11 Steps for Managing Garden Diseases in Humid Gardens

Nothing is more disheartening as a gardener than losing the battle with plant disease, reducing your harvest after a long season of hard work. Diseases in the garden can affect gardeners in any region, but they’re particularly devastating for those of us who live in humid climates.

Fungal and bacterial growth are two of the leading causes of plant diseases. Wet, muggy conditions speed up the growth of these pathogens, leading to more frequent disease issues in humid gardens. Powdery mildew, downy mildew, anthracnose, and blight are just a few of the problems that humid-area gardeners can expect.

So, what can you do to keep these diseases at bay? Fortunately, there are plenty of steps you can take to minimize fungal and bacterial growth, possibly stopping diseases in their tracks.

Step 1: Choose Disease-Resistant Varieties

Before you even start planting, do your best to choose disease-resistant varieties. This is a preventative step that can make an enormous difference, especially if you know what diseases are prevalent in your area or in your garden.

Of course, no variety is truly 100 percent resistant to disease, but one labeled “disease-resistant” will be much less likely to suffer. These varieties are more likely to be hybrids, while it’s more rare for heirloom varieties to have strong resistance.

Seed companies will proudly label their seed packets, helping you choose. For example, these classic ‘Sun Gold’ pole cherry tomatoes are labeled “disease resistant” in the description. In the “variety info” section, you’ll see that these tomatoes are resistant to Fusarium wilt and tobacco mosaic virus.

Keep a garden journal each year. If your plants fall prey to a mysterious disease, do your best to identify it and then record that information in the journal. This way, you’ll be able to plan future garden varieties based on pathogens you suspect may be problematic in your yard.

Did your zucchini succumb to powdery mildew? Try this ‘Emerald Delight’ summer squash, which is resistant to powdery mildew among other viruses. Were your bush beans struck with curly top virus? Check out this ‘Gold Rush’ cultivar, which is both curly top and bean mosaic virus resistant.

Step 2: Select the Proper Spot

Often we don’t have much of a choice when it comes to site selection, as we already have a raised bed in place or are simply stuck with the conditions of our yard. If possible, selecting the proper spot can be a game changer.

If you have multiple in-ground garden options, if you are selecting a site for a new raised bed, or if you otherwise have some selection control, keep these two critical factors in mind: drainage and sun exposure.

Well-draining soil is absolutely crucial. Moisture contributes to fungal and bacterial growth, and humid-area gardeners are already predisposed to these issues. Don’t make them worse by planting in a spot with wet, poorly-draining soil. You will likely struggle with root and crown rot issues if you do.

Additionally, choose a sunny, open site. Many common garden veggies prefer full sun exposure, so this should already be on your mind. In humid environments, direct sunlight is even more important. The sun will encourage excess moisture to evaporate faster, reducing fungal and bacterial growth. 

Ventilation is also a concern. Presumably, if you’re in a well-draining, sunny spot, the airflow is likely quite strong. If not, consider planting elsewhere.

Step 3: Rotate Crops

While it may be tempting to plant tomatoes in the same spot again and again (“after all, they did so well last year!”), you must resist the urge. Pathogens can actually build up in the soil, especially when you plant the same family of plants in the same spot repeatedly.

To avoid this problem, you can plant your favorite crops in different spots each year. Or if you’re limited to just one space, you may have to switch between plant families each season.

Step 4: Space Appropriately and Provide Support

Don’t underestimate how important it is to provide each plant with enough space and support to grow. Plants need enough space to grow so that they don’t compete for nutrients, water, and sunlight. In humid areas, they also need space to help minimize the spread of disease.

A lot of diseases are foliar and spread quickly from leaf to leaf. Planting too densely and forcing the leaves of nearby plants to touch is a foliar disease waiting to happen. 

Research the amount of space each plant needs, which is often noted on seed packets, and consider giving them a few more inches of space, if possible. This will reduce the frequency of leaves touching. Also, good ventilation between plants reduces moisture and pathogen growth.

Providing support in the form of a trellis, stake, or fence is also pivotal. Your plants may have had enough space when initially planted, but as they grow, they may branch out and rub leaves with their neighbors.

Vining plants most likely need support as they get bigger. Indeterminate tomatoes are troublemakers in this arena. Keep vines and branches carefully attached to stakes or cages to avoid this phenomenon.

Step 5: Water Properly

Since excess moisture in the air is what incites so many issues in humid gardens, improper watering can make problems even worse. Your plants need to be watered, but you don’t want to introduce any more water than is necessary.

Wet leaves can easily spread foliar diseases, which is related to the spacing issue in Step 4. In humid gardens, water strategically to minimize excess moisture that creates conditions for diseases to develop. 

While you can’t exactly prevent rain from falling onto your plants’ leaves, you can at least stop watering them from above with a hose. Avoid using sprinklers, too, which also get water on leaves and contribute to waste.

Drip irrigation or soaker hoses may be your best option. These methods slowly drip water at the base of the plants, avoiding their leaves. Plus, if you set up drip tape on a timer, you can strike “water the plants” from your list of chores. Just don’t forget to turn it off during times of heavy precipitation.

If drip irrigation doesn’t make sense in your garden for whatever reason, you can still water by hand. Just try to avoid watering the leaves. Water the base of plants; do not water from above. Wand hose attachments make base-watering easier.

Step 6: Mulch to Create a Barrier

Mulch is an asset in the battle against garden diseases in humid regions.

