Controlling Blackspot Without Toxic Chemical Sprays

Blackspot.

The very name of it evokes feelings of gloom and doom born from a sense of frustration and helplessness.  It's the nemesis of every rose gardener I've ever met. 

A week before a garden tour or rose show, the presence of even a half dozen classically spotted leaves on a top performer can spark panic in the heart of  the most stalwart rose gardener.  Once you see the telltale signs on one rosebush, you need to act quickly or the unsightly infection can rapidly spread throughout your entire rosebed.

For newcomers to rose gardening, blackspot is a fairly ubiquitous fungus that presents as round to irregular black spots on the leaves and canes of roses. 

The spots can be so numerous, they can coalesce into irregular black patches and they are eventually followed by yellowing and death of the leaf.  Left to its own devices, the fungus can defoliate a rose shrub in a matter of days to weeks, depending on the weather and severity of the infection.

Blackspot is not usually lethal in and of itself, but it can severely weaken a rose to the point where it won't survive unfavorable weather (protracted drought conditions, for example) or a very harsh winter.  Since it affects the leaves, plants that struggle to produce adequate food can't produce the same number of buds and those it does produce are often smaller and not the best form. The telltale affected leaves are an unsightly blight in the garden and the infection can spread from roses to other garden plants as well.

Caused by the fungus Diplocarpon rosae, blackspot strikes when there has been an extended period of rainy, damp, or very humid weather, especially when temperatures hover between 65-75 degrees Fahrenheit -  typical New England weather in the spring and early summer.  

Once summer temperatures hit the mid-80's with low humidity and little or no precipitation, blackspot will not be as much of a  problem unless you help it along by watering your rosebed at  dusk, when the leaves may remain moist for several hours and evening temperatures fall as the sun sets.

Because the spores can travel from a neighboring garden and linger in the environment, unless preventive measures have been taken, a few days of ideal weather conditions will bring a new wave of infection to your rosebeds. It takes a mere seven hours in a favorable environment for the spores to germinate.  More than one gardener has set his sprinklers to run after dinner and found the beginnings of blackspot in time for breakfast.

Prolonged periods of rainy or very humid weather can quickly turn a gorgeous, infection-free rosebed into a spotted, defoliating nightmare even in the best of circumstances.  For this reason, many rose growers routinely treat with fungicides to prevent infections when the weather turns favorable for the fungus to thrive.

In both of my former rose gardens, I  used rose dust and sprays and enjoyed the pest-free benefits of both topical and systemic chemical applications. But as anyone who has a water feature with live fish knows, fish are exceedingly sensitive to chemicals and chemical spraying is simply not an option around a koi pond.

Many experienced rosarians will tell you that spraying with chemical fungicides on a strict schedule is as essential as water to maintain healthy roses. Statements like that distress me and if I had let myself be swayed by that myth, we would never have even tried to grow roses.

In addition to our koi pond and water garden, our property abuts a conservation area consisting of over 55 acres of open land, much of which is considered wetlands.

We take our responsibility to the environment very seriously. Toxic sprays would harm not just our fish but would be a hazard to that ecosystem and to the bees who visit us daily from another neighbor's apiary as well. 

For those naysayers who have said to us, "Sure you can spray that around fish. Just don't do it on a windy day. And don't spray in the direction of the water." I say, try it in your yard, but we are staying away from chemicals here. (We actually had one person suggest that we just cover the pond with a tarp while we spray.)

Despite what you may have read, I'm here to tell you that while blackspot is definitely a scourge, there are safe and effective alternatives to toxic chemical sprays. We are successfully raising more than 180 roses representing over 75 different varieties without chemicals. If you have young children,  pets, or a water garden with fish, don't let the specter of toxic sprays deter you from the joy of growing and enjoying roses!

Here are the key elements we utilize for a chemical-free, healthy rose garden.

1. Good gardening practices and grooming of roses and rosebeds.

Healthy roses are better able to fight off the effects of pests and diseases and recover from infections when they occur.

You can minimize the incidence of infection in your gardens if you keep your roses in optimal health by regularly feeding, watering, and pruning them.  Following our initial spring pruning, feeding, and weeding, we spread a layer of compost and manure and then top off the beds with compostable black mulch for weed suppression and moisture retention.

Cleaning up any damaged or infected leaf litter goes a long way toward helping keep fungal infections like blackspot from spreading through a garden.

