Tips For The Control Of Downy Mildew

Tips For The Control Of Downy Mildew

A common but under diagnosed problem in the spring garden is a disease called downy mildew. This disease can damage or stunt plants and is difficult to diagnose. But, if you are familiar with the different ways this disease presents itself and with the conditions it which is can grow in, you will be better able to take steps to control downy mildew in your garden.

What is Downy Mildew?

Often times, when gardeners hear the name downy mildew, they think this disease is related to another common garden disease called powdery mildew. While the two have very similar names, they are two very different diseases.

Downy mildew is caused mostly by organisms that belong to either the Peronospora or Plasmopara genus. While powdery mildew is cause by a true fungus, downy mildew is cause by parasitic organisms that are more closely related to algae.

Because it is closely related to algae, downy mildew needs water to survive and spread. It also needs cooler temperatures. You are most likely to see downy mildew in your plants in the spring, where rainfall is frequent and temperatures stay cool.

Symptoms of Downy Mildew

One of the tricky things about downy mildew is that it can appear different ways, depending on what kinds of plants it is infecting. Most often, an infection of downy mildew will also include a fuzzy, soft looking growth that can be white, grey, brown or purple. This growth is most commonly seen on the lower leaves of the plant. This growth is where this disease gets its name from, due to its downy appearance.

Other common symptoms for downy mildew include mottling or spots on the leaves. The spotting will be yellow, light green, brown, black or purple. In some cases, the mottling may look like chlorosis.

Plants that are affected by downy mildew, may be stunted or have leaf loss.

Controlling Downy Mildew

The best control of downy mildew is to make sure that your plants do not get it in the first place. Because downy mildew needs water to survive, the very best thing you can do to prevent downy mildew is to water your plants from below. Water that sits on the leaves of the plant gives the downy mildew a way to infect and spread on the plant. The spore of downy mildews spreads by literally swimming through water until they come across live plant material to infect. If there is no water on your plant leaves, the downy mildew cannot travel to or infect your plants.

Good garden hygiene is also crucial to stopping downy mildew from developing in your garden. This disease overwinters on dead plant material, so removing dead plant material from your garden in the fall will help prevent the disease in the following spring.

If your plants become infected with downy mildew, the organic control of downy mildew is your best bet. The reason is that once a plant is infected with downy mildew, there are no effective chemical controls, though if you have a reoccurring problem with downy mildew, there are some preventative chemicals you can use. Downy mildew is not a fungus, so fungicides will not work on it.

Once your plants have downy mildew, the best thing you can do is to try to eliminate moisture and humidity around the plants. As mentioned, make sure your are watering from below. If possible, try to improve air circulation through selective pruning. In enclosed environments, like in the house or in a greenhouse, reducing the humidity will help as well.

Regardless of what you do, downy mildew normally clears itself up in the outdoor garden once the weather warms up, as this disease does not survive well in warm temperatures. If your plants only have a mild case of downy mildew, your best option may be to simply wait for warmer weather.

Downy mildew diseases are caused by oomycetes or water molds. They are fungus-like, but more closely related to algae.  

There are 2 types of downy mildew spores. One type, zoospores, can be splashed up by water or spread by the wind. The other type, oospores, reside inside the plant tissue and can spread rapidly and over-winter. At this point, there is no evidence that this particular mildew affecting impatiens is doing that, but why take chances.

Downy mildew is more prevalent in the spring and fall when the cool, wet or humid weather provides ideal conditions.

What is Downy Mildew?

A lot of people may confuse this disease with powdery mildew. Although the names are almost similar, it is important to note that the two are different in more ways than one. One of the major differences between the two is that true fungus causes powdery mildew. In the case of downy mildew, on the other hand, the cause is not true fungi but parasitic organisms. The main culprit for downy mildew are microorganisms that belong to the genus of Peronospora or Plasmopara.

It is a foliage disease that can easily spread from one plant to another through airborne spores. Insects and rain can also be potential carriers of the disease, as well as garden tools that are contaminated. As a wet weather disease, the infection will easily spread during the times wherein plants have wet leaves. Moderate temperatures and high humidity are some of the external conditions that will be favorable for downy mildew. The spores will be more active in production if the temperature is cooler than 65 degrees Fahrenheit. Most of the time, the damages are only on the physical appearance, but in severe cases, it can escalate into a bigger problem, especially for commercial growers.

