux mixture and repeat twice at 10 day intervals. (This treatment advice comes from the Rodale Organic Gardener’s Handbook of Natural Insect and Disease Control.)

Bordeaux mixture is a mixture of copper sulfate and slaked lime (calcium hydroxide) and water and it is used as a preventative spray to prevent infestations of downy and powdery mildew and other fungi. It is not effective once a fungus has become established. The percentage of the weight of copper sulphate to the weight of water used determines the concentration of the mixture. A 1% Bordeaux mixture is common and would have the formula 1:1:100 – 1 kg copper sulphate, 1 kg slaked lime and 100 litres (100 kg) of water.