Managing Leaf Diseases

leaf disease on a rose

One drawback to living in a maritime climate, like the northwest, is that all of the extra moisture in the air encourages fungus and bacterial issues with our plants.  More specifically the foliage becomes susceptible to infections, such as black spot on roses, mildew on lupine, rust on hollyhocks and blossom and twig blight on cherries.  Currently all around town is a fungus called Sirococcus conigenus that occurs on blue Atlas Cedars.  What on earth is a gardener to do when this happens in their garden?


Controlling fungus and bacterial diseases are a huge challenge for most of us.  By the time we realize there is a problem, it is too late to stop it.  Take for example the Atlas Cedars that are just hammered right now with Sirococcus (which causes the needles on the new growth to turn a tan color and eventually fall off).  The time to control this disease would have been when the new growth was just emerging, about a month ago, before we saw any signs of trouble.  The same is true with blossom and twig blight on cherries.  The time to apply a fungicide would have been before, during and immediately after the tree bloomed (most often taking three applications of copper to manage this problem).  Timing is everything when it comes to managing diseases and prevention is the key to success.  Keeping a garden journal can help us remember to plan ahead for the next season.  


Here are several things we can do to minimize foliar diseases in our gardens.  

First and foremost, plant resistant varieties.  Second, do not crowd plants.  There should be good air circulation in and around the foliage (pruning will help here as well).  Third, plant the right plant in the right place (putting a sun loving plant in too much shade is a recipe for disaster).  Fourth, practice good hygiene.  Get rid of infected leaves by stripping them off and raking them up underneath plants.  Applying a fresh layer of compost over the soils every year will do wonders to minimizing diseases.  Fifth, treat the infected plants with a fungicide, either natural or synthetic depending on your preferences.  Synthetics tend to last longer and are often systemic, which means that they are absorbed into the plant’s vascular systems and provide more thorough protection.  Most natural products work only on contact and need to be re-applied weekly to be effective.  Again, the key to applying fungicides is to get them on before you see problems.  Sixth, do not expect perfection.  Some degree of foliar damage is normal and perfectly acceptable. Often, as the weather improves, a plant will grow out of the condition and the new growth will be just fine.  Seventh, be willing to do some serious pruning.  I have taken roses that were smothered with mildew, stripped off all the leaves, pruned them half way back to the ground, fertilized and mulched them and in 6 weeks they were in full bloom and gorgeous.


Pruning can work miracles, which is my cue to remind you that this coming Saturday, 10 a.m. here at the nursery, we are having our Summer Pruning class - which you won’t want to miss.  This is your chance to fix all the “screw-ups” from the last time you tried to prune your garden and to address the problems that Mother Nature heaped on us from this lousy wet spring.  Summer pruning is critical to a healthy garden and you can learn all about it this Saturday.  Hope to see you there.