Whether you are a newbie gardener or have years of experience, a vegetable garden will keep you on your toes. Pest and disease issues are inevitable, though they can be mitigated by being proactive, and steps can be taken to prevent recurrences. To share her insights and methods, my guest this week is Susan Mulvihill, whose new book “The Vegetable Garden Problem Solver Handbook” is out this week.
Susan is a Master Gardener who lives on five acres in Spokane, Washington, about 300 miles east of Seattle, in a region known as the Inland Northwest. She writes the blog Susan’s in the Garden and a column for Spokane’s Spokesman-Review newspaper. We are old friends, and I have long been a big fan of her gardening techniques.
“The Vegetable Garden Problem Solver Handbook: Identify and Manage Diseases and Other Common Problems on Edible Plants” is the follow-up and companion to her first book, “The Vegetable Garden Pest Handbook: Identify and Solve Common Pest Problems on Edible Plants,” published in 2021. Together, they are a one-two punch for quick and easy reference.
“Identify” is the keyword for me. For anybody who wants to really be an effective, organic manager of pests and diseases, identification is a really important step that can’t be stressed enough. Susan and I are of a mind on this topic.
“It makes all the difference,” she says of identification. “You need to know what you’re dealing with, whether it’s a critter coming to your garden in the middle of the night, if it’s some type of a disease. Certainly, insects are a big thing to identify before you decide what their fate is. And you just need to be observant, monitor your garden regularly — and then you’re off and running.”
Her new book focuses on the 28-plus most common diseases and challenges in gardens. The irony is, as an organic gardener who adheres to best practices, Susan doesn’t encounter all of these challenges all that often. She also benefits from living in a dry climate, where gardeners don’t encounter many plant diseases. To round out the chapters, she did a whole lot of research into the diseases that gardeners in wetter climates know all too well.
Before getting into my conversation with Susan, I want to take a second to remind you that I have a new book out of my own, “The Vegetable Gardening Book: Your complete guide to growing an edible organic garden from seed to harvest.” It’s chock full of insider tips and new-to-you information that will help you step up your gardening game and tackle challenges.
How Susan’s New Book Came to Be
On the heels of Susan’s book on pests, Jessica Walliser, the editorial director at her publisher, Cool Springs Press, offered her the opportunity to write a book on plant diseases next. Susan was feeling worn out after finishing her first book and wasn’t convinced that a book exclusively about diseases would interest gardeners as much as a book on pests, so she passed on the offer. But then Susan’s husband, Bill, suggested that she write about diseases along with all of the different types of problems that gardeners deal with. She thought about it and came up with a list of issues that can arise in gardens.
“I’ve spent my whole career sharing what I’ve learned with others because I want people to be successful,” Susan says. “I want them to grow their own food. I knew I had all sorts of material to share. And so Bill and I grabbed a pad of paper and we sat down one day and started listing all sorts of things like extreme weather conditions, maybe seeds not germinating or pollination issues with squash, plant disorders, dealing with different types of critters that come in the garden — we certainly get our share of those — and yes, vegetable plant diseases. So I put together this detailed outline of how I thought the book could be written.”
Jessica loved the outline, and Susan wrote a second book despite initially being reluctant to do so.
“I’m happy to say that this time I got to write during the off-season, and I’m so pleased with how the book turned out,” Susan says.
She wrote the book between October 2021 and February 2022, working on an accelerated timeline so writing would not conflict with the growing season.
Getting Your Plants off to the Best Start Possible
There are many new gardeners since the pandemic began who could use a primer on getting plants off to the best start possible, and even seasoned gardeners may forget some of the principles. Taking these steps early on will minimize pest and disease issues that could arise later.
“Make sure you pick a very sunny location for your garden,” Susan says. “Vegetable plants need at least six to eight hours of sunlight a day, and it is very, very important. There are a lot of crops, especially leafy ones, that will tolerate a bit of shade, but go for as much sunshine as you can get.”
Another important step is to space plants adequately.
