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.

Outlines

00:00

🌱 Understanding Powdery Mildew

Jason from Fraser Valley Rose Farm addresses a common gardening concern about white patches on plant leaves, identified as powdery mildew. He explains that this fungal infection affects a wide range of plants including squash, cucumbers, melons, grape vines, roses, and some perennials and trees. Jason emphasizes that the severity of the infection varies by plant type and the progression of the disease. While it may not significantly impact the health of most trees and perennials, it can be more serious for annual crops and ornamental plants, potentially ruining the crop or affecting the plant's appearance. He also discusses how powdery mildew spreads, the conditions it thrives in, and the importance of early detection and prevention.

05:00

🌿 Preventing Powdery Mildew

Jason provides insights on how to prevent powdery mildew, starting with choosing resistant plant varieties. He mentions specific cultivars like Jacob Kline for bee balm and variegata to Bologna for roses that show resistance. He also stresses the importance of good air circulation, proper watering, and feeding to establish healthy plants that are less susceptible to the disease. Jason advises on cultural controls like spacing plants appropriately, pruning for air flow, and using irrigation methods that minimize foliage wetness. He also touches on the use of less harmful solutions for disease management, such as neem oil and wettable sulfur, which are effective in preventing powdery mildew when applied early in the season.

10:03

💧 Natural Remedies for Powdery Mildew

In this segment, Jason discusses alternative, non-chemical methods for treating powdery mildew. He recommends a baking soda and oil spray, providing a recipe that uses potassium bicarbonate and horticultural oil to minimize phytotoxicity. He also suggests a milk spray, which, despite its unpleasant smell and the need for a high milk concentration, has been shown to be effective in controlling powdery mildew. Jason concludes by inviting viewers to ask further questions about powdery mildew or other plant diseases, indicating his willingness to continue the conversation.

Keywords

💡Powdery mildew

Powdery mildew is a fungal disease that appears as white patches or a fuzzy covering on the upper surface of leaves. It is a central theme in the video as the speaker, Jason, discusses its effects on various plants and how to manage it. The video mentions that powdery mildew can affect a wide range of plants including squash, cucumbers, melons, grapevines, roses, and certain perennials and trees.

💡Fungal infection

A fungal infection is a disease caused by fungi, which in the context of the video, refers to powdery mildew. The video emphasizes the importance of recognizing the signs of fungal infections on plants, as it can impact their health and productivity. Jason explains that the severity of the infection can vary greatly depending on the plant variety.

💡Susceptible plants

Susceptible plants are those that are more prone to contracting diseases, such as powdery mildew. In the video, Jason lists plants like squash, cucumbers, melons, and roses as being particularly susceptible. Understanding which plants are more vulnerable can help gardeners take preventive measures.

💡Prevention

Prevention in the context of the video refers to the steps gardeners can take to avoid or reduce the occurrence of powdery mildew. This includes choosing resistant plant varieties, maintaining good air circulation, proper watering practices, and using certain organic sprays. The video stresses the importance of prevention over treatment once the disease has established itself.

💡Cultural controls

Cultural controls are practices that gardeners can implement to manage plant diseases. In the video, Jason discusses various cultural controls for powdery mildew, such as planting resistant varieties, ensuring good air circulation, and proper watering to keep the foliage dry. These practices are part of a holistic approach to garden health.

💡Resistant varieties

Resistant varieties are plant types that have been bred or selected for their ability to withstand certain diseases. The video mentions that choosing resistant varieties of plants like monarda or roses can help delay or prevent powdery mildew infections. This is a key strategy in managing the disease.

💡Air circulation

Air circulation is the movement of air around plants, which can help prevent diseases like powdery mildew that thrive in damp, stagnant conditions. The video suggests installing fans in greenhouses and spacing plants appropriately outdoors to ensure good air flow, which is crucial for plant health.

💡Watering practices

Watering practices refer to how and when gardeners water their plants. In the video, Jason advises against overwatering and recommends using drip irrigation or watering early in the day to allow time for the foliage to dry. Proper watering is essential to prevent the conditions that favor fungal growth.

💡Organic sprays

Organic sprays are natural substances used to control pests and diseases in gardens. The video discusses the use of neem oil, wettable sulfur, baking soda and oil sprays, and milk as alternatives to chemical fungicides for managing powdery mildew. These sprays are highlighted as effective and less harmful to the environment.

💡Phytotoxicity

Phytotoxicity refers to the negative impact of a substance on plant health. In the video, Jason mentions that some ingredients in organic sprays can be phytotoxic, causing damage to the leaves. He recommends using potassium bicarbonate and horticultural oil in a baking soda spray to minimize phytotoxicity.

💡Disease management

Disease management encompasses the strategies used to identify, prevent, and treat plant diseases. The video provides a comprehensive guide on managing powdery mildew, including choosing resistant varieties, cultural practices, and the use of organic sprays, which are all part of a broader disease management approach.

Highlights

Gardeners often ask why their plants' leaves turn white, which is usually due to powdery mildew.