First, a nice layer of mulch will help retain uniform moisture in the soil, which means you will not have to water your plants as often. Less watering means less wet leaves. Less wet leaves means less foliar disease. Mulch also prevents erosion that can stress plants leaving them open to disease. 

Second, pathogens often live in soil. A layer of mulch on top of the soil creates a physical barrier between the soil and your plants. You’d be surprised how much soil gets displaced during heavy rain, splashing up onto your plants and their leaves and spreading pathogens. Mulch prevents this from happening.

A lot of plant pathogens that live in soil survive all winter long. When spring rolls around, you may think you’re starting with a clean slate. Unfortunately, last year’s pathogens could still be chillin’ in the soil, just waiting to make contact.

Finally, a lot of common garden edibles, including melons, pumpkins, and strawberries, grow in such a way that their ripening fruits sit right on the soil. If your soil has last year’s pathogens, you’re going to want a layer of mulch to protect these precious fruits.

Step 7: Prune Strategically

Regular pruning is related to the concerns of Step 4, in which we discussed how important it is to provide each plant with enough space and support so that they don’t touch each other. As your season goes on, pruning will likely be required as well.

No matter how well-spaced and supported your plants are, chances are that prosperous plants will simply get more out of hand as the year progresses. Usually, this is what you want. Bigger plants have bigger harvests.

But in humid areas, it’s truly vital that plants make minimal contact with each other. You can remove any leaves that are touching each other with clean pruners. You should also remove any older leaves that are touching the soil, where pathogens may be lying in wait.

Additionally, if you discover that any branches or leaves already look sick, remove them. Snip them off, because you don’t want them to spread disease ever further to your other plants. 

During the most humid time of year, make a habit of surveying the garden every couple of days. Prune away any signs of disease before they get out of hand.

Step 8: Fertilize Regularly

Fertility may seem unrelated to humid air, but healthy, well-fertilized gardens are the most resistant to diseases. Think about it like this: If there’s a flu going around, you want to be in the best shape possible, well-rested and with a strong immune system, right? Plants aren’t that different.

Ideally, you don’t want plant disease to spread at all, but in humid environments, the spread can be inevitable. Sometimes there is only so much you can do.

Not all plants must perish, however. A weak, struggling plant may be destroyed by disease, but a stronger plant will survive. A strong plant with a little powdery mildew can still offer plenty of harvestable fruit.

Start your plants in nutrient-rich soil, enriched with compost or other soil amendments as needed. Keep a fertilizer schedule throughout the season for plants the require extra fertility.

Perhaps you apply slow-release, granular fertilizer each month. Maybe you top-off plants with a little fish fertilizer every few weeks. The key is to fertilize regularly instead of waiting until your plants are nutrient-deficient to feed them.

There are many fertilizer methods. The best method is going to be the one that you stick to. You should also consider the general fertility needs of each plant. Figure out what works best for your garden and your schedule, and commit to it.

Step 9: Control Weeds

Stay on top of weeding your garden. Falling behind is easy, especially as you get late into the season. Letting weeds grow rampant can be the reason plant diseases spread in humid gardens, as the weeds themselves can host pathogens.

Foliar diseases can be spread by leaf-to-leaf contact between weeds and garden plants. Moisture collects within weeds, which continues to spread fungal growth. Weeds also play host to a variety of insects that spread pathogens.

As weeds get bigger, they compete with your chosen plants for nutrients in the soil. As we discussed in Step 8, your plants need plenty of fertilizer to stay strong in the face of disease. Don’t let weeds absorb the nutrients you so carefully supplied.

And finally, as discussed in earlier steps, you’ve probably put significant effort into selecting a site that gets plenty of sunlight and ventilation. Weeds reduce ventilation. You’ve probably carefully spaced out and trellised your plants. Now weeds are popping up in these empty spaces.

For many reasons, it’s best not to let weeds get out of hand. Make manual weeding part of your routine. If you’re using a solid layer of mulch, as described in Step 6, hopefully, your weeds will be minimal. Either way, weed regularly.

Step 10: Sanitize Tools

Another sneaky way that fungal or bacterial diseases may spread through your garden is through infected tools. If you are frequently digging, pruning, and generally poking around in your garden, your tools may have pathogens on them.

You can sanitize cleaning tools in between each use with rubbing alcohol. Fill a spray bottle with rubbing alcohol, and give your pruners or tools a spritz in between each use. Super easy and super quick.

Another method is to clean your tools on a regular basis, perhaps every two weeks, by leaving them in a bucket of hot water with dishwashing liquid. Whatever you do, don’t let disease build up on your tools all season long.

Lastly, don’t forget about the most central tool in your garden: your hands. If you are manually harvesting, pruning, or weeding, consider washing your hands in between plants. This may be unnecessary early in the season. Once disease sets in, you’ll want to take every measure possible to avoid the spread.

Step 11: Apply Preventative Fungicide

Sometimes, you take as many steps as possible, but it’s still not enough. You may discover that certain fungal and bacterial diseases keep coming back over and over again in your humid garden. In humid gardens, more aggressive measures may be necessary.

If you just know there’s that one disease that gets you every year, consider using some sort of preventative fungicide (or bactericide or pesticide, whatever is applicable to the malady on your mind). This preventative measure can stop the spread of persistent disease.

Research which fungicides can prevent the disease that plagues your garden, and apply the preventative product in advance. Hopefully, this will stop the disease from taking hold.

You can also have your fungicide ready, and wait. Perhaps this year will be your lucky year, and with the steps outlined above, the disease won’t come back. If not, you already have the product you need in the shed. However, note that fungicides are more effective before disease is established.