We all love growing our own vegetables, fruit and ornamentals in our gardens. However, when plants begin to yellow, drop leaves or show other signs of distress, it may signal that they are suffering from a common plant pathogen and you’ll need to act fast. The good news is that there are plenty of organic solutions for plant diseases that will have your plants looking their best in no time.
This guide will cover some of the most common plant diseases you may encounter in your backyard, as well as how to treat them organically. And, just in case, we’ll also talk about a few other non-pathogen issues that may crop up in your garden, so you’ll know how to recognize them too.
No matter your garden problem, there is always a solution. You just need to know where to look!
- Recognizing plant diseases in the garden
- 10 common plant pathogens and how to treat them
- 1. Powdery mildew
- 2. Downy mildew
- 3. Blight
- 4. Rust
- 5. Black spot
- 6. Mosaic virus
- 7. Damping off
- 8. Verticillium wilt
- 9. Fusarium wilt
- 10. Sooty mold
- Other common issues
- Best garden practices to prevent plant diseases
Recognizing plant diseases in the garden
Different plant diseases cause different symptoms in your plants, but there are some common signs to look for. As any gardener knows, it’s a good idea to check your plants from to time for any signs of distress. If you catch a plant disease early on, it’s much easier to treat than a well-established infection.
Common signs of plant pathogens include:
- Yellowing, patchy or otherwise discolored leaves
- Distorted growth of plant leaves, fruit or flowers
- Stunted growth
- Powdery residue on plant leaves
- Leaf drop
- Plant death
Any one of these symptoms can indicate that your plant potentially has a disease. However, these symptoms can also indicate other problems, such as nutrient deficiency and pest attack, so be sure to check your plants over carefully for other clues.
Because some plant pathogens can at first look quite alike, for proper identification, you’ll want to consider other factors, such as growing conditions, time of the year, how you’re watering your plants, and so on. Answering these questions will help you more easily determine what’s going on with your plant or garden beds.
10 common plant pathogens and how to treat them
Although there are other potential plant diseases that can occur in your garden, below are the most common types you may encounter. We’ve also included some suggestions for organic treatment options to help your plants and garden recover quickly.
1. Powdery mildew
Powdery mildew is a very common problem in backyard gardens. As the name suggests, powdery mildew is a fungus that presents as a powdery white coating on plant leaves and stems. Affecting vegetable, fruit and ornamental plants, it is a common sight on lilacs, peonies, phlox, cucumbers, apples, grapes and roses.
Most often, powdery mildew develops during the heat of summer and is caused by a lack of air flow and humid conditions.
The best way to deal with powdery mildew is to prevent it by ensuring your plants are properly spaced and pruning away leaves as needed for better air flow. Overhead watering in the evening is one prime cause of powdery mildew. So, do your best to always water in the morning and at the base of your plants to prevent wet leaves and allow adequate time for water to evaporate before evening.
If your garden does come down with a bad case of powdery mildew, prune away as much of the damaged foliage as you can and destroy it by burning or bagging and tossing it. Pruning can also increase flow, which will slow the spread of mildew. That said, once established, fungicidal treatments won’t cure powdery mildew, but they can stop it from spreading further.
Copper or sulfur-based fungicide can help control powdery mildew. If your garden previously suffered from mildew issues, applying these sprays every 7 to 10 days spring through fall can prevent the problem from reoccurring in future seasons. Solarizing your soil can also do wonders for preventing future mildew problems.
For a homemade solution, you can also try out a milk spray by mixing 40% milk with 60% water and spraying your plants down. Alternatively, a baking soda mixture of 1 teaspoon baking soda with 1 quart of water works well too.
2. Downy mildew
Another common fungal issue, downy mildew is a little bit more difficult to deal with than powdery mildew. This fungus affects ornamentals and vegetables, including columbines, impatiens, pansies, cucumbers and other plants. It is also most commonly discovered after periods of very wet weather.
Downy mildew causes plant leaves to discolor and often develop a mottled appearance. Gray or white mold may also be seen at plant bases. As the disease progresses, it will weaken plants, reduce fruit and flower yield and cause issues like stunted growth and leaf drop.
