Watch for plant diseases emerging this month

  • This is a good time to cut some of your outdoor flowers for indoor display.

    This is a good time to cut some of your outdoor flowers for indoor display. Courtesy of Chicago Botanic Garden

 
By Tim Johnson
Chicago Botanic Garden
Updated 7/4/2022 8:07 AM

This is the time of year to check closely for magnolia scale, an insect about the size of a pencil eraser. Typically found on the underside of magnolia branches, the scale is responsible for creating an overall thin condition of the tree.

The scale exudes a clear, sticky honeydew after feeding on the plant. Black sooty mold is a fungus that grows on the honeydew. Gardeners often notice the black mold on the magnolia leaves before noticing the actual scale.

 

The mold itself is not a real problem. On small trees, simply pick off the scale and squash them. Other control measures include spraying with summer-weight oil or applying an insecticide when the scale is in the active crawler stage, generally in early September.

For trees with a history of magnolia scale, a late winter to early spring application of dormant oil will provide good control for the scale. If your tree is affected, provide extra water during summer to minimize stress.

• If your hybrid roses have been losing their lower leaves and remaining leaves have yellowish foliage with dark spots, then it is likely you have blackspot, which is a common fungal disease. Begin a spray program with approved fungicides immediately.

The fungicides need to be applied once every seven to 10 days, as they work to prevent the disease and do not cure foliage that is already infected. Be sure to clean up any leaves that have fallen from the plants.

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Many landscape shrub roses are resistant to blackspot so they do not need to be on a spray program. Rugosa roses (Rosa rugosa) should not be sprayed for blackspot, as the fungicides can burn the foliage.

• This is a good time to cut some of your outdoor flowers for indoor display. It is best to cut the flowers during a cool part of the day and to put them in water as quickly as possible.

Remove any foliage that will be under the water in the vase. Adding a preservative to the water can lengthen the life of the flowers.

• Tim Johnson is director of horticulture at Chicago Botanic Garden, chicagobotanic.org.

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