Powdery mildew appears as white patches or a fuzzy covering on the upper surface of leaves.

Certain plants like squash, cucumbers, melons, grape vines, and roses are particularly susceptible to powdery mildew.

Powdery mildew can also affect perennials such as phlox and menardes, as well as broadleaf trees like horse chestnut and big leaf maple.

The severity of powdery mildew's impact on plant health varies by plant variety and the progression of the infection.

Most healthy trees and perennials can withstand powdery mildew without significant health issues.

Powdery mildew is more of a nuisance for ornamental plants, potentially reducing flowering but not usually threatening their lives.

For crops like squash, cucumbers, melons, and grapes, powdery mildew can ruin the harvest if not managed.

Powdery mildew spreads via fungal spores that travel in the air and thrive in humid conditions.

Greenhouses are at higher risk due to increased humidity, and early season conditions are particularly conducive to powdery mildew.

Aphids can make plants more susceptible to powdery mildew by creating openings for fungal spores.

Prevention of powdery mildew should start with choosing resistant plant varieties.

Cultural controls like good air circulation, proper watering, and feeding can reduce the risk of powdery mildew.

Resistant cultivars of plants like monarda and roses can delay or reduce the impact of powdery mildew.

Spraying with less harmful solutions like neem oil or wettable sulfur can be effective in preventing powdery mildew.

A baking soda and oil spray, or a milk spray, can be used as natural alternatives to chemical treatments for powdery mildew.

Proper watering practices, such as using drip irrigation or watering early in the day, can help prevent powdery mildew.

The video concludes with a call for viewers to ask questions about powdery mildew or other plant diseases in the comments section.

Transcripts

hi again it's Jason from Fraser Valley rose farm I want to answer a question I

get a lot from gardeners this time of year which is why are the leaves of my

plants turning white now the white they're talking about is like this and

I'm not sure if you can see this real close up so I'll get a close-up picture

of it but it's a white patches or white covering sometimes fuzzy on the upper

surface of leaves and it's actually fairly unmistakable

it's called powdery mildew and there are some very susceptible plants so if you

see this and it's combined with a plant that I list here pretty much it's

confirmed it's powdery mildew so let's talk about squash we talked about

cucumbers and melons grape vines get it quite frequently also my favorite roses

they get powdery mildew fairly frequently in perennials you'll

sometimes see it on phlox's and menardes and a host of other perennials as well

and sometimes you'll see it on broadleaf trees like this one here this is a horse

chestnut but also we get it quite a lot on our local big leaf maple so that's

what it is powdery mildew and it's it's over a wide range of plants let me talk

to you a little bit about how it spreads what are the conditions how you can try

to prevent it and how do you treat it if you have it on your plants so I've just

told you that your plant has a fungal infection on its leaves I guess your

next response should be to ask how serious is that for the health of my

plant and the answer is it varies an awful lot by the host variety the host

plant variety and how severe that infection progresses to be so on that

tree that I have out there and basically most healthy trees will never be set

back too far by a powdery mildew infection they'll come through it just

fine they're a long life cycle plant and one season with powdery mildew and

leaves or even every season with powdery mildew on the leaves isn't going to set

them back much like wise on my roses I have to say it's more of a small

nuisance than anything else I don't think any of my roses lives are being

threatened by a powdery mildew infection most it's going to do is it's gonna be a

bit unsightly it might reduce the deflowering on my plants for one season

if it gets particularly bad but typically it does not impact the health

of the plant significantly similarly with the perennials the phlox's and monardas

are can get powdery mildew every year later

in the season and still come back healthy the next spring so the ones you

really do have to concern yourself with a little bit more unless it gets to be

an out of hand infection is things like the squashes the cucumber's the melons

zinnias and grapes those things that you're trying to get a crop off of them

so you're trying to get the fruit or you're trying to get the flower from

them and they're fast growing and their inability to get that sunlight through

to those healthy leaves because powdery mildew is in the way and the health

deteriorates in the plant means that your crop can be ruined

dead stop so that's where it's more serious is usually in those short cycle

annual crops or those ones you're trying to get a-goin trying to get an actual

physical crop off of whereas for ornamental plants it's usually a little

bit less severe unless you're trying to sell them which I do so let's talk about

how you prevent it now the truth is it is a sport it's a fungal spore it

travels in the air it blows in from everywhere you can't really stop it from

coming into your greenhouse even if I grow in a greenhouse like this closing

the sides isn't going to keep it out in fact you're at a special risk in a

greenhouse because humidity levels are up so let's talk about the conditions

that powdery mildew likes it likes humid wet on the leaves and it's actually a

little bit better in cooler or moderate temperature conditions so early in the

season in a greenhouse like this you're very susceptible to powdery mildew

particularly on a cloudy day that's when it's really going to bring bring up the

high humidity inside the plants cooler temperatures you're going to have the

germination of those those spores on your leaves I will say one other thing

is they sometimes tag-team with aphids so if you get a lot of aphids a healthy

plant may be resisting the the germination of the fungal spores on the

leaf surface but once you get the aphids in there they stick their little their

little sucking parts into your leaves and it leaves an opening for those for

the powdery mildew to infect your plant so they sometimes go hand in hand all of

those stresses sometimes go hand in hand so any kind of a physical stress on your