You can prevent downy mildew by following good garden maintenance practices, like crop rotation and autumn cleanup. Planting resistant varieties, watering plants at their bases, only watering in the morning and ensuring plants have adequate spacing for air flow are all great ways to prevent this mildew problem.
As with powdery mildew, if infection does occur, fungicides don’t really cure plants, but they can prevent the disease from spreading. Carefully prune away and destroy damaged foliage and then apply a sulfur-based fungicide every 7 to 10 days for the rest of the season to prevent spores from spreading.
Solarizing your garden soil can help prevent spores from reinfecting next year’s garden too.
Blight is most common in tomatoes, potatoes and other members of the nightshade family. Caused by a fungus, blight produces dark brown to black spots on plant leaves and spots often have concentric circles around them. Fruit may also exhibit patches of dark or sunken areas.
Blight can be divided into two varieties: early blight (occurring in May to June) and late blight (often spotted in August). Both types of blight usually occur after periods of wet weather, while late blight is a bit more severe and can kill plants quickly, even overnight.
Like other fungal issues, good garden maintenance is important for prevention. You’ll also want to ensure your plants are spaced properly for air circulation and water your plants at their bases to prevent wet leaves.
If early blight occurs, prune away damaged foliage as best as you can and spray your plants every 7 to 10 days with a copper fungicide spray. Plants damaged by late blight should be removed and destroyed quickly. Don’t compost them as blight spores may be able to survive the composting process.
To prevent overwintering spores, solarization can help too.
Rust is a variety of fungus that causes rust-colored patches on plant leaves and stems. As the disease progress, spots may turn black. Plants most commonly damaged by rust include hollyhocks, roses and tomatoes, but this fungus can also damage lawn grass.
A good autumn cleanup in your garden is imperative for preventing issues like rust. Be sure to destroy all damaged plant matter via burning or tossing, and don’t compost any diseased material as pathogens may be able to survive composting process unless you’re able to get the temperatures very high with hot composting.
To treat existing infections, prune away damaged foliage and spray your plants with a copper or sulfur-based fungicide every 10 days for the rest of the season.
5. Black spot
Black spot most frequently targets roses, but it can damage other ornamental flowering and fruiting plants too. Caused by a fungus, black spot doesn’t kill plants, but it does weaken them, making them more susceptible to other pests and pathogens.
Occurring most often in cool, moist weather, black spot presents as dark spotting on plant leaves. As the fungi spreads, plant leaves will yellow and eventually drop off.
Good garden cleanup in autumn is a must for black spot prevention. Additionally, adding a thick layer of mulch around plant bases can keep fungal spores in the soil from splashing up on plant leaves and infecting them. It’s also a good idea to plant vulnerable plants, like roses, in sunny areas of your garden where morning dew will quickly evaporate in the sunlight.
Other key ways to prevent black spot from developing include planting resistant varieties and watering your plants at their bases.
If problems arise, prune away affected plant material and properly dispose of it via burning or throwing it in the trash. Then, apply copper or sulfur-based fungicides every 1 to 2 weeks through the rest of the growing season.
6. Mosaic virus
There are several types of mosaic virus, but the two most common varieties are tomato mosaic virus and tobacco mosaic virus. Tomato mosaic virus is common in tomatoes, peppers, potatoes, cherries, apples and pears. Tobacco mosaic virus, on the other hand, targets tobacco, tomatoes, peppers, lettuces, cucumbers, beets and petunias.
Outbreaks usually occur in the heat of summer and cause mottled green and yellow leaves, yellowing leaves, distorted or stunted plant growth and reduced harvest yields.
Sprays don’t really work on mosaic viruses and, because these viruses can live in the soil for years, planting resistant plant varieties can keep your garden safe. You’ll also want to rotate crops on at least a 2 year rotation and remove and destroy any infected plant material if problems arise.
Additionally, because these viruses can spread through cigarette and cigar tobacco, smokers should wash their hands before working with plants in the garden. Never allow smoking in or around your garden for the same reason.
