Horticultural Blights and Plagues

When a plant dies in a garden or nursery, my clients are often concerned that it will spread to all other plants in the area.

And this can happen in certain conditions. For example, the fungus Botrytis can blight many types of bedding plants in a greenhouse if the humidity is high and the leaves are staying wet.

Thankfully, indiscriminate blights are not usually a problem. Plant pathogens tend to be more host-specific. Take a look at this propagation greenhouse:Notice first that this greenhouse is well-maintained. There are no weeds, the plants are all the same age and it is organized. This is a good growing environment.

The Oakleaf Hydrangeas (on the right) have collapsed and are infected with a fungal pathogen.

It is significant that the Hydrangea macrophylla crop (with bright green leaves) is very healthy. They are growing right next to the diseased crop of Oakleaf Hydrangea (Hydrangea quercifolia) and yet are showing no signs of disease.

WHAT DOES THIS MEAN?
The take-home message is that if you are having a disease problem in your garden or nursery, don't fear that it will destroy everything.

If a large area of your garden, nursery or greenhouse is in decline, have the problem properly diagnosed. Then ask the following questions:

• Are the plants being over- or under-watered?
• Does the soil drain properly?
• Are the plants in the right amount of sunlight?
• What is the soil pH?
• Is there too much mulch around the bases of the plants or are they planted too deep?
  
• Is the irrigation water salty?

A fungus or bacterium may be the primary problem. But in my experience, if many different species of plants are affected, the growing conditions are generally to blame.