If you notice white spots on grape leaves, it’s probably powdery mildew. But don’t let this fungal disease get you down. It’s incredibly common in home gardens. Powdery mildew spreads through spores that cause white, fuzzy growth on susceptible crops.

Home gardens often contain plenty of moisture, shade, and debris that promote the spread of this disease. Humidity and heat are necessary for its growth. When the perfect conditions arrive, the fungus forms a patchwork of white spots on grape leaves.

A little bit of powdery mildew is okay, so long as you limit its spread. Once it gets out of control, it can prevent the leaves from photosynthesizing, causing a poor harvest later on. Tackle the issue as soon as you see it to prevent it from harming your grape leaves this growing season. 

What is Powdery Mildew?

Powdery mildew (PM) is a fungus. It spreads through fungal spores that infect healthy leaves. They germinate when they land on the leaves, where they then begin growing and reproducing to make more spores. When left to flourish, the condition will leave white spots on all grape leaves within the vicinity. 

Signs and Symptoms

Soft, fuzzy growth appears on struggling leaves.

Signs are outward displays of the disease, while symptoms are responses the plant has to the disease. Signs are objects like fungal bodies, bacterial slime, and fuzzy mildew. Symptoms are visible when the infected plant responds to the disease. They include responses like leaf withering, floppy stems, and cankers.

Powdery mildew signs are readily recognizable. Look for soft, fuzzy growth on the tops and bottoms of the leaves. The white fungus spreads under the right conditions, and the white spots on grape leaves will expand into irregular patches. The mildew may creep onto the skin of ripening grapes in large infections.

Symptoms are responses the grapevine has towards a pathogen. It’ll wither its leaves, grow slowly, and drop infected grapes in response to an infection. You’ll typically notice the PM signs first before the symptoms manifest in the grape leaves. 

Affected Crops

Keeping susceptible plants apart slows disease.

Grapes aren’t the only plants susceptible to powdery mildew! Many different fungal species exist with their own targets. Some attack grapevines, while others prefer squashes, roses, and lilacs. 

Each species flourishes in a specific set of conditions, which is why you may see the disease on your grapes and not your squash plants in the same vicinity. Once you see white spots on grape leaves, it’s helpful to identify the species in question so you know the other plants it may target in your garden. 

The powdery mildew that targets grape leaves also targets Boston ivy, Virginia creeper, and porcelain berry. Consider situating these plants far away from each other so they don’t act as disease vectors. An infection can creep from your Boston ivy onto your grapevine, and vice versa. 

Prevent Powdery Mildew

As we mentioned earlier, prevention is the best way to avoid issues with PM. A proper garden setup can dramatically reduce the instances of infection in your crops. Know what you’re dealing with, and you’ll be able to foolproof the site from pesky fungal diseases. 

Lower Humidity

Persistent leaf wetness encourages rapid fungal growth nearby.

The mildew prefers spreading when the humidity is high. It’ll rapidly reproduce, creating spores that waft through the wind and land onto other leaves nearby. 

Water is both damaging and helpful for the disease, depending on how you apply it. It disrupts reproduction by washing off all the spores. Without spores, the pathogen struggles to spread to nearby plants. It’ll stay within the original infected area. 

When the water persists in the area, it’s actually harmful. Persistent watering of the leaves may cause higher humidity levels, which lead to rapid mildew growth. It’s best to water in the morning using methods that don’t wet the leaves, like drip irrigation or olla clay pots. 

Grant Enough Sun

Direct sun for six hours keeps leaves healthy.

The humidity tends to be higher in the shade, where moisture persists for extended periods. Direct sunlight heats exposed areas, causing water to evaporate. When there isn’t sufficient sunlight, water will sit on the leaves and cause horrendous infections. 

Increasing light levels is easy for potted grape plants. Simply move them to a brighter location where they’ll receive full sun; grant them six or more hours of daily direct sunlight.

In-ground vines are a tad more difficult to manage. You may need to prune overhanging trees and shrubs, or consider transplanting the grape to a more accommodating location for the next growing season. 

Avoid Overfertilizing

A single spring dose produces balanced, healthy growth.

How often you fertilize your grapes may impact powdery mildew development. Overfertilizing with nitrogen early in the season leads to ample leafy growth at the expense of flowers and grapes. The more leaves the vines grow, the more surface area they have where spores can land, leading to white spots on grape leaves.

Not enough fertilizer also leads to issues. Weak, unhealthy plants are more susceptible to pests and diseases than healthy ones.

Grapes tend to do well without much extra fertilizer—more problems come from overfertilizing than underfertilizing. Apply a single dose of a well-balanced organic fertilizer when the buds break open in spring. 

Instead of fertilizer, you may also use compost or a similar organic mulch. Mulch amendments decay over time, producing a steady supply of nutrients that doesn’t overwhelm the grapevines. 

Clean Garden Debris

Removing debris far away keeps crops safer nearby.

Like other diseases, powdery mildew overwinters on garden debris. It sometimes persists on the bark of the grapevine. However, it most commonly resides on infected leaves that fall in autumn. When spring warmth arrives, spores spread from the old leaves onto new ones to start the infection again. 

The most helpful thing you can do this year to prevent PM next year is to clean your garden. Remove any infected leaves and dispose of them. Bury the debris a foot or deeper underground, hot compost it, or get rid of it far away from your fruit crops.

How to Fix Powdery Mildew

So, how do you fix PM? Your first impulse may be to find a treatment spray. Spraying is the last thing to do; start with cultural controls, then use narrow-range sprays that target the mildew without harming pollinators, crops, and beneficial organisms.

Remove Infected Foliage

Partial pruning conserves energy while controlling disease effectively.

All grape plants rely on their leaves to photosynthesize. Without enough foliage, they’ll struggle to generate energy and battle the infection. This is why it’s crucial to remove infected leaves while also ensuring your grapevine has sufficient foliage for photosynthesis.

If white spots are on a few grape leaves, remove them promptly. This is extremely helpful early in the infection before it begins producing more airborne spores. Once a white patch grows spores, the growth will quickly expand onto nearby leaves, stems, and grapes.

It’s not a good idea to remove the leaves if the fungus has grown on most of them. You’ll do more damage removing them than you would by leaving them be. 

Improve Airflow

Air circulation reduces the risk of fungal problems naturally.

Cramped, close conditions lead to rapid PM growth. The closer the leaves are to each other, the easier it is for the spores to spread onto them. A single white spot will produce spores that cause dozens more spots to grow throughout the vine.

Pruning helps improve airflow, as does removing weeds and putting rambling stems on trellises. Don’t let the grape stems grow on the ground, as they’re more likely to encounter the spores than if they were hanging up.

Also, ensure you’re spacing multiple grape plants far enough away from each other. Think of commercial vineyards; they have rows of grapevines growing a few feet apart from each other, with the stems growing on wire supports above the ground. 

Adjust Watering Regimen

Sprinklers running too long may invite unwanted fungal growth.

Overhead sprinklers cause spots to spread if they run at the wrong time. It’s best to turn on overhead irrigation in mid-morning when the sun is rising. This allows excess water to evaporate from the site before it creates the humid conditions that PM prefers. 

Avoid using sprinklers in the afternoon, at night, and in the early morning. The water may sit on the leaves and persist on the ground, leading to humid conditions. The water isn’t bad! It’s the humidity that accompanies it when evaporation is slow.

Use a Fungicide

Milk sprays help defend leaves against fungal attacks.

Fungicides prevent the spread of existing white spots on grape leaves. They don’t eradicate the disease. Rather, they prevent it from producing more spores. They also cover the leaves with a protective coating that prevents the spores from infecting them.