The damages are only on the physical appearance

Marian Lund, UW-Madison Plant Pathology

What is basil downy mildew? Basil downy mildew is a devastating disease that affects the leaves, branches, and stems of many types of basil (i.e., plants in the genus Ocimum) commonly used for cooking. Green-leafed varieties of sweet basil are particularly susceptible to the disease, while purple-leafed varieties of basil, Thai basil, lemon basil, and spice basil are less susceptible. Certain ornamental basils (e.g., hoary basil) appear to be highly resistant to the disease. Basil downy mildew was first reported in the United States in 2007 and has since spread widely to wherever basil is grown, including Wisconsin.

What does basil downy mildew look like? Symptoms of basil downy mildew typically develop first on lower leaves, but eventually an entire plant will show symptoms. Initial symptoms include leaf yellowing (which gardeners often think is due to a nitrogen deficiency) followed by leaf browning. Affected leaves also curl and wilt, and on the undersides of the leaves, a gray-purple fuzzy material will develop.

Where does basil downy mildew come from? Basil downy mildew is caused by the fungus-like organism, Peronospora belbahrii. This pathogen can be easily introduced into a garden each year via contaminated seed, on infected transplants, or via wind-borne spores (technically called sporangia). Once introduced into a garden the pathogen can spread by wind, by rain splash, or via items (e.g., hands, clothing, garden tools) that come into contact with infected plant and then are used to work with healthy plants. The pathogen thrives in humid, warm environments and can spread rapidly, decimating an entire basil crop.

How do I save plants with basil downy mildew? There is no known cure for basil downy mildew. If you see basil downy mildew, harvest any asymptomatic leaves on infected plants, as well as other healthy basil plants in your garden. Use these materials immediately (e.g., to make pesto). Remove and bag any symptomatic plant remains and dispose of this material in your garbage.

How do I avoid problems with basil downy mildew in the future? Avoid planting sweet basil if possible. Instead, plant other types of basil that are more resistant to basil downy mildew. If you decide to grow sweet basil, try growing the variety ‘Eleonora’ which has been bred for at least some resistance to the disease. If you grow basil from seed, check to see if the seed you are buying has been steam-treated to kill the downy mildew pathogen. Be aware however, that this information may be difficult to find, because steam treatment of basil seed is relatively new and the use of this technique is not widely advertised (at least to home gardeners).

Whatever type of basil you choose, try to grow your plants in a manner that will keep them as dry as possible, thus creating an environment that is less favorable for the downy mildew pathogen to develop and infect. Plant basil in a sunny location, space plants as far apart as possible and orient rows in the direction of prevailing winds to promote good airflow and rapid drying of plants when they get wet. Avoid overhead watering (e.g., with a sprinkler) that will wet leaves and spread the pathogen instead, use a drip or soaker hose to water.

Use of fungicide treatments to control basil downy mildew is NOT recommended. Products that currently are available to homeowners, even when applied in the best manner possible, will likely not control the disease adequately, if at all. Thus using these products would be a waste of time, effort and money.

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Downy mildew

Downy mildews are caused by fungus-like organisms that attack the leaves and other above-ground parts of plants. There are several different types and most are specific to 1 group or 1 type of plant.


Downy mildews, unlike powdery mildew that needs warm, dry conditions, thrive in cool, moist, damp conditions and loves young and unhealthy plants, or those under stress.

Some downy mildew simply cause leaves and stems to turn yellow, while others can prevent plants from flowering. Onion downy mildew, on the other hand, damages the foliage and the bulbs, resulting in loss of yield or even complete crop failure.

Plants affected by downy mildews include brassicas, Impatiens, lettuce, onions, peas and pansies.


Generally, the upper leaf surfaces develop yellowy, discoloured patches, which can extend across large areas of the leaf. These patches are matched on the underside of the leaf by areas of fuzzy white or greyish, fungal growth, which may turn purple as they age.

Treatment And Control

Remove and destroy all infected parts of the plant as soon as symptoms are seen, including any foliage on the ground.

Unfortunately, it is best to destroy severely affected plants.

