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BONNIE’S GARDEN – Vegetable Garden Diseases: What To Do, How To Prevent

By Bonnie Pega
Bonnie's Garden

I’ve already had several questions about how to avoid problems in vegetable gardens, so thought it seemed timely to repost this blog on preventing and/or treating problems.

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

  1. Diseases are opportunistic—tending to attack stressed plants.  Starting with healthy organic-rich soil is the first line of defense. Be sure plants are in the correct amount of sunlight and watered when needed—but not over-watered.
  2. There is a valid reason for crop rotation.  So many diseases can linger in the soil for up to three years so tomato family members (tomatoes, potatoes, peppers, eggplants) should not be grown in the same spot more than three years in a row.  Same for cucumber family members.  Rotate plants sooner if diseases appear.
  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 at soil level; 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 aggravate the situation 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. By the way, it’s a 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.  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.

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): Circular or irregularly shaped 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 fertilize until the first flowers appear.  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.

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.

Verticillium: 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.

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.

Powdery Mildew: A grayish white film appears on foliage.  Our high humidity is the culprit.  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.  Keep weeds and debris out of the garden to avoid spores wintering over.

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 calcium deficiency.  This is why I use Tomato-tone—it has added calcium.  Eggshells crushed and added to the soil will help—in several years once they have composted down.

Next week, I’ll talk about common vegetable garden insect pests.

7 thoughts on “BONNIE’S GARDEN – Vegetable Garden Diseases: What To Do, How To Prevent

  1. Thanks so much for your disease prevention advice.
    Last year I had early blight. Since I don’t have a big garden, but want lots of tomato varieties, I tend to put tomatoes fairly close together. How close can I put tomato plants from each other?

  2. I try to space my tomatoes about three feet apart, if I can. I don’t want them touching each other.

  3. Thanks so much for your reply. Mine are about 18″ apart. Should I dig them up? I just don’t have much room in my raised bed, and I love having lots of tomatoes.

  4. It’s hard to say whether or not you should spread them farther apart. It depends on how long they’ve been in the ground, etc. You may want to leave them, but keep a very close eye on them for problems. If you get blight again this year, next year do not plant another tomato family member in that spot! Plant them in pots, if you want (give me a call and I’ll tell you exactly how).

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