7. Damping off
Damping off is a fungal disease that destroys seedlings, most often when they’re grown indoors; however, it can occur outside too. Brought about by wet and humid conditions, damping off causes seedlings to rapidly collapse, wither and die -- usually overnight.
There is no treatment for damping off, although you can take certain steps to prevent it. If you had an issue with damping off in the past, use all new planting pots or disinfect your old pots with a 10% bleach solution. Be sure to use fresh seed starting mix, follow planting instructions to avoid overcrowding your seeds and consider adding a fan to your setup to increase airflow.
Hydrogen peroxide, the standard 3% solution you can purchase at pharmacies, has also been found to be helpful for preventing damping off. Simply mix 1 cup of hydrogen peroxide into a gallon of water and mist your seedlings and starting mix down.
8. Verticillium wilt
Many plants are susceptible to verticillium wilt, including trees, shrubs, vegetables and ornamentals. Caused by a soilborne fungus, verticillium wilt clogs plants’ vascular systems, causing sudden wilting and plant death. Affected plants may also exhibit leaf yellowing and leaf drop or stunted growth.
While fungicides don't work on verticillium wilt, a good garden cleanup in autumn can help prevent its spread. If problems do arise, be sure to destroy all damaged plant material by removing leaves with pruners sterilized with a 10% bleach solution. Plants infected with verticillium wilt often display green streaks in their woody stems, a telltale sign of infection.
It’s also a good idea to provide your plants with consistent watering to avoid watering stresses. Also, if you dealt with wilt in the past, solarizing your garden can destroy wilt spores in your soil.
9. Fusarium wilt
Another variety of wilt, fusarium wilt is also caused by a fungus in garden soil and is most likely to occur in hot weather. Targeting both ornamentals and edibles, like beans, tomatoes and peas, fusarium wilt causes wilting leaves, blackened stems, stunted growth and issues with root rot.
Sprays don't work against fusarium wilt, so the best solution is to plant resistant seedlings and practice good garden cleanup. If you’ve dealt with wilt in the past, don't plant the same variety of plant in that area of your garden for at least five years and consider solarizing your soil.
10. Sooty mold
Sooty mold is a secondary infection caused by a fungus that grows on the honeydew (or sticky secretions) produced by sucking insects as they feed on your plant. This mold blocks leaves’ ability to photosynthesize and can cause leaf drop and stunted growth.
Since sooty mold is caused by sap sucking pests, the first step towards treatment is to deal with the pests themselves. Neem oil sprays or organic insecticidal soaps should take care of pests, like aphids and leafhoppers. Then, just wipe off any remaining mold residue from your plant leaves and you should be good to go.
Other common issues
Plant pathogens, like fusarium wilt and mosaic viruses, can cause a whole host of problems in the garden. However, if you’ve read the above descriptors and the symptoms your plants are displaying don’t quite match up with any of the most common plant diseases, another issue may be occurring in your garden.
Below are some other common plant problems which may display some of the same signs as plant pathogens; however, they require different treatments.
From tomato hornworms to aphids and spider mites, there are lots of pests that may invade your garden and potted plants. Some pest infestations produce similar symptoms to plant diseases; however, they often exhibit key differences too.
Signs you may be dealing with destructive garden pests include:
- Visible pests or eggs.
- Deposits of frass, also known as insect poop.
- Holey leaves, shredded lace-like leaves or leaves that look like they’ve been chewed on.
- Completely missing plant leaves, roots or fruit.
- Leaves that are yellowing or have a mottled appearance.
- Stippling on plant leaves.
- Distorted leaf growth.
- Stunted growth.
- Shiny trails or squiggly lines on plant leaves.
- Webbing on leaves.
As you can see, some of the symptoms (like yellow leaves) are indistinct and can either be attributed to pests or diseases. On the other hand, signs like munched on leaves are very clearly the signs of pests.
If you’re unsure if you’re dealing with pests, inspect your plant carefully, turning over leaves and checking around plant bases too. Some pests can be very hard to find -- for example, slugs love hiding under nearby rocks or along raised bed sides. Although it may at first resemble damage caused by common plant diseases, stippled plant leaves is a sure sign that sap sucking insects have been at work.
