Black Spots on Raspberries | Causes, Prevention and Concerns

Garden Troubleshooting

Black Spots on Raspberries | Causes, Prevention and Concerns

For many gardeners, early summer marks beginning of raspberry season. Those juicy, red berries begin to appear all over raspberry bushes ready to be harvested and consumed. While nothing beats an abundant harvest of juicy, tart and sweet raspberries, sometimes black spots can appear on your berries making them an eyesore and more importantly, having you question if they should even be consumed.

So what are those black spots on raspberries? Black spots on your raspberries are just dead or decaying areas of the berry most likely caused by a fungal or bacterial infection. When an infection begins attacking the raspberry bush, it will create toxins and proteins that will kill the plant’s immune system. As a result, many of the plant’s cells die off leaving black spots on the berries.

Safety Concerns:

You’re probably wondering: Are raspberries with black spots safe to eat? The short answer is yes. The bacterial or fungal infection that have caused the raspberries to gain black spots have evolved to harm a plant’s immune system, but not a human’s.

The only downside to consuming raspberries with black spots on them is that it may not taste very good. After all, you are consuming dead or decayed parts of the berry, but that is really the only concern when it comes to eating raspberries with black spots.

As long as you rinse off your raspberries with some water prior to eating them, you should be fine. The real threat is the dirt and debris on produce that may be harboring harmful bacteria such as E. coli on its surface. However, this threat applies to most all fruits and vegetables. As long as you give your produce a good rinse off, you should be fine.

Discolored or rotten raspberries, however, are never a pleasant sight to see. But, with the right treatment and diagnosis of the plant, you can prevent your raspberry bush from producing anymore defective raspberries in the future. 

Fungal Infections

Fungal infections are notorious for causing discolored spots on raspberries as well as other fruits and vegetables for that matter. Harmful fungi can be harbored in the surrounding soil, weeds or even bugs and insects in and around your raspberries. 

Not all fungi is necessarily bad for you raspberry bush. In fact, some fungi such as mychorrizal fungi can be beneficial for plants in your garden by helping the roots take in more nutrients from the soil.

While some fungi can be good for your raspberry bush, many are harmful. Some of the more common fungal infections that negatively affect your raspberry plants and give the berries black spots include the following:

 

Cane Blight

A blackened section insdicating Raspberry Cane Blight.

This is a fungal infection caused by a fungus called Leptosphaeria coniothyrium. Black raspberries are especially susceptible to the fungus. 

Cane blight occurs in the first year of growth. It affects any present wounds on the raspberry canes. 

The fungus then grows into the vascular system of the plant. The affected cane overwinters, and in spring, the fungus spores are released. These spores thrive in wet weather. Factors like irrigation and splashing rain help to distribute the fungal spores to other uninfected canes

The spores then germinate in any present wounds and produce new infections. Cane blight symptoms are likely to appear beyond the wound in fall. Therefore, you are more likely to notice cane blight on pruning wounds when you peel back the cane.

The presence of vascular discoloration should confirm that your raspberries have cane blight. Also, if the infection is present, the lesions will have dark red spots with irregular purple borders and gray centers.

In the second year of growth, cane blight will prevent buds and flowers from growing. You can also experience raspberry fruits that are sunken and black. They often look like tiny peppercorns.

Managing and Treating Cane Blight

If you do not want to see black spots on raspberries, you can start by preventing your canes from getting wounds as much as possible. This can be difficult for you if you harvest your raspberries mechanically.

Also, plant your raspberries with excellent spacing. Reduce weeds in your garden as well. This will increase the airflow and exposure of the sun towards your plants. It will also help the canes to dry thoroughly.

Avoid pruning your raspberry plants during the wet weather, and do not use over-irrigation methods to water your plants. This will help keep your plants dry while simultaneously preventing the spread of any present cane blight spores.

Check your sanitation to prevent the infected plant material from overwintering. In case of infection, remove the flowers after your harvest, burn them, and then bury the remains. You should also prune any old canes.

The optimum height of the raspberry cane is 3 to 4 feet. Once your raspberries reach this height, prune about one to four inches off the tips. Do this when you are sure you will experience dry weather for at least three days.

Also, practice pest control measures. Pests such as cane maggots and borers will cause wounds that act as easy infection points. To prevent the occurrence of cane blight on your raspberries, plant versions of the plant that are resistant or tolerant.

Nova, Julia, and Newburgh raspberry plants are known to be highly tolerant to cane blight. Also, water and fertilize your plants properly to reduce stress. Stress makes plants more susceptible to fungus.