plants either by under watering or aphids can sometimes make them a little

bit more susceptible to powdery mildew all right let's talk about prevention

because honestly once there's a good covering of powdery mildew on your

foliage it's pretty hard to wipe it out and your consideration for prevention

should always start by looking at resistant varieties of plants let me

give you an example in perennials monarda or bee balm is quite widely

susceptible to powdery mildew but there is one cultivar one well-known cultivar

called Jacob Kline which is a beautiful red one that actually holds off the

powdery mildew or at least doesn't get it until much much later in the season

so you don't have to worry about it quite so much now in roses you there are

a lot of the newer varieties that are much more power really resistant you can

go on to the site that I've recommended before help me find roses and that one

will show you in the description whether the plants are resistant or not this one

here is a variety called variegata to Bologna which is a bourbon rose

unfortunately fairly susceptible to powdery mildew I grow it anyway for

historical purposes and because it has a beautiful flower but I'm aware of that

and so I always watch it for the symptoms of powdery mildew early in the

season you will also see that in squash cucumbers and melons that oftentimes

when you go to the seed catalogs they will actually relist in the name of the

plant PMR or powdery mildew resistant and if

they don't list it in that then it's well worth reading into the description

of the plant to make sure that it has good resistance to powdery mildew

because again it may not be a case of avoiding the disease entirely but if it

can hold it off into the later part of the season after you've already got your

harvest in place that's just as good or almost as good so I know I'm gonna have

to talk about spraying here at some point because spring is the go-to in

dealing with fungus but give me a couple more minutes to talk about cultural

controls here the importance of which the first of which is to talk about good

air circulation in a greenhouse here we manage that very carefully because we

know that humanity can build up around those plants so we install circulating

fans to keep the air moving outdoors you want to manage it with the same thing in

mind so don't plant your plants so tight

together there's no way circulation space them appropriately and

prune them for good air circulation through the plants I know this on roses

I teach this on roses is to prune it to allow air to flow through the bottom

edge of that plant so we don't get powdery mildew down low that then

progresses up high and the plant also let's talk about watering feeding and

establishing your plants well the healthier the plant the less susceptible

it is to powder and mildew and particularly on roses I will say that a

lack of water will lead to powdery mildew in susceptible varieties every

time so that may sound paradoxical to you to talk about watering a plant well

but then also not letting the foliage stay too wet ways you can manage that

use drip irrigation or some other form of irrigation that keeps the water down

low where it isn't getting onto the foliage or if you are using overhead

irrigation be a little strategic about it if you see a sunny day with a bit of

a breeze water early in the day when it has plenty of time to evaporate off

those leaves that's good enough all right now let's talk about spraying then

I don't really encourage the heavy chemicals two reasons first of all I

don't want to spray where a spray suit I don't want to expose my family and my

farm to heavy chemicals but the second thing I'll say is that the studies show

that there are some less harmful solutions that are just as effective as

the heavy chemicals two of those that you would use as directed on the bottle

so look for neem oil for horticultural use that can do a good job also wettable

sulfur does a good job of reducing the incidence and severity of powdery mildew

usually again as a preventive early in the season and both of those products

are good in the cooler part of the season not so great in the later part of

the season when it's warmer because they can be phytotoxic at higher temperatures

so a couple that I'm going to talk about here I talked about a baking soda and

oil spray that I've recommended before when it comes to black spot I'm also

going to talk about a milk spray so the baking soda spray by recipe and this

time I'll give it to you in gallons is if you had a gallon of water with one

tablespoon of preferably potassium bicarbonate baking soda and say three

tablespoons full of horticultural oil or you can use canola oil

as a substitute now the reason I say potassium bicarbonate preferably and

also horticultural oil preferably is that these two ingredients are proven to

be less phytotoxic they don't burn the leaves quite as much so if you do

substitute them with canola oil or with sodium bicarbonate go ahead and do so

just I would probably either cut back the recipe or dilute it slightly or test

it first so you don't see higher you can see how much phyto toxicity there is

this and again in studies that that preparation has been shown to work as

well as the heavier chemicals in reducing powdery mildew the final what

I'm going to talk about is milk now milk does work and the solution that I've

seen mixed again in the studies is that it's it has to be at a fairly high rate

somewhere in the range of about one part milk to two parts water about a 33%

solution of milk you spray that on preferably on a warm hot day it will

smell really bad it may leave in an unattractive coating

of its own on the foliage but it does a good job of holding off powdery mildew

only thing I would say about that is in comparison to the oil and baking soda

spray you have it's probably a little more expensive to prepare and apply on a

regular basis and you do have to deal with the stink so that's my conclusion

on the spraying all right I hope this answers some of your questions on

powdery mildew and if you have any other questions on powdery mildew or other

plant diseases please drop those below the video