Choosing varieties of roses that are known to be naturally resistant to blackspot can also help, but resistant does not mean immune and even the highly resistant Knock-outs can develop blackspot in the "right" (or wrong) circumstances.

2. Apply cracked corn to beds prior to mulching in the spring and fall.

Cracked corn is an inexpensive source of the beneficial fungus Trichoderma which research  has shown to be an effective fungal biopesticide.  We can attest to its efficacy. We have been treating our rosebeds with cracked corn since 2005 and have noticed a dramatic decrease in the incidence of blackspot when we put it down in early spring compared with those years when we either didn't use it at all or put it down much later in the season.

To apply, sprinkle liberally over the rose bed and cover with 3-4  inches of compost and/or mulch. An occasional kernal may sprout if the mulch is applied too thinly.  We repeat the process when applying mulch in the fall.

Although this somewhat duplicates Step 3 (below) as it is a method of applying Trichoderma, a two-pronged approach also maintains a reservoir in the soil to fight the fungus both in the environment and on the rose.  We have also heard anecdotally from others who use this that over time, it can also help with weed control, although this benefit reportedly takes 3-4 years to manifest.

3. Spray with a biological fungicide  for added protection by Trichoderma.

We began spraying with Rootshield, a patented formulation of Trichoderma in a wettable powder in the late summer of 2009. We used it last year for the entire growing season without the concomitant use of cracked corn. The results were dramatic and more effective than what we previously saw with cracked corn alone.

This year we began the season by applying cracked corn, intending to combine both sources of this fungal biopesticide to maximize the benefit of both soil and foliar applications. 

Our usual practice is to spray with Rootshield WP, a product manufactured by BioWorks, Inc.  We spray a solution of 1 Tablespoon in one  gallon of water in early May and again in September,  and a solution of 1/2 Tablespoon in a gallon solution in midsummer (July).

4. Regular spraying with a solution of soap, oil, bicarbonate, garlic oil, and mint and citrus teas.

Spraying regularly with our home made solution of peppermint or spearmint tea, canola oil, homemade garlic oil, clear soap, and other all-natural insecticidal agents helps to control not only blackspot but powdery mildew and insect pests as well. (You can see our entire protocol HERE.) 

Because of our koi, we can't use pyrethrums in our garden either. For us, weekly spraying with either a garlic oil or mint based solution that includes baking soda adequately treats such pests as thrips, aphids, and spider mites and keeps a lid on powdery mildew as well. 

 5. Treat signs of blackspot promptly.

Preventing blackspot is much easier than treating it. We do our best to stay ahead of it, a daunting task when you have as rainy a spring as we have had this year.

This became even more of a nightmare for us recently when we bought several new roses from two local nurseries only to find them full of blackspot just a few days after planting them.  Even more troubling was finding that two of the worst offenders were 'Gemini' and a 'Double Knock-out' which are known to be some of the most blackspot resistant roses available.

The hygiene and care these roses received at the nursery was clearly substandard but we should have examined the plants more carefully before bringing them home and setting them into the rose beds.

Once you discover blackspot, your best recourse is to act quickly to remove all infected foliage and follow with another application of a general garden spray (I include both garlic oil and peppermint) with baking soda and Rootshield.

If you catch the infection early, you can simply prune off the affected leaves or branches but a severe infection may require pruning all the way back to the lower portions of the main canes. The canes on the right show signs of infection and we'll watch those carefully and likely prune them off as well, as the rose begins to recover.

After clipping and pruning away infected foliage and canes, every bit of leaf litter must be collected and disposed of, but not into your compost pile, where it can travel back into your rose garden on a windy day or remain dormant and ready to reinfect when you spread compost on your beds. 

We dispose of any infected plant matter with our non-compostable, non-recyclable trash. As an added precaution, after pruning and attending to infected roses, I clean my gloves and tools with a dilute solution of household bleach.

The capacity for roses to recover is remarkable if care is taken to catch and treat infection with blackspot early on and to treat the rose supportively as it is recovering and refoliating.  But an aggressive spraying routine with non-toxic, environment-friendly sprays can effectively prevent blackspot from developing even when extremes of weather make a fungal epidemic a foregone conclusion.

Author's Note:  BioWorks can provide you with a list of distributors of their products in your area. Contact them via the on-line form on their website. If you live in New England, Rootshield WP is available at Grriffin Greenhouse & Nursery Supplies in Tewksbury, MA