Blackberry leaves with signs of disease.

Like most food plants, blackberries can be targets of fungi and bacterial diseases. Here’s how to recognize and deal with these diseases.

Blackberry anthracnose

Blackberry anthracnose is a common fungal disease plaguing many home gardeners. Anthracnose can cause poor fruit quality and production, and in severe infections, weaken or even kill canes.

The first signs of a blackberry anthracnose infection will be in the spring, usually on the shoots of the new canes. Small purple spots will appear, which will increase in size, become oval shaped, and eventually turn a gray or buff color. You may also find small spots with light gray centers and purple margins on the leaves. In heavy infections, the spots on the canes and stems can grow in numbers and merge together, covering the canes and appearing as large cankers with cracks. This can girdle the cane, causing dieback.

This disease is caused by the fungus Elsinoe veneta. It overwinters in diseased canes and then releases spores during rainy periods the following spring and summer. The greatest risk of it infecting blackberries is between bud break and pre-harvest, as the fungus targets mainly the new growth.

The recommended blackberry anthracnose treatment is simple. Make sure to properly space and prune your plants. Erect varieties of berries are less susceptible to anthracnose than spreading types. Remove any wild brambles in the area, which can harbor the disease. Remove weeds in your berry patch and prune back blackberry bushes to promote good air circulation and light penetration. This will allow faster drying of foliage and canes.

After harvesting blackberries, and during the dormant season, remove and destroy any canes that have been infected.

These cultural practices may be sufficient to control blackberries with anthracnose, but using a delayed dormant spray may also be necessary. Before growth starts and while temperatures are still cold, apply a lime, sulfur, copper hydroxide, or a foliar fungicide. The recommended type may vary depending on your area, so check with your county extension office for the latest information.

Blackberry agrobacterium diseases

There are a few agrobacterium diseases of blackberries: cane gall, crown gall, and hairy root. All are bacterial infections that enter the plant through wounds and create galls or tumors on either the canes, crowns, or roots. Cane galls occur most commonly in the late spring or early summer on fruiting canes. They are long swellings that split the cane lengthwise. Crown galls are warty growths found at the base of the cane or on the roots. Both cane and crown galls on blackberries become hard and woody and dark in color as they age.

Hairy root appears as small, wiry roots that grow either alone or in groups from the main root or the base of the stem.

Galls interfere with water and nutritional flow in the vascular system of plants, seriously weakening or stunting the brambles and making them unproductive.

There are no chemical controls for the eradication of agrobacteria. It is important to examine canes prior to planting for any evidence of galls or hairy root. Only plant nursery stock that is free of galls and do not plant in an area of the garden where crown gall has occurred unless a non-host crop has been grown in the area for two-plus years.

Solarization may help kill bacteria in soil. For this, place clear plastic on tilled, watered soil from late summer to early fall. Also, be gentle with the canes when training, pruning, or working around them to avoid any injury that will act as a portal to bacteria.

Only prune the canes during dry weather, and sanitize pruning equipment both before and after use. If only a few plants are affected, remove them immediately and destroy them.

Orange Rust

Blackberry bush showing signs of orange rust.

This fungus causes plants to become stunted and weak with poor fruit production. The disease is systemic, and remains throughout the plant so just removing infected leaves will not solve your problem. Orange rust of blackberries is most infectious when temperatures are cool and wet with high humidity.

Initial symptoms of orange rust of blackberries are yellow or discolored new growth; spindly, wilted, or sickly appearance of whole plant; and stunted, twisted, or deformed foliage and canes. Waxy blisters may form on the margins and underside of foliage. These blisters eventually turn a bright, shiny orange color as the disease progresses.

The orange pustules then release thousands of fungal spores which can infect other blackberry plants. Infected leaves may wilt and drop, spreading the disease into the soil below.

Dig up and remove infected plants and destroy nearby wild brambles. Remove plants before the spores are discharged if possible.

Phytophthora Root Rot 

This soil-borne disease thrives in poorly drained soils and can live in the soil for years. Above ground symptoms include pale or reddish leaves, small leaves, defoliation, branch die back, stunting, and death. Remove infected plants.

Powdery mildew

Powdery mildew occurs on the top of the leaves in humid weather conditions. The leaves appear to have a whitish or greyish surface and may curl.

Avoid powdery mildew by providing good air circulation for the plants by good spacing and pruning. Contact your Cooperative Extension Service for fungicide recommendations.

Blackberry fruit rot

Blackberry fruit rot is caused by Botrytis cinerea, a fungus that can affect nearly every part of the plant. Fruit rot favors humid environments. It is especially prevalent when weather is wet before and during blooming, and again when berries ripen. The fungus overwinters on plant debris and weeds.

In spring, the spores spread via wind and water, including moisture from dew, fog, rain, or irrigation water, or by direct contact with plants. Once fruit rot of blackberry finds its way into your garden, it can be treated and reduced but not eradicated.

If your blackberries are rotting from botrytis, the blackberry fruit rot displays as a watery rot followed by a hairy, gray, or brown fungal growth. Flowers will appear brown and shriveled. Blackberry canes may look bleached with whitish-brown lesions. Small, black patches may appear on any part of the plant. Unharvested berries left on the vine become mummified.

