A Better Solution to Mildew and Blackspot on Roses

There are gardeners in bits of the country who find that the air is always so dry and the sun so bright that any rose they plant will thrive and not be touched by any fungal disease. But not all of us can live where the air is dry.  After three years of living in the mountains of Arizona, where the relative humidity hovers around 20 percent through most of the growing season, going up to about 60 percent in July and August, I know that even here it is important to be effective at battling fungal disease.

Those of us who collect roses and have many dozens of cultivars will usually find that despite our diligence in choosing disease-resistant roses, a rose with some susceptibility has crept into our garden when our guard was down.  There are gardeners who meticulously and unsentimentally remove such problem roses from the garden when they fall prey to any fungal disease, regardless of how much those roses might please them otherwise. Not all of us have the discipline to discard a rose just because it suffers a bit from blackspot of mildew.  After all, sometimes roses are more prone to disease while they are settling in.  Or sometimes seasonal conditions or cultural practices one year contribute to the problem and under other conditions the rose will be fine.

At the other end of the spectrum are those who have the will to exert complete control over their gardens, keeping their roses from being even occasionally touched by pests or diseases.  They purchase the latest fungal protection sprays from Rosemania and they spray their roses every two weeks, using complex chemicals to kill fungus.  Not all of us have the discipline or the inclination to spray this frequently. And not all of us really want to use chemicals that we reasonably fear might hurt us or the environment.

It is possible to grow a lot of very nice roses wherever one lives by steering a middle course.  Especially if one has succeeded in cultivating disease-resistant plants in the garden, one can use spot treatments as needed when problems crop up. And one can use relatively safe chemicals to control rose fungus.

Baking Soda Solution: The simplest solution is to add 1 rounded tablespoon of baking soda to a gallon of water, then add 2 tablespoons of horticultural or vegetable oil and 3 drops of Dawn dishwashing soap to 1 gallon of water.  The oil acts as a thin barrier, making it more difficult for fungal spores to establish. The detergent helps the mixture stick to the rose, which is especially important when fighting powdery mildew since it tends not to wet very well – especially in advanced stages.   The solution works best if applied once or twice per week.  Slightly  stronger solutions can work up to a week, but they may tend to damage roses. Note, too,  that sodium build-up in the soil can be a serious threat to soil fertility. This is not a solution that you want to spray twice per week across a huge garden over a long growing season.

Sodium Acetate Solution:  This is another solution that uses kitchen food ingredients to kill fungus – namely baking soda and vinegar.  Mix 2 tablespoons of baking soda in a gallon of water, along with 2 tablespoons of vegetable oil  and 3 drops of Dawn dishwashing detergent to a gallon of water.  Then stir in 1 tablespoon of distilled vinegar.  The baking soda reacts with the acetic acid in the vinegar to produce sodium acetate which is sometimes used as a preservative in foods.  It has roughly the same advantages and disadvantages as the baking soda solution, though its chemical action is a little different.

Potassium Bicarbonate Solution:  As mild as the baking soda solution is, there is one solution that proves even gentler to the rose, potassium bicarbonate. Potassium bicarbonate is a chemical analog of baking soda. It is used in wine, club soda, and pharmaceutical products, so one can feel pretty safe about using it in the garden.  One can either purchase it as Green Cure or Kaligreen which are preparations that contain both detergents and potassium bicarbonate  so they can be simply added to water.  Or, if one has hundreds of roses,  one can buy the potassium bicarbonate in bulk and make a mixture that contains horticultural oil and soap that is just like the baking soda mixture: to a gallon of water, add 1 rounded tablespoon of potassium bicarbonate, 2 tablespoons of vegetable oil, and 3 drops of dishwashing liquid soap.

In this case, a small build-up of potassium in the soil actually works as a fertilizer element.  So this mix – which is a little more effective than sodium bicarbonate is also a little more environmentally friendly. One could spray it weekly all season long without much reason to fear wrecking the soil.

Serenade: Who would have thought that fungus was so perverse that a good serenade could drive it from the garden?  In this case it’s not a song; Serenade is a brand name for a bacterial control agent, bacillus subtillus.  It is also available as Plant Guardian.  One nice thing about this solution, is that it does not build up in the environment, so it is quite sustainable.  Another nice thing about using a bacterial control is that it tends to be a self-stable solution. That is, as fungi make genetic adaptations that give them an advantage, the bacteria also tend to make adaptations that neutralize those advantages.  As with any spray, be sure you wear breathing protection; inhaling these bacteria can induce cold-like symptoms.

