Downy Mildew: How To Stop This Plant Disease Before It Starts
Many people confuse powdery mildew with downy mildew. However, powdery mildew is a completely different issue. Both are common problems in gardens.
The purpose of today’s lesson is to provide you with all you need to know about downy mildew. I’ll tell you how to treat it, but more importantly, I’ll teach you how to prevent it from happening again!
What Is Downy Mildew?
People who live in humid climates are likely familiar with downy mildew. With a preference for wet situations, this mold grows in soil and later colonizes the leaves of plants, causing severe damage.
There are multiple varieties of the Peronosporaceae family, including this downy mold, which belongs to them. They are classified as water molds.
It is also possible for Bremia and Basidiophora to cause problems for gardeners, particularly on ornamental plants. Peronospora and Plasmopara have a particular danger for gardeners.
It is not technically a fungus, but the downy mold family exhibits similar traits to fungi. However, these oomycete microbes do not produce any new cells after growth, which is why we treat them as fungi in the garden.
This fungus grows on the leaves and in the roots of plants, consuming the plant’s water supply. Although small amounts of downy mildew are not harmful to your garden, they are a sign that a larger issue is occurring with water.
A variety of foodstuffs as well as some flowers and shrubs are susceptible to this fungus.
It rarely injures more than 25% of a crop field in agricultural surroundings, but in the home garden it can be quite destructive. Get it out of the garden before it can spread too widely!
Life Cycle of Downy Mildew
Fungal spores in the soil are what start downy mildew. The spores can appear almost anywhere, due to their windblown nature.
The soil splashed onto the leaves of your plants can help move the spore onto the leaves. Once there, the spore latches onto the underside of the leaves. It penetrates the leaf’s surface with its mycelia and begins to grow.
It causes spots on the leaves as it grows, and a mat of sporangia appears beneath the leaf, ranging from a pale yellow to a dark grey or even tan hue. These spores eventually release spores that spread throughout your yard.
It is possible for spores to colonize leaves if the conditions are right. However, they only survive for a short amount of time.
Infected soil may remain infected for five years if moist plant debris isn’t removed. Spores overwinter in the soil and can reappear when soil adheres to leaves.
It spreads through the plant’s stems, as well as its mycelia, which look like fungi. Fresh growth may already be infected if it has had problems with the mildew in the past.
Peronospora’s mycelia can infect plants in such a way that they produce damaged fruit and show signs of distorted growth.
IDuring hops cultivation, cones can become colonized, effectively killing them before they fully develop.
If cucumbers aren’t grown on a trellis, they can suffer from the same problem with young fruit.
Keep an eye out for damage in areas with a history of downy mildew, especially in places where it has been a problem in the past!
Symptoms of Downy Mildew
The symptoms of downy mildew have been discussed in broad terms, but let’s take a closer look.
Early Downy Mildew Detection
The underside of the leaf is initially covered with a colony of mildew. The underside of the leaf may appear whitish, purplish, blue, or bluish-gray.
The top of the leaf will yellow in spots when the sporangia have locked into the leaf structure. The sporangia themselves look like a white or blueish-white spore as they begin to generate new spores.
In addition to colonizing the leaf, downy mildew can produce sporangia that are fuzzy in appearance.
Later Downy Mildew Progression
The mildew sporangia turn grey and powdery as the leaf spots die, and the spores fall in the wind to infect other leaves. Branches will be distorted or die off entirely if the spores aren’t contained.
When moisture conditions are right (85% or higher) at the soil’s surface, the process can be accelerated. New spores appear 7-10 days after infection, but can begin forming as early as 4 days.
A plant may die if untreated or constantly reinfected. However, the more likely scenario is for the plant to become colonized by mycelia.
Consequently, the plant can sometimes be a danger to others in the garden if an outbreak occurs again. However, effective deterrent measures reduce the likelihood of that happening. Control measures may be able to eradicate the infection.
Controlling Downy Mildew
Plant diseases like this can be tricky to control, and we are still learning quite a bit about them.
Downy Mildew Treatment
In order to prevent downy mildew from destroying other parts of the garden, it is essential to strike quickly once the mildew has begun to colonize a plant.
The essential oil of the neem tree is essential for fighting off downy mildew, but won’t affect the disease once it has colonized a plant.
It is common to use copper-based fungicides throughout the Pacific Northwest as a preventative and a fungicide, while treatments based on phosphorous acid have become increasingly popular.
Downy mildew can be combated with chemical treatments, but these are often hard to obtain for the typical home gardener. These are generally used for large-scale agriculture, and could be hazardous without proper safety precautions.
Home gardeners typically have access to only a few chemical versions, including chlorothalonil fungicides. This can work, so if none of the organic options work, you might be able to use it.
Prevention is the best approach to treating downy mildew.
Preventing Downy Mildew
In order to prevent downy mildew, you need to pay attention to your watering habits.
You should only water at the beginning of the day so that your plants have enough time to dry during the day.
If you notice any visible damaged leaves, remove them. This will help to prevent further spread into the plant’s system. Make sure to prune the plants to increase air circulation, as this also helps wet leaves to dry.
Make sure plants are staked or secure so air can circulate around all surfaces. If using a cage, thin out branches so you have good air circulation. This is crucial for plants like tomatoes, which may form dense growth.
Use of a soaker hose instead of a spray nozzle will reduce the amount of soil splashed onto your plants’ foliage, preventing any damage to the leaves.
By increasing airflow in a greenhouse, you will be able to prevent problems with powdery mildew and certain pests, such as whiteflies, and reduce humidity. In recent years, many plant varieties have been developed that are resistant to downy mildew. If possible, choose those varieties to avoid the problem altogether.
Maintain your plant’s bottom area as clean and debris-free as possible. You can mulch if necessary, but try to avoid accumulating fallen leaves which may harbor spores.
It is a good idea to remove weeds as soon as they appear in order to minimize airflow issues at the base of your plants. Removing weeds as they appear in subsequent years also reduces their appearance.
Finally, you should remove severely diseased plants from your garden and dispose of them to prevent them spreading to the rest of your garden.
Even though no one wants to lose a prized plant, the long-term effects of downy mildew in the garden can be devastating to your other plants. It is better to remove diseased plants to ensure others do not get it.
It all adds up to a good garden that’s maintained regularly and is unlikely to suffer from severe downy mildew problems.
During the warmer months, it is far less likely to appear, giving you the break you need to recuperate from the colder months!