Once you’ve identified what pests you’re dealing with, opt for organic control methods for treatment as these are safer for home use and won’t harm bees or pollinators when properly used. Effective pest control measures include using floating row covers, treating with neem oil or organic insecticidal soap sprays, BT thuricide and kaolin clay.
Sunburn and sunscald
Sunburn usually occurs on plant leaves and can cause leaf margins to turn brown and crispy, or leaves may appear blanched and experience color loss.
Sunscald, which is also caused by an overexposure to bright light, refers to damage to plant fruits, like watermelons and peppers. Damaged fruit may exhibit areas of necrosis, soft or mushy spots and brown patches.
Because they’re caused by too much bright sun, sunburn and sunscald only occur on areas of plants that are exposed to bright light. That means that if you’re seeing damage on lower, shaded leaves, too much sunlight is unlikely to be the problem.
Sunburn and sunscald can be prevented by ensuring that your plants are well watered in the morning and by keeping plant leaves dry by watering at the soil line. Over pruning plants, like peppers, can expose the delicate fruit to too much sun, so be sure to leave some sheltering leaves on your plant to protect your fruit.
If you’re experiencing a particularly hot and dry spell, placing an umbrella or shade cloth over your most susceptible plants can help. In a pinch, a bit of cheesecloth carefully secured over a few wooded stakes can shield your plants. Just remove the covering after the heatwave is over so your plants still have access to plenty of light.
Nutrient deficiencies in your garden are pretty common and frequently present as discolored plant leaves. Most often, leaves may begin to lose their color and look blanched, while leaf veins become more prominent due to the color contrast. Some nutrient deficiencies can also cause dark blotches to appear on leaves, leaf or blossom drop, stunted growth and other issues.
Testing your soil at the beginning of each gardening season is always a good idea as it will alert you if your soil is lacking in any particular nutrient. Even better, adding an annual layer of compost and mulch to your garden is a perfect way to prevent most plant nutrient deficiencies.
If your plants appear to have some sort of deficiency, run a soil test if you can and amend the soil accordingly with a good quality organic fertilizer. If you don’t want to run a soil test, feeding your plants with a balanced liquid organic fertilizer will help correct most nutrient deficiencies too.
Because deficiencies are so common, it’s a good idea to research a bit about each type of plant you plan on sowing before actually planting anything. Some plants, like pumpkins and cabbage, are very heavy feeders and need a fertilizer application every few weeks to maintain proper growth and prevent deficiencies. Try to plan out your fertilizing schedule beforehand so you know what plants need additional fertilizer throughout the growing season.
Underwatering plants can cause leaves to brown and turn crispy at their margins. When drought conditions intensify, plants may wilt, drop leaves and eventually die. Conversely, overwatering can cause issues with root rot, which presents as mushy roots and stems and plants that wilt and die.
One particular type of watering issue, blossom end rot, frequently occurs in plants like tomatoes and spoils fruit with mushy, dark spots. Although it can be caused by a calcium deficiency in your soil, it is more often triggered by irregular watering which changes plants’ ability to absorb calcium efficiently.
Since different plants have different watering requirements, it’s a good idea to research how much water your individual plant varieties need. In general, most plants require about 1” of water a week, but if you’re unsure if your plant needs water, just insert your finger into the soil. If the top 1” of soil feels dry to the touch it's usually time to water.
For gardeners who travel a lot or who tend to be forgetful about watering, adding a dripline system can fully automate your garden’s watering needs. This will take all the guesswork out of watering and simplify your gardening process immensely, so you won’t need to worry about over- or underwatering your plants ever again.
Everything in moderation, as they say, and that holds true in the garden too. Although we mean well when we fertilize our plants, avid gardeners often have a tendency to “over love” their gardens and a well-meaning fertilizer application can have disastrous results in already thoroughly fertilized garden.
If you know you’ve overfertilized a plant, sometimes flushing the soil with lots of water can correct the issue. That said, often we don’t realize if we’ve overfertilized something until the plant starts exhibiting signs like yellowing or browning leaves, crispy leaf margins, leaf drop or plant death.