“If you crowd them together, there are a few things that happen,” Susan explains. “For one thing, they’re competing with each other for moisture and nutrients — and space, of course.”
Crowded plants don’t have good airflow between them, so when their foliage gets wet, it stays wet for a long time, and that makes the plants more susceptible to diseases. And Susan points out that when plants are crowded together it makes it easy for pests to move from one plant to the next. Plant tags and seed packets offer spacing guidance, and so does Susan’s new book.
It’s vital to get watering right for plants to succeed.
“Don’t overwater them, don’t underwater them,” Susan says, offering this advice: “Just poke your finger into the soil up to the second knuckle and see what it feels like. Does it feel bone dry? That’s not a good thing. Does it feel sopping wet? That is horrible. That’s the fastest way to kill a plant — is to have the roots wet all the time.”
I’m a big proponent of the finger test, and I know other writers and speakers who are in the know always recommend that as the No. 1 way to tell if your soil needs water or not. Some people who are looking for something more sophisticated may be disappointed to hear that, but it really is the best way I know.
Another big one that you hear me talk about often is to take good care of your soil.
Susan’s recommendations include adopting no-till gardening to avoid disturbing beneficial microorganisms in soil and adding a top dressing of compost to your garden beds in early spring and again in fall and letting the nutrients work their way down on their own.
Another practice Susan follows is to give plants support in the form of cages or trellises that will keep their foliage off the soil, which harbors plant pathogens. “By getting their foliage off of the soil surface, you’re making it harder for those pathogens to get onto the leaves,” she said.
Susan uses cattle panel arbors for her winter squash and pumpkins to keep the fruit and leaves off the ground and to thwart hungry critters. I also like arbors because they provide great light and air circulation.
Put down organic mulch on the soil surface to create a barrier between foliage and soil-borne pathogens. The mulch will stop foliage contact with soil and prevent splash up of soil on plants when it rains or when the garden is watered. Additional benefits of organic mulch include suppressing weeds, retaining soil moisture and improving soil fertility as the mulch decomposes.
Susan also says to be smart about using fertilizers. Different varieties of fertilizer serve different purposes, so be really cognizant of what the products do and choose a fertilizer based on the specific needs of the crop.
“Is it something that needs to bloom and set fruit? Is it something that’s going to do a lot of leafy growth like lettuce? Is it something that’s going to form a root that you’re harvesting like carrots or parsnips?” Susan asks.
She devotes a lot of space in her book to this subject because she knows it trips up many gardeners.
One of the most important things to remember is to refrain from overextending yourself.
“We get so excited, we want to plant all these different things, but especially when you’re new you might not realize how much time it takes to tend to a garden,” Susan says.
People are working full-time, they’ve got other things going on and they don’t realize that gardeners need to devote some time to the garden in order for it to grow successfully, she continues. “If you plant too much early on in your gardening career, then what can happen is you’re going to be totally overwhelmed. You’ll have difficulty keeping up with it, and then it’s not a good experience at all. So that might sound like something that everybody would think of naturally, but don’t do more than what you can keep up with.”
Plants can grow beyond your wildest imagination, and if you have more plants than you have time to keep up with — weeding, pruning, watering, harvesting, etc. — it can be demotivating. And when pests and diseases start to show up, it gets even more challenging to stay on top of caring for a garden.
“A stressed plant is not a happy plant,” Susan says, “and it’s sending out little chemical signals that will attract insects. And certainly, when plants are stressed, they’re not strong enough to be able to withstand issues like diseases or insect problems.”
Not only do the plants send out signals that say, “Hey, I’m low-hanging fruit here, come get me,” insects can do the same thing. The pests can call their buddies and say, “Hey, I found some easy pickings here — come on in everybody. The party’s over here.”
In an established garden, crop rotation is an important practice for mitigating pest and disease issues before they become big problems that require more intervention.
When the same type of crop or plants from the same family are grown in the same spot season after season, plant pathogens build up in the soil. For instance, if you plant nightshades (tomatoes, eggplants, peppers, potatoes) in the same garden bed for consecutive years, nightshade diseases will be prevalent in that bed and recur each growing season.
