Raspberries and white spruce share a disease called late leaf rust.
Mid to late summer, spores of this fungal disease infect red and purple raspberries. Unlike yellow rust, which produces yellow to orange pustules in early to mid-summer, late leaf rust pustules are generally not seen until mid to late summer.
Late leaf rust symptoms
The first symptom of late leaf rust (Pucciniastrum americanum) is small yellow (chlorotic) spots on the top of older leaves. These spots generally start to appear in lower portions of the plant, slowly spreading upward, into younger leaves. As the disease progresses, reproductive uredinia form on the underside of leaves, containing masses of spores.
Extreme infestations can defoliate the entire plant. Fruit and flowers can also be infected, which causes them to rot. Of course, if all the leaves have fallen off due to disease, there probably won’t be any fruit or flowers. If the fruit has already formed, late leaf rust will appear as tiny orange spots on individuals drupelets. Affected fruits will also ripen unevenly.
Late leaf rust lifecycle
Spores of this disease overwinter in white spruce and infected raspberry canes. Spores can also be spread by wind, so the health of your upwind neighbor’s raspberries can have a direct impact on yours.
Late leaf rust control
Like other rusts, late leaf rust is best avoided by pruning plants for good airflow and avoiding overhead watering. Also, be sure to remove spent canes each year. If any of those canes have been infected, removing them will break the disease triangle.
Fixed copper sprays may help prevent and treat late leaf rust.
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