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HomePlant ProblemsPowdery Mildew in Tomatoes: Identification and Prevention

Powdery Mildew in Tomatoes: Identification and Prevention

We’ve in all probability all been standing in our backyard at one level or one other, pondering to ourselves, “What’s this white powder on my tomato leaves?”.

Whereas it might look like a innocent eyesore, powdery mildew impacts many alternative crops and may unfold rapidly, wreaking havoc throughout your backyard plots.

Discover out as soon as and for all what causes powdery mildew to look on tomato vegetation, and what you are able to do to forestall it.

What’s Powdery Mildew?

Close-up of tomato bushes affected by powdery mildew. The bush is lush, has many complex leaves, which consist of medium oval green leaflets with slightly serrated edges. They are arranged alternately along the axis, forming a pinnate leaf structure. The leaves are covered with a white powdery coating.
Powdery mildew is a harmful fungal illness that steals vitamins from host vegetation and spreads globally by way of spores.

Powdery mildew is a widespread and damaging fungal illness attributable to Oidium neolycopersici, an obligate parasite that survives as mycelium, dwelling on the vitamins of a bunch plant. Mainly, this fungus lives on or contained in the plant matter of a bunch plant, exploiting the dwelling cells by stealing vitamins and dying when the plant dies.

The fungus travels by way of wind or rain, lands on a plant host, and rapidly germinates asexually by producing conidia on the plant floor. Every time these spores (conidiophores) are dislodged from the plant and journey to a brand new one, a brand new an infection cycle begins. So not solely does the conidia play a task in replica, however it’s also a part of the pathogen that bodily travels to a brand new host plant and continues to unfold

Oidium neolycopersici can have an effect on a broad vary of host vegetation and sadly, its origin is just not fully understood neither is the explanation for its excessive worldwide unfold that occurred a few years in the past.


Conidium: a kind of asexual reproductive spore of fungi (kingdom Fungi) often produced on the tip or facet of hyphae (filaments that make up the physique of a typical fungus) or on particular spore-producing constructions referred to as conidiophores. The spores detach after they mature. The plural type of conidium is conidia.


Mycelium is the white, hair-like root system of fungi you would possibly see throughout your soil floor and whenever you dig beneath your vegetation. They’re answerable for the decomposition of matter, function meals for a lot of soil creatures, and their development helps assist in carbon dioxide launched into the ambiance. The plural of mycelium is mycelia.

Crops Weak to Powdery Mildew

Close-up of the leaves of a tomato plant affected by a fungal disease powdery mildew. Leaves are oval, dark green, with slightly serrated edges, covered with white powdery spots.
Powdery mildew impacts varied plant species and may quickly unfold between crops.

Powdery mildew is most frequently discovered on cucurbits (cucumbers and squash), nightshades (tomatoes, peppers, and eggplant), and legumes (beans and peas).

It’s believed to have an effect on over 60 species in 13 plant households. Sadly, because the spores of powdery mildew can journey rapidly from one plant to the subsequent and may leap round totally different crops, they’ll destroy a plot fairly rapidly if situations are ideally suited.

Many business growers count on powdery mildew to have an effect on sure crops so they have a tendency to counterbalance this by planning successions all through the season of crops reminiscent of basil, cucumbers, and summer season squash.

This enables them to take away contaminated vegetation, begin contemporary in a unique space of their farm and hopefully break that cycle of unfold.

Perfect Powdery Mildew Circumstances

Close-up of a tomato plant with ripe fruits in a sunny garden. The plant is affected by powdery mildew. The leaves are large, pinnately compound, composed of oval dark green leaves covered with a gray-white powdery coating. The fruits are medium, rounded, bright red.
Powdery mildew thrives in heat, dry situations with excessive humidity, preferring cloudy days over wet durations or excessive warmth.

Whereas many fungal ailments thrive in moist, humid situations, powdery mildew prefers heat and dry. Perfect temperatures are between 60°-80° with excessive humidity across the vegetation, and it does finest on cloudy days.

It slows its unfold in wet durations and excessive warmth (above 90°) and doesn’t have an effect on vegetation in direct solar as a lot as these within the shade. For these causes, spring and fall are the most effective occasions for powdery mildew to point out up.

Professional tip: Discover a spot in your backyard that provides plenty of solar for plant species extremely vulnerable to powdery mildew.

Due to the spores’ wind-bourne touring nature, water is just not required for powdery mildew to unfold, germinate or infect, making it extra worrisome than another fungal ailments.

Identification & Signs

Close-up of a powdery mildew tomato leaf against a blurred background. The leaf is medium in size, oval in shape, dark green in color with serrated edges. The leaf surface is covered with white powdery spots.
Powdery mildew causes dusty spots on leaves, beginning mild and turning purple, primarily on leaf tops.