Likewise, nightshade pests that overwinter in soil as pupa before emerging in spring will readily attack any nightshades planted in that spot again. But plant something other than nightshades and the pests will move on (or die from lack of food) and the nightshade pathogen population in the soil will diminish each year that it lacks a host plant.
Crop rotation means switching to a different plant family in a garden bed each year, on a cycle of every three to five years. Think nightshades the first year, then beans, then cucurbits, then brassicas, then alliums, then back to nightshades.
Susan keeps really good records to track where she has grown which plant family to ensure she is rotating crops properly.
Dispose of Infected Plant Material
When plants do become diseased, it’s important to take diseased plant material out of the garden. Fallen leaves, clippings and spent plants should be disposed of far from your plants.
Diseased plant material, even after it is long dead, carries bacteria or fungal spores that will lead to new infections if not removed from the garden. It can go out with the trash but should never be placed in your compost pile.
In large-scale composting operations, the compost can reach a temperature high enough and sustain it long enough to kill pathogens. But Susan points out that most compost piles that home gardeners have will not get hot enough for long enough to neutralize the pathogens. “So err on the side of caution,” she says.
She advises bagging the plant material up right there in the garden rather than dragging it across your yard and risking it reaching other areas.
Practice Intercropping to Reduce Pest and Disease Problems
Intercropping is mixing up what plants are grown together, rather than planting rows of the same species of plant. Intercropping is a good pest prevention method because it can confuse and repel insects.
Plants that appear and smell different from one another are less appealing to pests than a monoculture of identical crops that they can hone in on. For instance, garlic, onions or aromatic herbs next to tomatoes can deter tomato pests.
Insects are vectors for many plant diseases, so reducing pest activity will also have the effect of reducing the occurrence of disease infections. For example, beet leaf hoppers that spread beet curly top virus are very attracted to a solitary planting of beets — but they have a hard time finding and infecting beets that have been interplanted with crops that have leaves that look nothing like beets.
Flea beetles, cucumber beetles and thrips are three more big disease vectors that you don’t want to see in your garden.
Inspect Plants Before Bringing Them Home
If you go to a plant sale or garden center and see plants that you want to take home and put in your garden, be sure to inspect them first for pests, eggs and diseases. If the plants are carrying pests, you can remove any eggs, larvae and adults after a thorough search and bring the plants home. If the plants show signs of disease, just put them down and walk away. You don’t want to introduce that disease to your plants at home.
Sanitize Garden Tools
When using tools to prune plants and work the soil, sanitizing them regularly will go a long way toward reducing the spread and recurrence of plant diseases.
To sanitize pruners and other metal tools, Susan says to use rubbing alcohol rather than bleach. Bleach will cause pitting in metal, which creates little pockets that pathogens can go into.
I have a holster for my spray bottle of alcohol so I am always ready to sanitize my pruners between cuts when removing suckers and diseased foliage
Nonmetal planting containers such as plastic seed starting trays can be cleaned with bleach solution with no concern about pitting. And there’s always good old soap and water.
Pick Resistant Varieties
If you have trouble in your garden with a certain plant disease you can save yourself the headache by choosing disease-resistant varieties. Seed packets, seed catalogs and plant tags are often coded with letters that indicate what diseases a plant variety is resistant to, such as “A” for alternaria and “VF” for verticillium and fusarium.
Choosing a resistant variety can be an easy way to resolve a pesky issue, but there may not be a readily available variety for the specific disease that is troubling your crop. If that’s the case, Susan says the best solution may be to skip growing that crop for a few years to break the reinfection cycle.
Water the Soil, Not the Plants
“I know we all say, ‘oh, I gotta go water my plants,’ but what you’re trying to do is water the soil,” Susan says. “The plants don’t need the water, the soil does. And when you get your plants real wet, that makes it much easier for disease pathogens to spread.”
