Roses have long been thought to be fussy and fastidious but in reality, it's just a handful of reasons why old-fashioned Rose bushes earned this reputation! After all, Wild Roses grow and ramble through ditches alongside the road, and many old farmsteads still bloom reliably for decades.
The main reasons that Roses have earned this bad rap are poor site selection, and a few Rose diseases like Black Spot.
All About Black Spot
Sounding like something out of the Dark Ages, Black Spot is actually a fungus called Diplocarpon rosae. It gets onto foliage and stems due to poor sanitation and cross-contamination, infecting leaves and greatly reducing plant vigor. Plus it forms its namesake - sooty black spots and smudges all over your beloved Rose bush! Especially during warm, wet weather that we typically see in the spring and fall.
Like a certain virus we have all come to live with, this fungus adapts fast and changes often, making it difficult to keep up with in terms of prevention and finding treatments. Despite the best efforts of modern Rose breeders to create new and resistant Rose varieties, this means that the resistance bred into new cultivars may not last as new strains of Black Spot adapt to overcome it. Spurring plant scientists to continue their efforts.
Black Spot Identification
- Circular, black spots with fringed margins
- Fast, expanding purple to black patch on the upper surface of the foliage
- Black blistering on stems
- Leaves turn yellow around the patches - almost like yellow halos
- Occasionally visible fungal strands (mycelium) visible like wispy or foamy cotton
- Leaf drop
- Small, scabby lesions on young stems
- In worst-case scenarios, entire plants look defoliated and have blackened stems
- Defoliate by midseason
- Weakened or reduced flower bud set
- Poor flower quality
Affected Roses also have a higher susceptibility to environmental stress and winter injury. Drastically shortening their lifespan.
Black Spot Look-Alikes
While nothing damages your Roses quite like Black Spot can, there are a few issues that may have similar characteristics.
Powdery Mildew
A non-fatal fungus that can grow a bluish, whitish, or gray fuzzy powder on leaves, shoots, and buds. This can lead to leaf drop and dusty-looking plants.
Downy Mildew
Another fungus that causes leaves to develop purple to red blotches on the upper surface. Has a similar effect on the plant but is not fatal.
Rusts
Yet another fungus (see a theme going on here?) forms small, orange blisters that develop on both leaf surfaces in early spring. In late summer or early autumn, these pustules turn black and are full of overwintering spores that develop when temperatures are cool with high moisture or high relative humidity.
Anthracnose Mosaic
Rose Mosaic is caused by a complex of several viruses that infect the plant and cause highly characteristic symptoms on the leaves,including wavy or zigzag, yellowed lines, rings, spots, and/or mottled patterns, yellow watermarks, and blotches. Incurable, the viruses are spread by poor handling of plant material at the source and poor sanitation when pruning or grafting Roses.
Botrytis Blight
Often occurring during periods of cool, cloudy, humid weather, Botrytis blight forms gray-brown, fuzzy growth on all above-ground surfaces of infected plants, plus tan flecks or patches on flower petals and flower buds which may droop or never open.
Cankers (Brand and Common)
A fungal stem canker that infects Roses through wounds from unsanitized pruning, insect damage, and other types of mechanical injury. Cankers begin as small, yellow, or dark-reddish spots on the green stems and expand with brown or reddish-brown in the centers. The canker margin appears reddish-brown or purple or dark-brown.
Crown Gall
The only bacteria on this list, Crown Galls or swellings appear on the main stem at or slightly below the soil line at the crown of the Rose and even on the roots. Depending on the severity of the infection and where the galls appear, some Roses look stunted and can have sickly-looking foliage and flower production, while other Roses are killed.
Black Spot Prevention & Treatment
The above Rose issues and the fungal spores of Black Spot are all easily prevented with proper sterilization of garden tools, not working on Roses when their foliage is wet or during wet periods of weather and high humidity, and proper sanitization in the garden.