To prevent fertilizer issues, always be sure to follow the manufacturer’s instructions when applying any commercial fertilizer and opt for organic options (like kelp or alfalfa meal) whenever possible as these are usually less harsh on plants. Using compost, aged manure or worm castings in your garden is also a very safe bet as these products are much less likely to cause issues if you accidentally apply too much.
Herbicides are generalized chemicals, meaning they will kill almost any plant they come in contact with, not just weeds. If you are applying herbicides in your yard, overspray can be a real issue as windblown droplets of herbicides can find their way into ornamental beds and vegetable gardens.
Even if you don’t use herbicides in your garden, if you live near a large scale farm or your neighbors use herbicides, overspray can drift in the wind into your garden too. If you apply manure to your garden beds, it can sometimes contain herbicides as well from the feed the animals ate.
Signs that plants have been exposed to herbicides include deformed growth, leaves growing in cup shapes, yellowing or browning leaves, leaf veins turning unusual colors (like yellow or red) and plant wilting and death.
To prevent issues with herbicides, try to keep your garden as organic as possible. If you’re dealing with a weed issue and hand pulling isn’t an option, consider smothering weeds with a weed barrier cloth or several layers of cardboard, try out a weeding torch or spray weeds with a horticultural grade vinegar.
If you believe herbicides are being blown onto your property from nearby residences, have a chat with your neighbor and offer some suggestions of other chemical-free products they could try.
Best garden practices to prevent plant diseases
Although there are products, like copper and sulfur fungicides, that can help treat plant pathogens once they occur, for a healthier garden the best option is to prevent problems before they arise. To this end, practicing good garden maintenance is key, while other tips (like purchasing resistant plant varieties) can help too.
Fall garden cleanup
Cleaning up your garden in autumn is an essential part of garden maintenance and one of the best ways to prevent pests and diseases from occurring year after year.
How much or how little you clean up your garden in fall really depends on you. Some gardeners choose to leave some debris in place to serve as a sheltering spot for pollinators and other beneficial insects, while other gardeners clean up everything.
However, if any of your plants suffered from any diseases or pest infestations during the previous growing season, it’s important that you remove all affected plant matter at the end of the season. This guarantees that when new plants emerge next spring, they won’t come in contact with old plant material that may spread fungi spores or other diseases.
Properly dispose of old plant matter
Pulling out old, diseased plant debris isn’t enough. If last year’s crop suffered from wilt, blight or another problem, you’ll want to make sure you dispose of that old plant matter properly.
Burning diseased plant material is one good option to ensure pathogens cannot overwinter. Alternatively, bagging and throwing away yard waste will stop disease spread too.
While composting is a great solution to reduce yard waste, green your garden and provide you with lots of nutrient-rich compost, you’ll want to avoid composting diseased or pest-infested plant matter. Although some hot composting piles achieve temperatures high enough to sterilize compost, killing off pests and pathogens, cold compost piles don’t generate any heat. That means that any composted material has the potential of spreading disease into next year’s garden.
Rotate your crops
Because many pests and pathogens can survive for quite a while in garden soil, it’s a good idea to rotate your plants every year, never planting the same type of plant in the same area twice. Additionally, because some pests can target plants from the same family, avoid planting similar plants in the same area too. For example, if you planted tomatoes in one section of your garden, don’t plant tomatoes, peppers, eggplants or other nightshades there the next year.
Different pathogens can stay in the soil longer than others. Fusarium wilt, for instance, can stay viable in the soil for up to five years, while mosaic viruses may last only two years or so in the soil. For this reason, try to rotate your plants around your garden on at least two year cycles.
Solarize your soil
Soil solarization works on a number of pathogens and pests, from downy and powdery mildew to squash vine borers. It’s a fancy name, for a simple process, but it works well to destroy many problematic fungi spores and destructive insects.
To solarize your soil, first thoroughly water your empty garden beds and then simply cover them up with sheets of thick, clear plastic. Anchor the plastic sheets down close to the soil line with landscape staples or heavy rocks or bricks around the sheet’s edges. Then, just leave your plastic sheets in place for about a month.