Drip irrigation systems and soaker hoses are both good alternatives to overhead watering. They will get the water on the soil rather than the plants. Another option is a watering wand, which attaches to the end of a hose for hand watering and makes it easy to reach into a garden bed, get past the foliage, and apply water directly to the soil.
Monitor Your Garden Regularly
“It’s fun to walk out to the garden because you never know what you’re going to see,” Susan says. “You might see you’ve got your first ripe tomato of the season or something really cool like that. But by going out in your garden preferably every day and just doing a little cruise through, you will see things, problems that are just starting, and you have far more options if you are spotting them so early.”
Biofungicides for Plant Disease Prevention & Control
“There are some organic products that are used either as a preventive measure for diseases or they can also be used for controlling diseases,” Susan says.
Biofungicides are an example of preventative products. “They contain different types of living organisms that will do wonderful things,” she says.
Biofungicides include beneficial organisms such as Bacillus subtilis, Bacillus amyloliquefaciens, Bacillus licheniformis and Streptomyces griseus. If you have had problems with a certain fungal disease, then the following growing season a biofungicide can be applied early to stop a recurrence.
“You can buy both a foliar spray and a root drench, depending upon what you’re dealing with,” Susan says. “And the way that the biofungicides work is some of those organisms are root dwellers and so they compete with pathogens that are down in the soil. Some of these organisms will parasitize pathogens, which makes me very happy, and other ones will help trigger the plant’s immune response.”
Susan likes biofungicides because they won’t harm birds, fish or other wildlife.
“It’s cool to know that there are these beneficial types of bacteria that are doing so many good things to protect our plants,” she says.
And unlike copper fungicides, they won’t build up in the soil. (Copper toxicity in soil can prevent seed germination and stunt plant growth.)
Pros and Cons of Horticultural Oil
Horticultural oil is often used for insect control, particularly on fruit trees, because it smothers insect eggs and scale. A less-known use of horticultural oil is it can be used to control fungal diseases.
Beware that horticultural oil can interact with sulfur sprays, so the products should not be used on the same plants.
Horticultural oil can also burn plant foliage, so it’s always smart to test a product on a small area before using it on a whole crop. Give it a day or so and check to see how the test area reacted.
There are a number of organic control products derived from plants, including neem oil, thyme oil and garlic extract.
“Plant extracts can be used as a control and as a preventive,” Susan says.
One example is garlic for controlling and preventing powdery mildew. And then there is neem oil for controlling and preventing anthracnose, powdery mildew, rust and scale — but neem oil is also toxic to pollinators and aquatic creatures.
As Dr. Jeff Gillman has told me, people think if a product is organic you can use as much as you want — but snake venom is organic, and do you really want to drink it?
Neem oil should only be applied away from a body of water and only early in the morning or late in the day when pollinators are less active.
Thyme oil is used for buckeye rot, damping off disease, gray mold, downy mildew, powdery mildew and late blight. It does not raise concerns the way neem oil does.
Sulfur fungicide is another organic preventative control for stopping recurrences of a fungal disease from a past gardening season.
“It will control quite a few different types of fungal diseases, but it is toxic to insects, to amphibians, to fish, to mollusks, and it can damage the foliage,” Susan says.
She adds: “You don’t want to use sulfur fungicides in conjunction with horticultural oil because they do not play nice together.”
Powdery mildew is one of the most common diseases gardeners encounter. It is caused by several different types of fungi, Susan points out. It looks like someone has sprinkled bread flour all over the foliage of a plant.
Squash plants get powdery mildew most often but it can affect artichokes, beets, cabbage family crops (brassicas), carrot family crops (umbellifers), bean, peas, lettuce, nightshades and so on.
“Oftentimes what happens is it, it occurs late in the growing season. And for me, if that happens in my garden, I tend to just ignore it because I know we’re almost to the end of the season, so the plants are almost finished,” Susan says. “I’m just waiting for those last winter squash and pumpkins to mature.”