As downy mildew like moist conditions, avoid dense planting and control weeds, to provide good air circulation around the plants.

Water plants early in the morning, like watering plants in the evening, leads to high humidity that persists throughout the night.

Chemical Control

There are no fungicides available to home gardeners for use against downy mildews.

MSU Extension

Mary Hausbeck, Michigan State University Extension, Department of Plant, Soil and Microbial Sciences - July 2, 2014

Wet weather favors downy mildew on cucumbers. Preventive fungicide sprays are recommended.

Michigan cucumber and pickle growers have battled downy mildew, incited by the water mold, Pseudoperonospora cubensis, since 2005. Resistant cultivars are not currently available and fungicides have been the only effective means of controlling this disease. This downy mildew pathogen is resistant to commonly used fungicides including Ridomil Gold-based products and the strobilurin fungicides (i.e., Cabrio, Quadris and Flint). Results from our research have identified a limited number of fungicides that are effective, but must be applied every five to seven days to control downy mildew on cucumbers when weather favors disease (Table 1).

Downy mildew spores detected

A significant number of airborne downy mildew spores were detected on June 18, 2014 in the pickle-growing region of Bay County. This was the only day in June with relatively high spore counts. With spore detection and the favorable environmental conditions for downy mildew that we’ve had this summer, Michigan State University Extension suggests a fungicide program take place for cucumbers. There are no confirmed occurrences of cucurbit downy mildew in Michigan or nearby states. There have been outbreaks in Florida, North Carolina and South Carolina.

The downy mildew pathogen does not overwinter in Michigan or nearby states. It reproduces via tiny, microscopic spores that act as seeds of the pathogen. The spores travel on air currents from areas where it can overwinter (i.e., southern areas of the United States or protected cucumber production greenhouses) into Michigan each growing season. To help growers determine the level of downy mildew threat, a monitoring system has been in place for Michigan since 2006. In May 2014, spore traps were placed in Monroe, Saginaw and Bay counties, and they continuously sample the air and collect spores into the trap. These spores become imbedded on a sticky film that is removed and taken to the laboratory. A compound microscope is used to identify any downy mildew spores that are present on the tapes.

The numbers of spores per day are uploaded to my “For Growers” webpage at This webpage also features current Extension articles about downy mildew, fact sheets for commercial growers and home gardeners, fungicide recommendations, photos of downy mildew-infected cucurbits and look-alike diseases, and links to other websites, including the MSU Enviro-weather Vegetable page, which also has pest management recommendations for cucurbit downy mildew and the Cucurbit Downy Mildew Forecast website.

The spore traps alert growers to any influx of spores into Michigan’s production regions, but are not the only tools used to time fungicide sprays. Since we do not have a trap in each field or in each county, it is possible that an isolated spore mass coming into a particular region may be missed. In each year that we have trapped downy mildew, spores have been detected before the first case of downy mildew disease was confirmed. In the past, downy mildew spore numbers have been detected by the traps weeks prior to the confirmation of disease in each location.

Table 1. Fungicide recommendations for downy mildew on cucumber

  • ** Previcur Flex 6SC – two-day pre-harvest interval (PHI)
  • ** Ranman 3.6SC – zero-day PHI
  • Gavel 75WG – five-day PHI
  • Presidio 4FL – two-day PHI
  • Tanos 50WG – three-day PHI
  • Zampro 4.4C – zero-day PHI

Alternate products and mix each with either:

  • Dithane (mancozeb) – 3 pounds
  • Bravo (chlorothalonil) – 1.5 pt

* Before downy mildew is confirmed in the area, products should be applied at seven-day intervals, and at five-day intervals after disease is confirmed. Use of the highest labeled rate of products is recommended.
** These fungicides are especially effective under heavy disease pressure.

2013 fungicide research

The Hausbeck lab has been working to ensure that pickle growers will continue to have the most effective fungicide products available. Through our research, the pickling cucumber growers and allied industries will know in advance whether previously effective strategies have remained durable or if alternative products and strategies are needed. In 2013, we tested fungicides, new and unregistered, for efficacy at low and high rates when applied alone or in a rotational program with other downy mildew fungicides Table 2 lists the fungicides that were included in our research effort. FRAC codes are used to determine the mode of action that the active ingredient has on the pathogen.

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