It’s generally a good idea to begin spraying your garden at least with horticultural oil early in the season to get ahead of the curve a little.  And it can be helpful to use several of these solutions in rotation in the garden when conditions for fungal disease are favorable.  You may find that spot treatment or local treatment suffices if conditions are right.

Know your Fungus:  Knowing a bit about the conditions favorable for fungal infection can   help one be effective in fighting it. Four fungal infections are responsible for most fungal damage to roses: black spot, powdery mildew,  downy mildew, and rust. We will talk here about the two most common ones: powdery mildew and blackspot. Blackspot is most prevalent in the eastern US.  Powdery mildew is most prevalent in the western US.  In the west it is uncommon for blackspot to kill roses.  In the east it is uncommon for mildew to kill roses.  But blackspot definitely does kill roses east of the Mississippi, and mildew can kill roses in damp, foggy areas along the west coast.  So effective control of one or the other can be important.

Powdery Mildew:  The characteristic feature of powdery mildew is a white powder that spots or covers the surface of the leaves.  It can also cause leaves to curl up before there is much sign of white powder on the surface.  The mildew is most active between the temperatures of 60F and 80F, with eight or twelve hours above 90F sometimes being enough to kill the fungus. It thrives between 50% and 90% relative humidity, and is especially active at night.  Evidently, liquid water on the surface of leaves makes it difficult for the fungus to establish, so some advocate overhead watering.  If one does water roses where powdery mildew is a problem it is important to water before noon so that foliage is dry and there’s not a pocket of high humidity around the plant going into the evening.

Mildew tends to strike quickly growing young leaves first. Many roses that are susceptible to the disease will only have infections on new foliage. Similarly, shade tends to cause the disease to spread most easily.  One rose in my garden set a long cane this year, half of which was in sun, the other half in shade.  The half that was in shade was a mess. The half in sun was untouched by mildew.

Recommendations:

  • Plant mildew-resistant varieties if you live where mildew is a problem,
  • Remove especially mildew prone roses,
  • Do not use fast-release fertilizers that would cause growth spurts.  Be especially careful to avoid them when atmospheric conditions are most conducive to mildew,
  • Be doubly careful to plant mildew-susceptible plants where they get plenty of direct sunlight,
  • Water early in the morning, spraying off the surfaces of the leaves.

Black Spot   The characteristic sign of black spot is round, black spots on leaves. After infection, the leaf tends to yellow and die.  The disease tends to infect oldest leaves first, so it starts from the lowest leaves on the plant and works its way up.  As with powdery mildew it depends on high humidity.  Unlike powdery mildew, it tends to be most active when the temperature rises above 90F during the day and does not drop below about 80F at night.  Also, unlike powdery mildew, roses that tend to fall prey to this disease do so unconditionally – so long as the temperature and humidity levels are favorable.  Shade may exacerbate the problem.

Recommendations

  • If one lives where blackspot is likely to be a problem, be careful to plant only blackspot resistant varieties.
  • Remove blackspot-prone varieties.
  • Inspect daily. If one notices blackspot on any rose in the garden, it is useful to remove and destroy the leaves immediately, don’t throw them on the compost heap.
  • If one lives east of the Mississippi, one should at least spray all the hybrid tea roses bourbon roses, miniature roses, and other roses that are known to get blackspot.
  • Spray regularly when conditions are most conducive to blackspot.
  • Water early in the day so that foliage can dry out before nightfall.

The good news is that many roses that are susceptible to fungus are susceptible only to blackspot OR powdery mildew. I have grown about two hundred fifty cultivars and only Merveille de Lyon has shown susceptibility to both blackspot and powdery mildew.  And it appears to have survived its first encounter with them.  Cultivars with shiny leaves tend to be more resistant than dull-leafed cultivars.  But if you live where fungus is a problem always check to make sure a rose going into the garden is resistant to any diseases it is most likely to encounter.

It is possible to raise roses in most parts of the US without resorting to complex chemicals. By choosing cultivars well suited to the local environment one can almost eliminate the need for chemical interventions of all kinds.  When one grows a lot of roses in one place, though, it is almost inevitable that something will go wrong, and in such cases there are sensible interventions that use low-toxicity chemicals or biological controls.