Get It Growing: Fungal disease causes and treatment

Fungi are everywhere; on our bodies, on our food, on our plants and in the air we breathe. In the garden, we encounter fungi on vegetables, soft fruit and orchard fruit as well as some ornamentals. To understand why we get these infections we need to look at the life cycle of a fungus.

Fungi generally reproduce by producing spores. Spores can be circulated by air, raindrop impact, weed transmission, infected soils and aerosol formation.

Spores such as those responsible for powdery mildew or apple scab tend to land on leaf surfaces where, given the right conditions of humidity and temperature, they germinate much as a seed does. The spore sends out a root-like hypha which then gains access to the interior of the leaf through the stomatal pores used for gas exchange. Once inside, it drills into cells and extracts the nutrients it requires.

Finally, the fungal network produces fruiting bodies — these are the “fuzz,” or surface eruptions, we see in diseased plants.

These then supply spores for the next generation.

Fungal diseases usually show up as circular areas of affected tissues, often with colored halos around them but some varieties of mildew do not. As an example, roses affected with black spot do show the characteristic circular infection sites.

The description of the life cycle above gives us valuable insight in how to negate or at least control fungal attack on our plants. Some of the methods to reduce disease are cultural, good growing practices, but most involve the use of fungicides.

So what is fungicide? A fungicide is a treatment, frequently chemical but biological products are also available, which will stop the attachment, growth or development of fungal spores which lead to disease.

The single most important thing to know about fungicides is that they must be applied prior to development of disease. Once disease is established, fungicides offer only poor disease control at best. This is why many fruit trees get sprayed while dormant and no leaf surfaces are exposed to spores. Literature or knowledge from past years will determine when to spray.

There are different agents applied for different reasons. Some stop attachment of the spore to the leaf, some suffocate the spore before it can germinate while others chemically inhibit germination and growth of the spore.

The most frequently used fungicides are the horticultural or “dormant” oil sprays. They suffocate the spore by coating it to stop the gas exchange needed for germination.

Many herb-derived oils such as clove, peppermint, thyme, rosemary and sesame work in the same way. All of these treatments are typically applied before bud burst.

“Lime sulfur” is another popular fungicide but is caustic and very smelly — it prevents germination as do neem oil sprays. Mineral oil, kaolin clay and canola oil may work differently by coating the leaf surface, blocking spore attachment and suppressing growth.

Organic control using approved materials such as neem oil, Serenade sprays (B. subtilis, a bacterium), bicarbonate and some copper- based sprays can be very effective.

Cultural methods can also reduce disease incidence. Fungal disease frequently occurs during cool, damp weather but a few have developed to attack in dry, warm weather such as squash mildews.

Avoid over-watering to help suppress disease and space your plants to allow good air circulation.

Avoid overhead watering as drip irrigation lessens the amount of fungal spread.

Another important control method is to immediately remove any infected fruit or diseased leaf tissue to minimize disease spread.

The most commonly used chemical fungicides include sulfur (as a dust or in lime sulfur), chlorothalonil, captan and myclobutanil. The latter two are frequently used in the home orchard to control mildew and scab.

When using chemicals such as the above, follow the directions for application rate on the fungicide container, wear the recommended protective clothing and follow any recommendation for delaying harvest after spraying.

Use fungicides with caution! Do not overuse or drench the soil under the tree or around the affected vegetable. At the roots of plants, fungi called “mycorrhizae” are essential to plant growth and over-use of fungicides may impact them adversely.

Publications outlining fungicide usage are available as free downloads from university extensions: “Managing Diseases and Insects in Home Orchards” publication EC 631 at and “Organic Fungicides for the Home Garden” publication FS128E at

Bob Cain is a veteran Clallam County Master Gardener.

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