I (yes, it’s me, HerbDoc) had an interesting question from a beekeeper the other day and thought others might have the same problem. The beekeeper was looking for a control for late leaf rust which would not harm the pollinators in the garden. He indicated that his property also contained white spruce and that they had lost their entire crop of raspberries last year.
Late leaf rust is a fungus which is heteroecious, meaning that it attacks two different hosts at different stages of its life cycle. The rust fungus produces two types of spores only on red raspberries. The alternate host for the rust is white spruce on which the aeciospore is produced. These are released from infected white spruce in mid-June to early July and are capable of infecting the raspberries at that time. The spores start to form on the undersides of the infected raspberry leaves and flower parts. Another type of spore develops on the infected leaves and canes in the fall and serves as the overwintering form of the fungus. These develop the following year and infect the white spruce needles.
New studies indicate that most of the fungus is overwintering on infected canes so it is wise to cut them down. All of the canes and anything that fell to the ground should be disposed of in the garbage that leaves the property.
The best advice includes making sure the site has good air circulation and full sun exposure. A tree canopy can cause fungus problems. Row width should be 1-2 feet and cane density should not exceed 3-4 canes per square foot. Control fertilizer to prevent excessive growth and keep weeds under control. At the end of the season remove and destroy old fruited and infected canes.
The only treatment that is considered organic is fixed copper (Bordeaux mixture – a mix of hydrated lime and copper sulfate). It would be applied in dormancy and at first bloom. Most farm supplies carry this product and Gardens Alive! Online carries what they call Soap-Shield which is copper-based and non-toxic to bees and other pollinators.