Controlling Black Spot

What Is This Disease?

If you grow roses, you’ve probably encountered black spot. Caused by the fungus Diplocarpon rosae, this disease causes circular black spots of various sizes on plant leaves. Infected areas start small, but if left untreated, will enlarge and increase in number. Eventually, infected leaves will turn yellow and fall off. Black spot usually isn’t serious enough to kill your roses on its own, but if no action is taken to control this disease, defoliation will continue, weakening the plant and reducing flower production. Weakened plants become much more susceptible to other diseases, winter injury and possible death. The good news is the Blue Bottle offers do-it-yourself solutions to get rid of black spot and protect your roses.

Signs of Infection

This disease usually begins on the upper sides of leaves. You’ll first notice small black spots, often fringed with yellow margins. They appear in various sizes, some as small as 1/10 of an inch, others can measure up to ½ inch in diameter. Areas adjacent to the black spots turn yellow and leaves drop prematurely, usually beginning at the bottom of the plant and progressing upward. Spots can also occur on stems, and plants can be completely defoliated.

What Causes Black Spot?

This fungal disease overwinters in diseased canes and infested fallen leaves. The fungal spores germinate in the spring and spread through splashing water droplets. Black spot is most prevalent in moist weather with frequent rain. During wet periods in spring and early summer, black spot can easily spread from leaf to leaf and even to nearby plants. Once an infection is established, the fungus produces more spores that splash to new plant tissue, spreading the disease. Fungal spores on the leaf surface must be continuously wet for at least seven hours for infection to occur. New spots develop in 5-10 days. Not surprisingly, extreme summer temperatures and drier conditions during July and August help limit development of this disease.

Where is Black Spot Found?

Black Spot is one of the most common diseases in the garden and found everywhere roses are planted. In addition to roses, black spot can also affect tomato plants, fruit trees, plus ornamentals and other flowers. If you notice black spots on your leaves, your plants may be infected.

Tips for Prevention

Preventing black spot begins with:

A fall/winter clean-up – rake up, collect and remove fallen leaves, prune dead leaves and diseased canes and discard.

Keeping the foliage dry – fungal spores need a wet surface to germinate and cause infection. Water the base of the plant, not the leaves, whenever possible. If overhead watering is your only option, water early in the day so foliage is dry by nighttime.

Pruning – allowing more air circulation in and around plants helps keep foliage dry.

Spring mulching – a 2-3 inch layer of mulch helps prevent splashing water, which is how these fungal spores spread.

How to Control Black Spot

If your plants are already infected, you can take steps to get rid of black spot and protect your plants. Here are helpful tips from our experts.

Remove diseased leaves – prune infected leaves from the plant, collect dead or defoliated leaves from the ground, and discard or destroy them.

Avoid overhead watering – keep leaves as dry as possible as black spot spread through water droplets.

Use the Blue Bottle – BioAdvanced offers multiple solutions that are all easy to use. All-In-One Rose & Flower Plant Spray controls black spot and other listed fungal diseases. It also controls listed insects like adult Japanese beetles, aphids and more. Or choose All-In-One Rose & Flower Care which offers systemic disease and insect control, plus includes fertilizer for strong roots and bigger blooms. It’s available in both concentrate and granules.

Increase sunlight – if your roses are shaded by nearby trees or shrubs, consider pruning these plants to increase the amount of sunlight your roses receive. Roses grow best in full sun. Or consider moving your roses to a sunnier location in your landscape.

Replant using more disease-tolerant rose varieties – Many yellow and copper-colored rose cultivars seem to be more susceptible to black spot. Roses reported to be highly resistant to black spot are: ‘Fortyniner,’ ‘Coronado,’ ‘Carefree Beauty,’ ‘Simplicity,’ ‘Bonica,’ and ‘Grand Opera.’

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Controlling Black Spot

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