Did somebody come by way of your backyard and sprinkle your vegetation with flour? In all probability not! It’s extra possible that you’re seeing early indicators of powdery mildew, as that’s the way it’s typically described. It begins out trying like little dusty circles or spots on leaves, therefore the identify powdery mildew. Fruit is never affected.

Gentle yellow and inexperienced circles will seem, and as they develop, they might flip purple with necrotic facilities. Leaves will sometimes solely present white powder on the tops of leaves; signs are a lot much less possible on the leaf underside. They may generally yellow and dry up, and if the illness is extreme sufficient, they are going to twist and should even fall off the plant.

Your vegetation are most vulnerable when they’re younger and weak, however breakouts can happen at any life stage.

Plant Disposal

Top view, close-up of tomato leaves with a fungal disease - powdery mildew. The leaves are large, pinnately compound, composed of oval dark green leaflets with slightly serrated edges. The leaves are covered with a white-gray powdery coating. The beds in the garden are covered with floating row covers.
Get rid of contaminated vegetation to forestall powdery mildew unfold.

If you wish to wipe powdery mildew out fully, take into account destroying contaminated vegetation so powdery mildew doesn’t unfold all through your entire backyard. Since this pathogen can journey by wind and rain, in addition to from plant to plant, including the plant to a burn pile or trash is finest. In any other case, the spores could make their method again over to your backyard.

To completely and correctly kill the late blight pathogen, pull out contaminated vegetation on a sizzling and sunny day, lay them out within the solar, bag them, or cowl them with a tarp so they can’t journey and unfold. Use a preventative measure on the remaining vegetation to cut back spreading dangers.

Professional tip: If the breakout is just not extreme, you might be able to merely take away the affected plant components. Many growers have had success with DIY fungicidal preventative sprays. Extra on {that a} bit later.

Stopping Powdery Mildew in Tomatoes

Fungal ailments unfold rapidly and have an effect on many alternative crop households, making them very troublesome to eliminate as soon as they’re current. Prevention is essential!

Prune For Enough Airflow

Pruning a tomato plant with black and green secateurs, against a blurred background of a green garden. Close-up of sharp pruner blades between which is a thin green slightly hairy tomato stalk. Small green fruits of a rounded shape ripen on a tomato bush.
Correct spacing and pruning are important to forestall illness unfold.

You should comply with correct pruning practices to get good airflow, as overcrowding of vegetation and dense foliage makes it very simple for ailments of all types to unfold.

You should definitely present your vegetation with enough spacing. Tomatoes should not have any lower than 12 inches between each other. Sure varieties would possibly require as much as 36 inches on both facet. Ideally, leaves shouldn’t be touching these of one other plant close by.

As tomato vegetation develop, it’s best to prune off any outdated and pointless leaves towards the underside, as this opens up numerous respiratory house for the plant. Many growers take away any shoots that come off the principle stem on the underside foot of the plant.

You should definitely do a weekly “suckering” of the vegetation. Tomato suckers are these pesky growths that type between a leaf and the principle stem. They might turn into full-size stems and set fruit when you allow them to, however this simply crowds your backyard and creates competitors for assets.

Bear in mind, a plant solely wants about ⅔ of its foliage to stay wholesome and viable, so you may take away fairly a bit when pruning. 

Professional tip: You should definitely rid your backyard of any contaminated leaves and suckers, as they might be harboring fungal spores. Take them straight to the burn pile!

Hold Gardens Clear and Instruments Sanitized

Close-up of the disinfection of secateurs. Female hands hold blue secateurs and a cotton pad with a sanitizer on a blurred background.
Apply correct device sanitization after pruning to forestall illness unfold.

Any time you’re in your backyard to prune, remember to follow correct sanitizing of your instruments afterward to keep away from any potential illness unfold.

You would possibly even take into account wiping shears down after pruning every plant, because the illness will switch very simply at a wound website. Merely seize a Clorox® wipe or spray the shears down with a 70% isopropyl alcohol answer and wipe clear.

Making sanitization a part of your pruning routine helps you get used to doing it every time. This manner, that every time you seize your shears, they are going to be clear and protected!

Develop Powdery Mildew Resistant Cultivars

Close-up of ripe Indigo Rose cherry tomatoes in the garden. The fruits are covered with drops of water. The fruits are small, round, dark purple, almost black, arranged in a cluster on an elongated stem.
Select powdery mildew-resistant cultivars to reduce the danger, though full prevention is just not assured.

You possibly can choose tomato cultivars which can be extra resistant or tolerant of powdery mildew than others. On many seed firm web sites, you may filter for particular illness resistance (search for the code PM when you don’t see a powdery mildew choice). Under are a number of examples of those cultivars bred for powdery mildew resistance.

