BONNIE’S GARDEN – Diseases In The Garden

Our garden vegetables are prone to certain diseases. Our best defense is knowledge and good cultural practices.

Learn About Vegetable Garden Diseases

  1. Diseases are opportunistic—tending to attack stressed plants.  Starting with healthy organic-rich soil is the first line of defense.  Make sure plants are in the correct amount of sunlight and water when needed—but do not over-water.
  2. There is a valid reason for crop rotation.  Some diseases can linger in the soil for up to three years so tomato family members (tomatoes, potatoes, peppers, eggplants, tomatillos) should not be grown in the same spot more than three years in a row.  Same for cucumber family members (cucumbers, squash, pumpkins, melons).  Rotate plants sooner if diseases appear.  If you had problems with tomatoes, for example, you can plant cucumbers there next year, but plant tomato family members elsewhere.
  3. Keep your garden free of weeds and debris—summer and winter.  Weeds and debris can host diseases.  Weeds also force the veggies to compete for water and nutrients—which stresses the plant. And they can provide cover for insects to hide and lay eggs.
  4. Allow for adequate air circulation around your plants.  When you have a limited amount of space, it’s tempting to plant as many plants as possible.  But with our humidity, we are just asking for fungal disease.  Planting too close together also allows insects and diseases to move from plant to plant.  When tomato plants are around two feet tall, remove any leaves touching the ground; then mulch lightly.  This can help with pathogens that splash up on the foliage during a rain.
  5. Avoid wetting the foliage when watering your garden.  With our high humidity, why add to the problem by getting water all over our plants?  A soaker hose puts the water right where you need it, and less evaporates so it’s better for your water bill, too.  Once plants are well-established, water less often, but deeply–rather than light sprinkles every day or two.
  6. Use lower nitrogen fertilizers–particularly on tomatoes.  I use Tomato-tone because it has calcium which can help prevent Blossom-end Rot—a disease which can affect tomatoes, peppers, and eggplant. But, it’s also good all-purpose fertilizer that can be used on your entire garden—not just tomatoes.
  7. Stay on top of insect problems as insects can transmit diseases.

Despite our best efforts, sometimes we still have problems.  So what can we do?  If we are fairly certain it’s a fungal disease then garden sulfur may help.  A fungus is common early in the season, particularly when we’ve had a cool, damp start.  It’s also common in later summer when the humidity has been off the scale.  Since so many diseases start with brown and/or yellow spots, however, try to go online and look-up tomato diseases to see if your particular problem looks more like blight, Septoria, Verticillium or Fusarium.

One more thought—even a fungicide can kill a honeybee or butterfly if sprayed directly on them.  Spray just before dark for best results—this gives pollinators a change to go home first.  And avoid fungicides containing copper.  Even though copper sulfate is listed as an organic fungicide, there is evidence that it is more toxic to bees than is sulfur.

For most diseases, the only thing we can do is to remove the infected plant as soon as possible and dispose of it.  DO NOT compost it.  Put it in a plastic bag, tie it up and toss it before it has a chance to spread.  And don’t plant another member of that same vegetable family there again for several years.

Common Garden Diseases

Here is a brief description of some of the more common garden diseases—for a better diagnosis, check online for a picture of your problem.

Blight (Early or Late)

Dark spots surrounded by yellow on older leaves first.  Stressed plants are most at risk.  Do not allow plants to wilt.  Water thoroughly without over-watering.  Do not feed until the first flowers appear and use a lower nitrogen fertilizer (like Tomato-tone).  Blight can be easily transmitted when foliage is wet so avoid handling plant.  Pick off the affected foliage and treat the plant with a mild organic fungicide.

Blossom End Rot

Ever gone to pick that beautiful ripe tomato only to find the bottom is black?  That’s a disease called Blossom End Rot.  Tomato family members are prone to it.  It is caused by a calcium deficiency and/or watering issues that prevent the plant from obtaining calcium.  This is why I use the fertilizer Tomato-tone—it has added calcium.  Eggshells crushed and added to the soil will help—in several years once they have composted down.

Fusarium Wilt

Yellowing and wilting of leaves tend to occur on one side of the plant.  In early stages, top growth may wilt in sun and recover in the evening, regardless of whether or not soil is moist.  Planting in a well-draining spot is essential.  Remove and destroy the infected plant.

Powdery Mildew

A grayish white film appears on foliage.  Our high humidity is the problem.  When watering, avoid getting water on the plant foliage.  When it first shows up, remove the worst of the affected foliage and apply an organic sulfur-based fungicide—spray very early in the morning or just before dark to avoid sunburn.  Don’t use copper fungicides, as it is toxic to pollinators.  Keep weeds and debris out of the garden to avoid spores wintering over.

Septoria leaf spot

Small spots with darker brown margins appear.  Heavily infected leaves will turn brown and fall off.  Wet foliage and prolonged spells of wet cooler weather can affect.  Do not use overhead watering.  Remove and destroy infected plants.

Tobacco Mosaic

Yellowish green and dark green patches. New leaves may be “ferny” and distorted in appearance.  Infection of a garden plant can be caused by smoking near tomato family members.  Do not smoke or handle tobacco products near tomato or related plants.  Mosaic can also show up in cucumber family members.


Yellowing and wilting of leaves will occur all over-usually starting at the bottom.  Top growth may wilt in sun, early on, and recover in the evening.  Plant in well-draining soil only and remove and destroy infected plants.

This is by no means a complete list.  There are many more so garden defensively.

If you have any problems or questions, come see us!

To read more from Bonnie, visit our blog

4 thoughts on “BONNIE’S GARDEN – Diseases In The Garden”

  1. Hey Bonnie,
    That picture at the top… looks exactly like the stuff on half of my squash
    and cucumber leaves! My hubby has sprayed neem oil on them (though
    I’ve tried to remove the leaves that are coated that much, wiping down my clipper blades with alcohol after each cut), but I’m not sure that neem oil is helping. The weather has been crazy in Fxbg; hot and humid interspersed with lovely, dry, springlike days. I’m doing my best not to over water, and to keep it off the leaves. Not sure if my first attempt at a veggie garden is going to produce anything this year!

  2. Hi Kay, unfortunately squash and cukes are prone to powdery mildew and our 80% summer humidity doesn’t help! What you may try is growing a climbing cucumber and trellis it up, hereby keeping it off the ground and adding fabulous air circulation. I only get very very mild cases–or none at all–when I trellis my cukes. Squash don’t trellis well but trellising your cukes gives more ait circulation to your squash by getting the cukes out of the way.

  3. Just now seeing your response Bonnie, thanks. Yes, the humidity is the hardest part of summer for me. We’d already put up a trellis for the cukes, and they don’t seem to be as bad as the squash. Finally have some blossoms on the cukes and have been tapping them to try and encourage pollination and hopefully get some fruit, but the squash don’t seem to have anything but male flowers, and the bees and butterflies are not visiting the garden at all. We want to be all organic so the only thing my husband did was spray the neem oil, which we were told was okay for organic gardening. This is certainly a learning experience! Hopefully next year will be better.

  4. Hi Kay–yeah the humidity is a bear here! When you spray neem oil, by the way, spray about eight or eight-thirty at night–since bees and butterflies go home just before dark, it gives the good a chance to get out of there first. Otherwise, it’s a matter of waiting until the plants begin to produce some female flowers,too.–and yes, they will, sometimes it just takes a couple of weeks. But if you see no bees or butterflies, you’ll have to step in and pollinate the flowers yourself.

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