Black spots on raspberry plants or the fruit itself are common signs of a fungal disease, which is a widespread challenge for berry growers. These spots are symptoms of a few different cane diseases that thrive in moist, humid conditions. Understanding the specific appearance of these lesions helps identify the exact pathogen, which is the first step toward effective management. The fungi responsible survive the winter on the canes, leading to recurring problems if not addressed.
Identifying the Main Fungal Culprits
The two most frequent causes of black spots on raspberry canes are Anthracnose and Spur Blight, each with distinct visual characteristics. Anthracnose, caused by the fungus Elsinoe veneta, is most damaging to black and purple raspberry varieties. The infection begins on young primocanes as small, reddish-purple spots.
These lesions then enlarge, developing a noticeable ash-gray or white sunken center surrounded by a distinct, raised purple border, often described as a “pit lesion.” If the infection is severe, these spots can merge, girdling the cane and leading to the condition known as “gray bark.” Anthracnose can also affect leaves, causing small spots that eventually dry and fall out, creating a “shot-hole” appearance.
Spur Blight, caused by the fungus Xenodidymella applanata, is typically more problematic for red raspberries. This disease is characterized by dark, purplish-brown or black blotches that appear on the first-year canes (primocanes). These blotches are usually centered where a leaf stem attaches to the cane, known as the spur. The fungus infects the leaves first, then moves through the leaf stem to the cane tissue around the bud.
As the season progresses and the cane goes dormant, the purplish area often fades to a pale gray or silvery color. Tiny black dots, which are the spore-producing structures of the fungus, may be visible within this discolored area. The primary damage from Spur Blight is to the buds, which often fail to grow the following spring, resulting in bare sections of cane and a reduced yield.
Preventative Cultural Practices
Managing the environment around the raspberry patch is the most effective long-term defense against these fungal diseases. Fungi require extended periods of wetness on the plant tissue to germinate and infect, so the goal is to promote rapid drying. This begins with proper site preparation, including planting in full sunlight and well-drained soil.
Trellising and support systems are recommended to keep the canes upright and prevent them from sprawling on the ground. A simple post-and-wire or T-trellis system supports the canes, allowing air to circulate freely around the foliage and fruit. This improved air movement quickly evaporates moisture from dew and rain, reducing the window of opportunity for fungal spores to cause infection.
Judicious pruning is another practice for promoting good air circulation and removing sources of disease. For floricane-fruiting varieties, the old canes (floricanes) that have finished producing fruit should be cut down immediately after harvest. Removing these canes eliminates the primary structure where Anthracnose and Spur Blight fungi overwinter.
It is important to thin the new canes (primocanes) to a manageable density, ideally maintaining no more than six to eight of the strongest canes per foot of row. Switching from overhead sprinklers to drip irrigation minimizes wetting the foliage and canes, preventing the splashing of spores from the soil onto healthy plant tissue. If starting a new patch, consider planting resistant cultivars, such as ‘Heritage’ or ‘Nova’, which have shown lower susceptibility to these cane diseases.
Immediate Intervention and Fruit Safety
If black spots indicate an active infection, immediate sanitation is necessary to curb the spread of the disease. Any canes showing severe symptoms, such as extensive gray bark or dead buds, should be pruned out completely. The cut must be made several inches below the visible lesion, and the removed cane material must be destroyed (by burning or deep burial) to eliminate the fungal source.
A preventative application of a dormant spray in late winter or early spring is one of the most important chemical controls. This application, typically using liquid lime sulfur or a fixed copper fungicide, must be applied before the new leaf buds show more than one centimeter of green tissue. The spray’s purpose is not to cure the current infection but to kill the overwintering spores on the cane surface before they infect the new growth.
If the disease pressure remains high during the growing season, a preventative fungicide program may be warranted, particularly during wet weather periods. Home gardeners often use a copper-based fungicide, applying it when the new canes are approximately eight to ten inches tall and repeating the application as recommended on the product label. Fungicides work preventatively by creating a protective barrier on the plant tissue, meaning they must be applied before an infection takes hold.
Black spots on the raspberry drupelets themselves are generally not a toxicity concern for human consumption. Fungal infections that cause cane spots, like Anthracnose, can spread to the fruit, resulting in berries that are dry, seedy, or small, which is primarily a quality issue. Fruit showing signs of soft rot or fuzzy mold should be discarded, but berries with only cosmetic spots are typically safe to eat, though they may have an inferior flavor and texture.