Plum/Sauce Granadero F1
Slicer/Beefsteak Geronimo
Climstar F1
Frederik F1*
Cherry Indigo Rose
Heirloom Striped Stuffer
*Good choice for greenhouse growers

Bear in mind that you could be nonetheless expertise powdery mildew even when rising a so-called resistant cultivar as a result of the resistance bred into these cultivars makes the vegetation extra resilient, however is not going to 100% stop the pathogen.

Apply Crop Rotation

Close-up of young tomato seedlings in the garden against the backdrop of raking the soil with a large garden rake. The tomato seedling has vertical low stems with complex pinnate leaves. The leaves consist of oval leaflets, bright green in color with serrated edges.
Powdery mildew can survive in particles or soil, however training crop rotation may help stop its reappearance.

Sometimes powdery mildew solely lives and thrives on dwelling vegetation, however generally it could possibly survive in overwintering particles or plant matter. If you happen to dwell in a hotter area the place frost doesn’t happen, you might should be involved concerning the fungus dwelling in your soil. In any other case, it’s not possible that the fungus will reappear within the spring from the soil.

That being stated, it’s at all times a good suggestion to follow crop rotation so pests and ailments have a harder time discovering crops they have a tendency to contaminate when it’s time to get up within the spring.

Your crops will most certainly turn into contaminated with powdery mildew by way of wind or rain transport or different contaminated vegetation, presumably introduced in from an out of doors supply.

Rising in a Greenhouse or Tunnel

Greenhouse for growing tomatoes. Many bushes of tomatoes with ripe fruits in a large spacious greenhouse. The bushes are tall, with upright stems and pinnately compound leaves, consisting of oval, slightly serrated leaflets. The fruits are medium in size, round, bright red when ripe and green when unripe.
Good airflow and early illness detection are key to stopping widespread greenhouse fungal ailments.

Because of the nature of rising in a greenhouse, fungal ailments reminiscent of powdery mildew are simply unfold and quite common. Airflow could be improved by correct pruning methods and by conserving followers calmly blowing always. Bear in mind, spores of those fungi journey simply in water and wind, so the followers may pace up the unfold of any illness already current.

Simply comply with all of the methods recommended for creating good airflow, preserve your tunnel tidy, and scout early for illness, and it’s best to have a profitable season.

Therapies & Natural Sprays

Prevention earlier than signs is essential with most fungal ailments, together with powdery mildew. Do your analysis and have a plan in place earlier than the season begins.

Copper-based remedies, neem oil, and bicarbonates are natural prevention therapy choices. Sulfur mud has additionally been proven to kill powdery mildew spores which have but to contaminate vegetation and has been broadly utilized in vineyards because the 1850s. It’s a reasonable choice and has a low threat of resistance. Nonetheless, it might have unfavourable results on gardening associates, reminiscent of useful pests and fungi, so use it with warning.

One factor to notice is that it’s necessary to not combine and match remedies with out offering a niche between them. Mixing neem oil and sulfur on the similar time can improve the plant’s threat of sunscalding; equally, mixing oil with copper will do the identical factor. Go away a two-week window between totally different therapy varieties to permit the prior technique to interrupt down within the daylight.

And as with all therapy plans, remember to learn all labels, comply with directions and preserve out of the attain of youngsters and animals.

Selfmade DIY Therapies

It’s suggested to check the next remedies on a single take a look at plant as therapy outcomes can differ and rely on exterior components reminiscent of daylight, temperature, and stage of illness.

Combine 1 half milk to 2 components water and spray on vegetation weekly. Whereas this technique remains to be being studied to gauge why it’s so efficient and to find out the best ratios to make use of it in, cow’s milk is a dependable DIY powdery mildew therapy. Full-fat milk is more practical than skim milk or whey, however all varieties are efficient.

A take a look at from the College of Minnesota combined 1 half mouthwash and three components water and sprayed leaves had good success charges. New foliage could be broken, so watch out with youthful, much less established vegetation.

Combine 3 tablespoons of sodium bicarbonate (baking soda) in 1 gallon of water. Evenly mist your vegetation with this answer. However use a light-weight hand for this therapy; you shouldn’t overuse baking soda as it could possibly accumulate within the soil. For this technique, slightly goes a good distance!

Last Ideas

Powdery mildew is usually a devastating fungal illness in yard and business gardens, however it’s preventable and treatable within the early phases. Simply know what to look out for and scout your vegetation typically.

If you happen to haven’t already, make associates together with your native Extension Workplace agent, as they’re glad to assist and an excellent useful resource to native gardeners and farmers!



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