Plant in full sun because powdery mildew does not tolerate heat, and avoid overfertilizing with nitrogen because tender new growth is most susceptible to powdery mildew. Look for resistant varieties, and always dispose of infected plants in the garbage, not the compost.
Some summer squash varieties have silver patterns on their leaves that look like they could be powdery mildew — but it’s just the natural pattern. Just rub your finger across the leaf. If the silver color doesn’t brush off, it’s not powdery mildew.
Blossom End Rot
Blossom end rot is one of the most complex and misunderstood disorders, Susan says. It affects tomatoes, primarily, but also zucchini, peppers and eggplants.
Blossom end rot happens at the end of the fruit opposite the stem. It looks brown and leathery. “When you’ve seen it, you’ll never forget it cuz it’s pretty ugly,” Susan says.
Blossom end rot is caused but calcium deficiency, but the answer is not more calcium in the soil. Most soil has plenty of calcium.
“In order for plant roots to take up the calcium out of the soil and deliver to the plants, the soil needs to be consistently moist,” Susan says. “I don’t mean sopping wet, just lightly moist. If we’re inconsistent about watering our gardens, that can be a real problem.”
Social media posts about blossom end rot recommend using antacids, Epsom salts or even milk to prevent it. This is bad advice.
“My thought is these people who think they have come up with the solution are probably having good luck because they also happen to water their plants regularly,” Susan says.
Foliar calcium sprays don’t help either because leaves cannot take in calcium, and calcium has no way to move from leaves into fruits. Calcium must move from the soil up to the fruit via water, so, again, keeping the soil moist is key.
Too much nitrogen fertilizer promotes leafy growth, and calcium will end up in the new leaves rather than the fruit.
If someone waters regularly and still gets blossom end rot, Susan recommends they get a soil test to get to the bottom of the issue.
I hope you enjoyed my conversation with Susan Mulvihill and benefited from her advice. If you haven’t listened yet, you can do so now by scrolling to the top of the page and clicking the Play icon in the green bar under the page title.
What are your vegetable garden problem solver tips and tricks? Let us know in the comments below.
Links & Resources
Some product links in this guide are affiliate links. See full disclosure below.
joegardener Online Gardening Academy™: Popular courses on gardening fundamentals; managing pests, diseases & weeds; seed starting and more.
joegardener Online Gardening Academy Master Seed Starting: Everything you need to know to start your own plants from seed — indoors and out.
joegardener Online Gardening Academy Organic Vegetable Gardening: My new premium online course.
joegardener Online Gardening Academy Beginning Gardener Fundamentals: Essential principles to know to create a thriving garden.
joegardener Online Gardening Academy Growing Epic Tomatoes: Learn how to grow epic tomatoes with Joe Lamp’l and Craig LeHoullier.
joegardener Online Gardening Academy Master Pests, Diseases & Weeds: Learn the proactive steps to take to manage pests, diseases and weeds for a more successful garden with a lot less frustration. Just $47 for lifetime access!
joegardener Online Gardening Academy Perfect Soil Recipe Master Class: Learn how to create the perfect soil environment for thriving plants.
SusanInTheGarden.com – Susan Mulvihill’s website
Disclosure: Some product links in this guide are affiliate links, which means we get a commission if you purchase. However, none of the prices of these resources have been increased to compensate us, and compensation is not an influencing factor on their inclusion here. The selection of all items featured in this post and podcast were based solely on merit and in no way influenced by any affiliate or financial incentive, or contractual relationship. At the time of this writing, Joe Lamp’l has professional relationships with the following companies who may have products included in this post and podcast: Rain Bird, Corona Tools, AeroGarden, Milorganite, Soil3, Greenhouse Megastore, PittMoss, Territorial Seed Company, Earth’s Ally, National Wildlife Federation and TerraThrive. These companies are either Brand Partners of joegardener.com and/or advertise on our website. However, we receive no additional compensation from the sales or promotion of their product through this guide. The inclusion of any products mentioned within this post is entirely independent and exclusive of any relationship.