You can also apply lime sulfur during early spring to reduce the chances of the fungal infection occurring. Make sure the buds are less than half an inch long.

Anthracnose

Antracnose on Raspberry leaf

Anthracnose causes black spots on raspberries as a result of the fungus Elsinoe Veneta. The fungus overwinters in infected plant tissues. In spring, the mature spores are released from the buds on the raspberry plants.

This often happens in wet weather since the splashing rain, insects, and wind distributes the spores. The fungus mostly affects the young green raspberry plants. Once the plants are affected, it only takes between three and twelve hours for the spores to germinate.

Within the first week to the end of one month, lesions will start appearing on the plants. The fungus will remain in the plant until the second round of harvest if not treated or eliminated. 

You can tell that your raspberries have anthracnose when your canes start to stunt and lose vigor. Additionally, the lesions will be evident on your plant but most prevalent on the canes. The fungus lesions are often sunken and oval or round.

They also have a dark purple border with a gray or tan center. If the lesions affect a vast region, the condition is known as gray bark. The affected bark will eventually crack.

In the beginning, the anthracnose fungus is yellow and small. Eventually, the fungus becomes large and looks like cane lesions. The center might also drop out, giving way to a shot-hole appearance. As a result, any infected fruit will be dry and hard. 

Management and Treatment of Anthracnose

Start by spacing your raspberry plants properly. Also, ensure that you continuously control the appearance of any weeds around your raspberry bushes. This will increase the airflow and sun exposure that your plants experience.

Do not use overhead irrigation since it soaks the foliage and helps the spread of the fungal spores to unaffected plants. Make sure you plant your raspberries when the sun is fully up. Remove any wild plants that might be growing near your raspberries since they can spread the disease.

Because the fungus overwinters in any infected plant material, make sure you have the best sanitation system. Also, remove your floricanes and infected canes after harvest. Burn and bury them to prevent the reoccurrence of the infection.

Make sure that you prune your raspberries before the new canes sprout. Also, remove any stressed plants since they have a high likelihood of being infected by the disease. Pests such as maggots and borers will also distribute the fungal spores.

Hence, control the pests using pesticides. You can also opt to plant raspberry varieties like Nootka, Heritage, and Chilcotin that are resistant or tolerant to Anthracnose.

Bacterial Infections

Like fungal infections, harmful bacteria can also attack your raspberry bushes and potentially give your raspberries black spots. Likewise, these bacteria can also be lurking within the soil, water and surrounding weeds of your garden. Because of this, its important to identify bacterial infections early on in order to prevent significant harm to your raspberry bushes.

Fire Blight

Fire blight on apple tree

The bacterium Erwinia amylovora causes fire blight. You will notice the presence of fire blight on raspberries through a sudden withering and dying of your flowers, leaves, branches, fruit spurs, and twigs.

The appearance of brown to black spots on raspberries confirms the presence of fire blight as well. The most affected plants will look as though they are scorched by fire hence the name fire blight.

You will also notice dark brown or purple-black lesions that are sunken, cracked at the margin, and encircled on the twig, trunk, and branch of your raspberry plant. If this occurs, it becomes difficult for your plant to survive.

Your raspberry fruits will also get water-soaked, turn brown or black, and become shriveled. During the wet and warm spring season, you will notice droplets of the bacteria on the surface of your fruits.

Insects, rain, and wind carry the bacteria to other unaffected plants and fruits. Once the bacteria enter the vascular tissues of your plants, the raspberry starts to darken and die. This usually happens in the early summer and late spring.

If the infection is not removed, the bacteria will overwinter at the trunk and branch margins of your raspberries, and the process will repeat itself on the next blossoming spring season.

Management and Treatment of Fire Blight

No flight blight cure currently exists. But, you can take some protective measures even though they are limited. The best thing that you can do for your garden is to remove any presence of the flight blight bacteria.

Do this by pruning or cutting the affected plants, and use bleach or alcohol to disinfect the pruning equipment after each cut. This will prevent the bacteria from spreading. However, it will not control the most severe attacks

Avoid overhead irrigation since it can create wounds in plant tissues that become easy targets for the infection. You can also start your raspberry garden by planting the resistant varieties. Such raspberries include Avon, Nova, and Ruby. Do not plant the Boyne raspberry plant since it is susceptible to flight blight.

Colored raspberries like the Honey Queen also show high resistance to the bacteria.

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