METHODS OF PREVENTING POWDERY MILDEW ON MONARDA
Monarda is a valuable plant in my gardens and have grown it for most of my adult life in many different places in B.C., from the wet southern coastal region to here in dry Kamloops. I enjoy & appreciate many aspects of this plant: the shape and colours of the flowers, the scent of both the leaves and flowers, it attracts pollinating bees, butterflies and hummingbirds, the leaves can be used to make tea, the flowers are deliciously edible, and its overall striking beauty. Monarda has a lot of value to me, and it makes me smile.
There are a few problems that can be associated with growing Monarda: powdery mildew (Erysiphe cichoracearum fungus), rust, invasiveness, and crown die-out. Here in Kamloops our dry climate helps eliminate most of these. Powdery mildew is the most common issue, which I am addressing here. It starts early in the growing season. White fungal threads produce spores, which germinate, penetrate the leaves, and absorb nutrients. The grayish-white fungal patches develop on the upper surfaces of the leaves; eventually the leaves will yellow and die. It can be most severe in late summer and fall. The fungus can overwinter on plant debris and in buds, releasing spores the next spring to continue the cycle. The solutions are not complicated. With proper care and attention, and simple adjustments, the grower can continue to grow and enjoy it with greater success. There are also disease resistant varieties to choose from.
What factors can cause or exacerbate powdery mildew?
A) Poor ventilation (plants are too close or too thick).
Spacing the plant is a priority, allowing space for air to move around and through it. Don’t have any other plants too close, and if necessary, thin the stems growing from the crown to increase air circulation.
B) Water on the leaves caused by overhead watering.
Eliminate overhead watering. Install drip irrigation if possible. If hand watering, use a wand extension to get water directly onto the soil and water early in the day so any water on the leaves can dry quickly. Watering late in the day promotes the problem, as the temperature drop at night increases the humidity, which increases spore development.
C) Heat stress and high relative humidity.
They are not tolerant of dry soils or shade. Protect it from hot winds, and ensure it is near an adequate water supply and not out of reach of the watering system. Plant it where it will get at least 6-8 hours of sun a day. There is higher relative humidity in the shade, partly due to lower temperatures, both during the day and at night. The lack of sunshine also doesn’t allow moisture on the leaves to dry quickly.
D) Diseased parts (leaves, stems, flowers) remaining on or near the plants.
Proper hygiene is imperative. Inspect the plant frequently – early diagnosis is key. Promptly remove all diseased or dead parts and dispose of in the garbage. Never compost! Also sterilize any tools used, to prevent transferring the spores.
There is an option for spraying affected areas with fungicide. Personally, I will not use chemical fungicides. Other options are to spray with a baking soda mix (1.5 tablespoons baking soda per gallon of water) weekly, or a weak milk solution (1 cup milk to 9 cups of water) every 2 weeks. Some people will use a horticultural oil (3 tablespoons per gallon of water), spraying every 2 weeks. I have never used any sprays.
In conclusion, knowing Monarda’s ideal growing conditions and creating a healthy environment has made healthy plants which are resistant to disease. I have already addressed the stresses that can affect this plant, put best practices into use, and have eliminated powdery mildew from my Monarda.
This article also appears on the Laughing Swan Website @ www.laughingswanfarm.com