Solarization only works when temperatures are high, so you’ll want to do it in summer. Between the hot temperatures, the added moisture and the plastic barrier, soil temperatures beneath your plastic sheets will rise steeply. This essentially cooks any fungi or other pests living in the soil, making certain they won’t return in the future.
Once your solarizing sheets have been in place for at least a month, just peel them back and you’re ready to start planting again. While this method doesn’t work for all plant pathogens, it is an effective remedy for issues like mildew.
Many plant diseases are caused by soil dwelling fungi, which can be splashed up on plant leaves by raindrops and overhead watering. Applying a thick, 2 to 4” layer of natural mulch at the base of garden plants can help protect plants from fungal spread and also ensure better moisture retention, which can improve plants’ natural immunity.
When selecting mulches for your garden, natural mulch options are usually best, such as chopped autumn leaves, bark mulch, wood chips or salt marsh hay.
Additionally, to prevent issues like stem rot, never put mulch directly against plant stems.
Always bottom water
Overhead watering is an inefficient watering method, which wastes a lot of water to overspray and evaporation. But worse than that, overhead watering can increase humidity around plants too much, creating the conditions where diseases, like powdery mildew, can easily spread.
Instead of using an overhead sprinkler this year, look into dripline irrigation systems and soaker hoses that direct water straight to plant roots. If you don’t want to buy a new system, simply direct your garden hose towards the base of plants when hand watering.
Prune when needed
Fungi and other issues thrive in conditions with minimal air circulation. Pruning your plants as needed can increase air flow, thereby reducing humidity around your plants and creating a healthier growing environment.
With that said, not all plants will benefit from pruning, so be sure to do a little research before pulling out your garden pruners. Additionally, overpruning plants can cause other issues, like sunscald due to reduced leaf coverage, so don’t prune too much.
Follow planting instructions
Proper plant spacing is key for increasing air flow in your garden and preventing issues like mildew. When planting seeds and nursery starts, follow all planting instructions carefully, especially any notes about plant spacing. If instructions say to space plants 18” apart, don’t plant them all smushed together!
Look for resistant plant varieties
While not all plants are available in resistant varieties, many common plants (particularly vegetable crops) are. Plant scientists have been hard at work for years selectively breeding crops for natural disease and pest resistance.
For example, many varieties of tomatoes on the market today are resistant to mosaic viruses and/or wilt. Some types of cucumbers are also resistant to downy or powdery mildew.
While plant nurseries may have resistant plants available, often the best way to get your hands on resistant plants is to grow them from seed yourself. When ordering seeds this spring, check out seed catalogs for information about what seeds are resistant to. Sometimes seed catalogs note resistance in abbreviations like “PM” for powdery mildew resistance and “DM” for downy mildew.
Disinfect garden tools
When you’re working in the garden, especially if you’re working around crops you know have suffered from diseases in the past, cleanliness is very important. Fungal spores, like powdery mildew, can easily hitch a ride on garden pruners and spread from plant to plant.
While working, it’s good practice to disinfect garden tools, particularly pruners. To disinfect tools, you can:
- Soak tools in a 10% bleach solution for 30 minutes. Then rinse off your tools and pat them dry with a cloth. If you want to be extra cautious, you can dip your tools in bleach between each pruning cut.
- Dip, spray or soak tools in 70 to 100% isopropyl alcohol. This technique gets rid of any pathogens quicker than bleach.
While it can be frustrating to discover that your garden plants are suffering from some variety of pathogen, it’s not the end of the world. Sooner or later, plant pathogens will happen in the garden, but there are ways to control and eradicate those pathogens if they do occur.
Prevention is always better than treatment and, with the help of this guide, we hope you’ve learned some tips on how to avoid plant diseases. Traditional gardening tactics like fall cleanup and crop rotation have been used for generations for a reason: they work. While more modern methods, like the development of disease resistant plant seeds, can help keep your garden disease-free too.
By knowing the signs of common plant pathogens and how to prevent them, you’ll be a more effective gardener. And if problems arise, you’ll know exactly how to fix them!