Why Leaves Get White Spots: Powdery Mildew

Summary

In this video, Jason from Fraser Valley Rose Farm addresses the common issue of powdery mildew affecting plants, particularly during certain seasons. He explains the fungal infection's appearance on a variety of plants and emphasizes its varying impact on their health. Jason discusses the conditions that favor powdery mildew, such as humidity and cooler temperatures, and offers practical advice on prevention, including choosing resistant plant varieties and maintaining good air circulation. He also suggests less harmful alternatives to chemical sprays, like neem oil, horticultural sulfur, and even milk, for managing the disease.

Takeaways

• 🌱 Powdery mildew is a fungal disease that appears as white patches or a fuzzy covering on the upper surface of leaves.
• 🍃 Susceptible plants include squash, cucumbers, melons, grapevines, roses, phlox, menardes, and certain broadleaf trees like horse chestnut and big leaf maple.
• 🌡️ The fungus thrives in humid conditions and cooler temperatures, making early season greenhouses particularly prone.
• 🌳 While powdery mildew can affect many plants, its impact varies; it's often less severe on trees and perennials but can be devastating for annual crops and those intended for fruit or flower harvest.
• 🌱 To prevent powdery mildew, choose resistant plant varieties, such as the Jacob Kline monarda or powdery mildew-resistant (PMR) squash and cucumbers.
• 💧 Good air circulation, proper watering, and maintaining plant health are crucial for preventing powdery mildew; avoid overhead watering that keeps foliage wet.
• 🌬️ In greenhouses, use circulating fans to manage humidity and prevent the buildup of fungal spores.
• 🌱 Neem oil and wettable sulfur are less harmful alternatives to heavy chemicals for controlling powdery mildew, effective as preventive measures early in the season.
• 🥛 A baking soda and oil spray, or a milk solution (one part milk to two parts water), can also be used as home remedies to deter powdery mildew, though they may have drawbacks like cost or unpleasant smell.
• 🛡️ Once a significant amount of powdery mildew is present, it's challenging to eradicate, emphasizing the importance of early detection and prevention.

Q & A

What is powdery mildew and how can it be identified on plants?

Powdery mildew is a fungal disease that appears as white patches or a white, sometimes fuzzy, covering on the upper surface of leaves. It's fairly unmistakable and can affect a wide range of plants.

Which plants are particularly susceptible to powdery mildew according to Jason?

Jason mentions that squash, cucumbers, melons, grape vines, roses, phlox, menardes, and broadleaf trees like horse chestnut and big leaf maple are particularly susceptible to powdery mildew.

How serious is a powdery mildew infection for the health of a plant?

The seriousness of a powdery mildew infection varies by the host plant variety and the severity of the infection. For most healthy trees and perennials, it's more of a nuisance and doesn't significantly impact health, but for fast-growing plants like squashes, cucumbers, and melons, it can ruin the crop.

What conditions favor the spread of powdery mildew?

Powdery mildew thrives in humid, wet conditions on leaves and prefers cooler or moderate temperatures. It can also be more prevalent in greenhouses due to higher humidity levels.

How can aphids contribute to the spread of powdery mildew?

Aphids can make plants more susceptible to powdery mildew by puncturing the leaves with their sucking parts, creating openings for the fungal spores to infect the plant.

What are some preventive measures for powdery mildew?

Prevention includes choosing resistant plant varieties, ensuring good air circulation, proper watering and feeding to establish healthy plants, and strategic watering to minimize foliage wetness.

What is the role of resistant plant varieties in managing powdery mildew?

Resistant plant varieties can delay or reduce the severity of powdery mildew infections, which is beneficial especially for crops where the disease can impact the harvest.

Why is air circulation important in preventing powdery mildew?

Good air circulation helps to reduce humidity around plants, which is a condition that powdery mildew favors. It also prevents the buildup of fungal spores on plant leaves.

How can proper watering practices help in managing powdery mildew?

Watering practices should avoid wetting the foliage, using methods like drip irrigation or watering early in the day to allow for evaporation, thus reducing the risk of infection.

What are some less harmful alternatives to chemical sprays for treating powdery mildew?

Less harmful alternatives include neem oil, wettable sulfur, and homemade solutions like a baking soda and oil spray or a milk spray, which have been shown to be as effective as heavier chemicals in reducing powdery mildew.

How does the baking soda and oil spray recipe work for treating powdery mildew?

The recipe involves mixing one tablespoon of potassium bicarbonate (baking soda) and three tablespoons of horticultural oil or canola oil per gallon of water. This spray is less phytotoxic and can be as effective as chemical treatments.

What is the milk spray method and how effective is it against powdery mildew?

The milk spray method involves mixing one part milk to two parts water to create a 33% milk solution. It can be effective in preventing powdery mildew, but it may leave an unattractive coating on foliage and has a strong odor.