To prevent fruit rot, site blackberries where the plants are exposed to direct sunlight. Ensure the soil is well drained. Never plant blackberries in low areas where water pools. Spread a layer of straw or other organic mulch around blackberry plants to prevent fruit from direct contact with the soil. Space plants far enough apart to provide ample air circulation.

Avoid high-nitrogen fertilizers, especially in spring. The fertilizer produces dense foliage and shade, thus preventing rapid drying. Adjust your irrigation schedule, if needed. Water blackberries with a soaker hose or drip system and avoid overhead watering. Keep the plants as dry as possible.

Practice good weed control, as weeds limit air movement and slow drying time of blooms and fruit. Keep the area clean. Pick blackberries frequently and don’t allow fruit to over ripen. Harvest in the morning as soon as the plant is dry. Refrigerate berries as soon as possible. Discard rotten blackberries carefully. Never leave them in the garden and don’t place them on the compost pile.

Chemical fungicides may be effective when used in conjunction with the above techniques. Check with your local cooperative extension office to determine which product is suitable for your area. Don’t overuse fungicides. Strains are already resistant to certain fungicides in several regions.

Algal spots

In many cases, blackberries with algal spots will still produce a good crop of berries, but in the right conditions and when severe, the infection can really take a toll on canes. It’s especially important to look for signs of algal spot if you are growing blackberries in a warm and humid climate. A bad infection could kill your plants.

Very few species of algae infect plants, but one, Cephaleuros virescens, infects and causes damage in blackberry canes. When the infection is mild, it doesn’t cause much damage and the canes will still produce good fruit in abundance. However, when the infection spreads, it can girdle the canes or even cause splitting that allows other infections to set in, and ultimately this can damage the canes enough to kill them and severely limit the crop.

The infection causes light green to yellow or orange velvety spots to form on canes, which is why the disease is also called orange felt or orange cane blotch. The spots begin near the base of canes and will be more prevalent there than higher up. The spots get more orange in color and fuzzier as the infection continues.

When the conditions are warm and wet, the spots will merge and ultimately cover or girdle the cane. To distinguish between algal spots and a rust disease, rub the orange spots. If the color comes off on your fingers, it’s a rust disease. If it stays put, it is more likely Cephaleuros of blackberry.

The spots typically begin appearing in early summer and grow larger and merge in the fall. There are many methods of cultural control you can try to manage algal spots on blackberries before turning to chemical interventions. To prevent spreading the infection to new canes, for instance, trim back old canes after harvesting berries and destroy them.

Use drip irrigation instead of overhead to reduce standing moisture on the canes. Again, keep them well trimmed and keep the area underneath weed-free to promote air flow. Make sure your canes are planted in a spot that drains well. You can also help your blackberry canes fight off the infection by providing them with the appropriate soil amendments and fertilizer, as well as adequate water without over-watering. If the infection gets severe, you can try a copper fungicide spray.

Rosette (also known as “double blossom”)

Rosette, also called double blossom, is a severe fungal disease that can affect blackberries grown in the southeastern United States. Double blossom is a particularly nasty disease because it appears biennially, infecting plants in spring or early summer—but, symptoms don’t appear until the following year when new growth starts on fruiting canes. So, it’s easy to miss double blossom until it’s already widespread in your patch.

How to spot double blossom: Infected buds develop several branches, instead of the single branch that is indicative of healthy stems. Nodes will grow closer together than normal when infected. Young foliage of infected plants will start light green and grow to be yellowish-brown, compared to the dark-green foliage of healthy plants. And, finally, infected flowers will have more pink, purple, or red color, compared to the healthy flowers of the same variety—plus, flowers may appear ruffled or distorted, giving Rosette disease the nickname “double blossom.”

How to fight Rosette or “double blossom”: Here are five strategies for fighting double blossom with best practices and treatments:

• Plant disease-resistant cultivars: Most erect, thorny blackberry varieties are susceptible to Rosette—try planting a thorn-less cultivar that is resistant or somewhat tolerant of double-blossom disease.
• Destroy any wild blackberries: If you’re in the Southeastern U.S., you should remove any wild plants from the vicinity of your cultivated blackberry patch—wild blackberry plants are often infected, and are the cause of double blossom spreading through the air.
• Rigorous pruning of infected plants: With new plantings, prune out any stems with apparent Rosette symptoms before the infected buds open. You can save plants by pruning before blossoming, because the disease is not systemic within the blackberry plant—sometimes, removing the side stems with disease symptoms is good enough.
• Use organic fungicide: Try a Bordeaux mixture as a pre-harvest fungicide spray, applied only when the temperature is above 75 degrees F—otherwise, it may burn the foliage. Because of its corrosive potency, Bordeaux mixture should not be stored in delicate containers—and it can even damage spray nozzles.
• Mow severely infected plants: Badly infected blackberry plants should be pruned to about a foot above the ground by mowing immediately after harvest. If the plants are so severely infected that harvest is not feasible, mow them before the harvest. Remove diseased plant material from your patch, and fertilize with a complete fertilizer.

How do you deal with diseases in your blackberry patch? Please share your tips with us in the comments below.