How to Beat Powdery Mildew
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You’ll know powdery mildew has paid your plants a visit when it looks lot like confectioners’ sugar has been sprinkled on the plant leaves. At first it may be hard to spot as it might appear on just a small portion of the leaf appearing as an irregular circle. But it quickly spreads and soon appears on the surrounding vegetation. Soon the entire leaf is covered and at the same time colonies develop on the surrounding vegetation and in other areas of the garden.
So how does it all start? Well, the plant becomes infected when an airborne spore, or
conidia, lands on a leaf and germinates. It soon grows a guide tube that attaches tightly to the leaf. Then it pierces the plant cell wall and membrane and inserts a hollow tube that sucks up plant nutrients, weakening the leaf and slowing growth. Within a week the fungus produces tiny mushroom stalks that release millions of spores, ready to infect more leaf surfaces. The fungus also produces a secondary spore, which over-winters outdoors and may also hide in a greenhouse or indoor garden even after the crop has been harvested.
Powdery mildew is most likely to attack the young leaves, up to two or three weeks old.
A dozen or so different fungus fall under the heading of Powdery Mildew, but two different fungus species are the most likely culprits.
L. taurica, tends to attack warmer gardens. It prefers a temperature of about 77 °F (25 °C).
S. macularis prefers a cooler temperature; however, the more virulent stain found in indoor gardens today has adapted to tolerate more heat. Both strains thrive in moderate humidity and are not injured by water. Their conidia can live in water for short periods and are mobile in it. However, strong water sprays do destroy some conidia.
CULTURAL CONTROL
Heat
Powdery mildew is sensitive to heat. Neither species will grow at 90 °F (32 °C). and will quickly perish when above 100 ° F (38 °C).
To get a complete kill maintain the temperature for an hour. This may not be a feasible option in most indoor gardens for several reasons. The first is that it may be difficult to heat the space to such a high temperature. The second is that even a single peak of 100 ° F (38 °C) affects the growth of plants. Vegetative plants with flowers or fruits in mid stage growth (weeks 3-7) may stretch a little from the experience. The heat treatment has relatively little effect on first and second week flowers or flowers nearing maturity.
You can minimize heat’s impact on plants in several ways. Heat the garden at the end of the day, as the lights are turned off. Since the plants are not photosynthesizing, they have lower water needs.
If the plants are being grown hydroponically, lower the temperature of the water to 60 degrees. Keeping the roots cool will help the upper plant parts beat the heat. It’s not difficult to do this, even if you don’t have a water chiller. Just add ice to the reservoir or flow through system. Roots of plants growing in soil can also be cooled using thermal ice packs at the base of the stem.
The heat treatment should kill off most of the fungus and its spores. The chances are there will still be some fungal re-growth. These can be eliminated using spot treatments.
Pruning
If one particular plant seems to be infected with a few tiny white spots on a few of its leaves, get a bag large enough to drop the leaves into and then cut them off into the bag. Remove the bag from the room. This prevents spores, the white powder on top of the leaves, from becoming airborne while being removed. Remember to wash your hands and clean the scissors or knife with soap and water, hydrogen peroxide, alcohol or bleach. Spray the plant with one of the sprays listed below after pruning to prevent re-infection and encourage healing.
If, you notice a re-infection a few days later, there is a good chance that this plant is very susceptible to powdery mildew and presents a good location for the infection to start and spread from. The plant should be removed immediately by placing a bag over it and removing it from the space. Then the space should be sprayed with one of the sprays listed below.
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