Gone Gardenin’: Mildew doesn’t make for a pretty garden

Hey there, time traveller!
This article was published 01/09/2016 (3377 days ago), so information in it may no longer be current.

During the growing season this year, we have had an unusually high number of days with moderate temperatures and very high humidity. These weather conditions, coupled with many calm days with little air movement, have created the ideal conditions for the development of mildew on many of our garden plants.

Mildew is one of the most widespread and easily recognizable fungal diseases, and it can attack members of almost any plant family; few plants are immune from mildew.

It is recognized by the development of white or grey powdery spots or patches on leaf surfaces, and eventually, if unchecked, will cover entire leaf surfaces as well as stems and flowers.

Mildew is host specific, meaning that different species of mildew attack specific plant families. This is an advantage as the disease does not spread from plant to plant in the garden. For example, mildew on lilac leaves will not spread to neighbouring rose bushes as they are different plant families attacked by different mildew species.

Although mildew is rarely fatal — in fact, some plants, such as common lilac, get mildew every year with no apparent effect on the overall health of the plants — it does detract from the attractiveness of plants. It causes foliage to curl, turn yellow and brown, and the plants will prematurely defoliate.

Flowers will be deformed, bud drop will occur, and new growth, which is especially susceptible to mildew attack, will wither and die. New plant growth may become disfigured and distorted.

Some plants are more susceptible to mildew than others. Lilacs and Virginia creeper are two shrubs that get mildew almost every year. In the vegetable garden, vines such as cucumbers and zucchini, as well as peas, seem to be the least immune to infection.

Several perennials, notably delphiniums, perennial phlox, monarda, and roses are more likely than not to get mildew. Annual flowers are generally less likely to get mildew, although begonias and other shade-loving annuals can be prone to attack.

What can be done to prevent mildew from invading the garden? Remembering the conditions that favour its development — high humidity, moderate temperatures and poor air circulation — will provide some clues as to what might be done to cut down the chances of mildew attack.

Although we cannot do much about high humidity or outdoor temperatures, we can increase air circulation in our gardens. We might also resist the temptation to crowd plants together, thus allowing more air movement between individual plants.

Minimizing the amount of shade in the garden by trimming branches to allow more sun — and air — into all parts of the garden will make it less prone to mildew attack.

Many gardeners like to feed their plants but new growth is particularly susceptible to mildew so over-fertilizing is not a good idea. Using a slow release fertilizer rather than a high nitrogen fertilizer in one big dose will make plants less susceptible to mildew.

One other strategy is to select mildew-resistant cultivars. There are monarda, lilac, and perennial phlox varieties on the market that are resistant to mildew.

What can be done if mildew is already present in the garden? Firstly, early detection is important and damage from mildew cannot be undone.

Some remedies do not work as effectively if the disease is too far advanced. Fungicides are the best defence, and these can be either homemade remedies or commercial products.

Homemade fungicides usually have baking soda or potassium bicarbonate in the ingredients, while commercial fungicides are often copper based, with the main ingredient being copper sulphate — these are the same products that many gardeners use to combat blight on tomato plants. A regular — usually weekly — spray program will be necessary.

Although some people might think that spraying plants with water would be counter-productive in preventing mildew, that is not the case. Mildew spores are most likely to attach themselves to dry leaf surfaces so a strong spray of water will dislodge fungus spores from the leaf surfaces if they have not had time to get established.

Because high humidity is mildew’s friend, this spraying should be performed early in the day to allow the excess moisture to evaporate and dissipate from around the plants. Because it doesn’t take mildew spores long to set up shop on leaf surfaces, this control method will have limited success.

On some plants, such as lilacs, you might just let nature take its course. On many of your favourite ornamentals, however, you will be looking for a line of defence against this most pervasive and unsightly fungal disease so that you can enjoy the fruits of your labour and not let mildew ruin the appearance of your precious garden plants.

Albert Parsons lives, writes, and gardens in Minnedosa.

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