In last week’s article, I wrote about a couple of insect problems that I have been seeing in many gardens at this time.

This week, I would like to write about a couple of disease problems that you may be encountering in your own gardens as we head into the summer months and how best to treat them and/or how to prevent them from destroying your plants.

Over the past two to three years, boxwoods have been bombarded with several problems. If your boxwoods are not being attacked by the box tree moth (see my article from June 22 for a description), they could be suffering from boxwood blight.

Boxwood blight is caused by a pathogen called Calonectria pseudonaviculata. This fungal disease can cause widespread leaf drop and can eventually kill the plant.

This particular blight is also known to affect the Japanese plant pachysandra. The disease first presents itself on the leaves as circular lesions with dark brown edges and black striping on the leaf stem.

As the disease progresses the leaves will turn straw colour and drop off. The fungal spores can remain viable on fallen leaves for up to five years, so it is important to clean up any dead, fallen leaves to help prevent the spread of the disease.

The sticky fungal spores can be spread by wind, rain, birds, insects, animals, gardening tools or even on clothing such as shoes, making the transmission of this disease very easy. The fungus thrives in warm, humid weather.

There is no known spray that will stop the disease, but spraying unaffected foliage with a fungicide containing chlorothalonil every 10 to 14 days will help the spread of the disease. You can also contact your local tree care services.

Make sure that you sanitize your pruners with a bleach and water solution (one part bleach to nine parts water) after every time you prune your boxwoods as a precaution.  

Powdery mildew is another fungus that appears on the leaves of a wide variety of plants from annuals and perennials to shrubs and even shade trees.

It first makes its appearance with small patches of white film on the top of the leaves that can be wiped off with your finger. As the fungus spreads, it can cover the entire leaf surface.

As it continues to progress, the leaves will often turn yellow and then brown. Powdery mildew will slow down a plant’s growth and affect the overall health of the plant. In severe cases, it can even kill the plant.

The fungus is made up of spores that fly through the air due to wind as well as splashing water that land on other plants, making it highly contagious.

Unlike some fungi that require moisture to thrive, powdery mildew usually occurs after a long, dry period as we experienced several weeks ago.

Other factors that lead to the likelihood of mildew are poor air circulation around the plants and when leaves go into the evening with water on them.

The best way to keep powdery mildew from becoming a problem is by carefully monitoring your plants on a regular basis.

As soon as you start noticing some leaves with white spots on them, pinch those leaves off and discard them – do not compost them.

Also, try to improve the air circulation around and through plants by thinning out crossing branches. Keep plants that are susceptible to powdery mildew evenly watered especially during dry periods. Don’t let the soil dry out for a long period of time.

You can also use a garden fungicide spray every 10 to 14 days to stop it from spreading. Please note that with most fungicides and insecticides, you should not be using them when temperatures are over 25 C.

When shopping for new plants, look for varieties that are more resistant to powdery mildew.

Some plants that are more susceptible to powdery mildew are peonies, summer phlox, ninebark, gerbera daisies, zucchini, melons, and Norway maple.

These methods of treatment can also be applied to other fungal leaf issues such as black spot fungus.

Joanne Young is a Niagara-on-the-Lake garden expert and coach. See her website at joanneyoung.ca.