Powdery Mildew: Powder Coated Devastation
I am going to diverge from my blogs regarding nutrients and supplements for today and talk to you all about powdery mildew. Recently several gardeners I have spoken with have reported severe problems with powdery mildew and this is what is prompting this rant.
There are a number of fungi that can cause what is referred to as powdery mildew. As the fungus settles on its’ host plant(s) the tissue beneath the fungal growth dies soon after infection. Damage from powdery mildew includes the stunting and distortion of leaves, flower buds, growing tips, and fruit. Yellowing of leaves and necrosis of plant tissue can result in premature leaf drop. Nutrients are removed from the plant by the fungus while it attacks your plant. This will quickly diminish the plant’s ability to photosynthesize, and if left unchecked it can weaken or kill a plant. Once the disease is present immediate action must be taken.
Powdery Mildew presents as a dusty white to gray coating on the leaves or other plant parts; it almost looks like someone has lightly dusted the upper surface of the leaves with confectioners’ sugar. Powdery mildew will start as small circular powdery white spots on the top of a leaf. As the fungal spores spread they will completely cover the leaf surface.
This fungal attack all starts with an airborne spore landing on the leaf of a host plant. The spore germinates and attaches itself to the leaf with a tube that pierces the outer cell wall and starts drawing nutrition from the plant. Within a short time the fungus has reproduced to form a powdery white colony, and from there it starts manufacturing millions of more spores to infect surrounding vegetation.
The fungi which cause powdery mildew are spread by spores produced in the white powdery patches. These spores are dispersed by wind or physical agitation where they travel on air currents to a new host plant. The host plants for the fungi that cause powdery mildew are selective so one species of fungus can not attack all types of plants…they are plant specific. For example the species of fungus infecting begonias will not cause powdery mildew on cucumbers.
The fungi that cause powdery mildew proliferate most in periods of high relative humidity. To reduce your garden’s susceptibility to powdery mildew make sure you have good air movement in and around your plants and that plants are not crowded together. Overly dense planting can lead to a very humid environment below the canopy. With little air movement in the undergrowth it becomes a veritable breeding ground for powdery mildew.
There are a few cultural practices that can be employed to help combat powdery mildew. Heat is an effective control but at the temperatures necessary to kill the fungi it may damage your plants. Powdery mildew will not grow at 90 °F, and will die when above 100 °F. Temperatures only need to remain above 100° F for about an hour in order to kill most of the spores. If you do decide to go this route try to raise the temperature in you grow space right before lights out. If you are growing hydroponically keep your water/reservoir temperature as close to 60 degrees as possible to help reduce the shock to your plants from the high air temperatures.
Another cultural practice essential to combating powdery mildew is to have good hygiene. This it means you must keep a clean grow-space. All old dying leaves should be removed, but care must be taken to not spread the airborne spores. If you find a leaf where a white powdery fungal colony has emerged, remove the infected leaf (or leaves) by placing a plastic bag around the leaf and then gently cutting the leaf off into the bag. Seal the bag shut and remove it from the room quickly. Make sure you have turned off all you circulation fans before you attempt this so not to spread any spores that might become airborne.
I would be remiss if I did not mention that Ozone (O3) will kill the airborne spores of powdery mildew. There are two types of ozone generators: corona discharge and U.V. They both produce ozone but by different methods. Ozone or O3 is an unstable molecule and wants to return to its stable form O2. When this return to stability occurs, a single Oxygen molecule is released. It will find another molecule or in this case a mold spore to bond with effectively killing the spore.
Other Methods of Suppression:
Hydrogen Peroxide
Hydrogen peroxide sold as Via Oxy® will act as a contact fungicide and leaves no residue. When it comes in contact with the powdery mildew it will kill the spores. To treat plants with Via Oxy (17% pure hydrogen peroxide,) mix 2 tablespoons of Via Oxy per gallon of water and apply liberally by spraying infected areas.
Neem Oil
Neem oil has natural fungicidal properties: it interrupts the fungi’s metabolism on contact, and it stops spore germination. Neem oil is best used as a preventative measure because it works better as a preventative than a cure. It shouldn’t be used on flower buds or flowers.
pH Up
pH-Up is a general term for an alkali pH adjustment chemical. It usually is in the form of caustic potash (KOH). Because Fungi need an acidic surface to grow on by spraying pH-up (diluted in water) on the infected area of your plant it will effectively create an inhospitable environment where the fungi will fail to flourish. Mix enough pH-up to bring the water solution to a pH of 8. Then spray liberally on all infected areas. This is a very effective method of control.
Vermi - T or Serenade®
Vermi-T and Serenade® are liquid solutions of bacterial populations that attack the fungi. They are easy to use, quite safe and effective. Just spray them on infected areas of your plant. If using Vermi-T apply a mixture of 4 parts water to 1 part fresh Vermi-T and spray liberally.
Milk
Milk kills powdery mildew so well that professional rose growers all over the world use it as a fungicidal spray. Use one part milk to nine parts water. Use at first discovery of powdery mildew and then reapply weekly to stay off further infection.
Sulfur
Sulfur is a long standing control of powdery mildew. Sulfur sprays can be used but they leave behind a residue on the plant. In greenhouses gardeners use sulfur vaporizers that heat sulfur powder or prills until they vaporize. The sulfur leaves a fine residue of a very low pH on all plant surfaces which inhibit the fungal growth of powdery mildew. A problem with vaporized sulfur is it too leaves behind a fine residue of sulfur on all plant surfaces. To treat outbreaks of powdery mildew use your sulfur burner for 7 to 8 hours per night 1 to 2 times a week.
*As a side note if you choose to combat your powdery mildew problem using any of the above mentioned sprays please know that spraying liquids on your plants and flower sites will raise your humidity and can lead to mold problems. To counter act this, increase the air movement around the plants by adding extra fans. Also try to lower the ambient humidity with a dehumidifier if possible.
Fungaflor® TR
There is a new weapon out to fight powdery mildew called Fungaflor® TR. It is a dry micro release fungicide in an aerosol bomb. One Fungaflor TR bomb can treat up to 1,500 square feet and is easy and highly effective! Simply clear a 3 foot space around the can before releasing the spray, place the can at or above the height of the plant canopy, turn off all fans, and release after your lights go off. Leave the room sealed for a minimum of 3 hours or all night. When the lights turn back on go in and turn on the fans and that is it. Fantastic and easy!
There is mounting scientific evidence that the addition of Silica (i.e. Silica Blast, Rhino Skin, or Dutch Master Silica) can prevent and treat powdery mildew. Also there are new systemic treatments of powdery mildew and Pythium that use a chemically altered form of Phosphorus called Phosphites (Pure Flowers blossom enhancer) that seem to prevent and cure powdery mildew amongst other infections.