Black Spot spores, however, are primarily spread by wind, but these spores can’t germinate without the presence of water - whether that is in the form of dew, condensation, or moisture remaining on the leaves. These conditions, especially over long periods of time, are breeding grounds for Black Spot and other fungi.
Removing or treating infected plants or plant parts immediately helps keep the fungus from spreading further by wind. Keeping dropped leaves and other debris clear from the area around your garden, especially Roses helps too!
Good Air Circulation & Sunlight
Other methods of controlling Black Spot are reducing weed competition and opening up the area around your Rose so it has good air circulation. This helps dry the leaves faster and keeps them healthier. Overcrowding plants is another problem that can quickly lead to disease and pests.
Situating your Rose where it gets full sun all day, especially morning sun to dry the leaves quickly of dew. This moisture buildup from humidity and morning dew is where the fungal spores grow best, so limiting moisture on the leaves is vital. If you need to trim away a few shrubs or tree limbs to get more sun into the area, it will certainly help a lot!
Mulch & Good Watering Practices
While you can’t stop the rain, you can only water your Roses at the roots, or use underground or under mulch soaker hoses. Applying a 3-4 inch thick layer of clean mulch around the entire base of your Roses root system helps keep soil from splashing fungal spores and other issues onto the foliage and keeps your leaves looking cleaner and healthier. Not to mention all the other fantastic benefits mulch has to offer your plants. Water in the morning so any splashed leaves have a chance to dry up fast throughout the day.
Common Sense Garden Sanitation
One of the most vital prevention methods is buying quality Roses from a trusted source and having impeccable garden sanitation.
Sterilizing your pruners and other tools used around your Rose bushes, using clean soil and mulch, and removing dead foliage and leaf litter that collects in the interior and under your bushes all play an important role in preventing and limiting the spread of this fungus.
When wintering your Roses, only use clean and even sanitized insulation materials and clean dry leaves each year.
Bring In The Good Bugs
The spores can also be spread on the feet of visiting insects, while not intentionally, garden visitors inadvertently bring the fungal spores to you. However, killing every bug that dares touch your Rose is not the answer.
By encouraging or even introducing natural enemies (you can buy ladybugs and praying mantis!) you not only bring in an aggressive first line of control that helps everywhere in your landscape but also reduce chemical dependency and buildup in our environment. You can also use Diatomaceous earth and lures to organically help with pest control. Spray only to treat certain pests when they are present.
Fungicides
If chemical controls are used, they should be used only minimally and in a targeted manner. Otherwise, you can use a fungicide specifically labeled for the control of Rose Black Spot. Other organic control includes using, sulfur, neem oil, and copper.
Treatment Tips
From the moment you see even the smallest sign of Black Spot on your Rose bushes, unfortunately, all you can do now is stop the spread of infection to new leaves and stop it from infecting other Rose bushes in your landscape (and your neighbors too!).
- Treat early in the growing season, every growing season
- Apply a Dormant Spray over winter to kill overwintering spores
- Retreat according to product directions and after a heavy rain
- Spray the entire plant, including the undersides of the leaves
Black Spot Resistant Rose Varieties
As mentioned above, finding a trusted source for clean, disease and fungus-free Rose bushes with quality rootstock is essential for helping slow the spread of Black Spot! University studies have shown that the types of Roses with the most resistance to Black Spot are Floribundas, Landscaping Shrub Roses, and Climbing Roses, while Hybrid Tea Roses, Grandifloras, and Miniature Roses have varying degrees of susceptibility, but new and improved rootstock and breeding programs are being developed continuously.
Not only do we at Nature Hills pride ourselves on having in place common sense practices and sanitation to prevent the spread and shipment of infected plants, but we also use Plant Sentry™ to remain compliant with all state and federal agricultural laws and guidelines. Ensuring you receive the healthiest plants, delivered right to your door!
There are a wide variety of highly disease-resistant Rose bushes available to you that have these common diseases and Rose fungus built right in.
So don’t worry and plant happy with these tips and tricks to stop Rose issues in their tracks, and know what to do when an issue arises with the help of Nature Hills Nursery!
Happy Planting!
