Stopping Diseases in Your Garden

By Steve Reiners, Professor and Chair, Horticulture Section, Cornell University, Cornell AgriTech. This is part of a series of columns that he wrote about vegetable gardening during the pandemic that ran Spring-Summer 2020. Read more articles from 2020 | Read later articles in this series.

May 22, 2020

There could be a fungus among us, at least in our gardens.  Bacteria and viruses too, as well as water molds and nematodes.  Plants get sick, too. So, this week, we’ll focus on plant diseases and what you can do to keep them at bay.

According to my plant pathology colleagues, there are approximately 9,000 plant pathogens in North America.  While most plant species have 80 to 100 potential pathogens, only about 5-10 of these are common problems.  I promise you that at least a few will show up in your garden. Despite this ominous outlook, there’s no reason to wave the white flag.  There are a few simple steps you can take to keep your plants healthy.

For a disease to be a problem you need three things.  The presence of the pathogen, a susceptible plant and the right environment.  These are the three sides of the “disease triangle.” And you have control over them.

Plant resistant varieties

First, plant resistant or tolerant varieties, if available.  Vegetable breeders are constantly creating new varieties with better disease resistance.  For example, look for tomatoes resistant to FusariumVerticillium, late blight, early blight and Septoria. Look for powdery and downy mildew tolerance in squash and cucumbers. This is the simplest way to stop any garden disease.


The second thing is to keep foliage dry.  Plant diseases need moisture to thrive.  Space plants adequately so you have good air circulation around them to keep foliage dry..  

Keep plants off the ground. Support plants like tomatoes on stakes or cages.  Grow cucumbers on a trellis.  Space plants so breezes blow around leaves, which means not planting too closely.

This affects watering too.  If using a sprinkler, do so in the morning so leaves dry before nightfall.  Some foliar diseases overwinter in the soil and may splash up on leaves in a heavy rain. So, use mulch to prevent splashing. And stay out of the garden when it’s wet or you can spread diseases from plant to plant.


Finally, If you see a plant showing obvious disease symptoms, remove the infected leaves or the entire plant.  How to you know it’s a disease rather than some other problem like a nutrient deficiency?  Most diseases are species specific.  So if you see corn, eggplant and squash all yellowing, chances are it’s not a disease but more likely a nitrogen deficiency.

In general, damage from disease usually starts small and expands over time.    Cornell has a great website for vegetable diseases. While some of the recommendations are for commercial growers, the pictures and lifecycle information will help you identify particular diseases and understand how they infect and spread.  Also, contact your local Cornell Cooperative Extension office.  There are knowledgeable people in those offices that can help you identify your problem.

People sometimes ask, can you get sick from eating a diseased vegetable?  The short answer is no.  It may not taste very good, but we won’t get sick.  When you hear of people getting sick from eating fresh fruits and vegetables, it’s likely from human pathogens like Salmonella that have contaminated the food.

On to warm-season crops!

Spring has finally arrived.  I think we can stop worrying about frosts until fall.  Now through June is the time to plant your warm-season crops.  That includes tomatoes, peppers and eggplants; vine crops like cucumbers, squash and melons; snap beans, sweet potatoes, and sweet corn.

If planting corn, don’t plant one long single row as this leads to ears with few kernels.  Pollen from the tassel at the top of the plant must fall on the silk at ear level.  If planted in one single row, most of the pollen falls to the ground or on plants in the next row.  Plant corn in a block with at least three to four rows so that your chances of successful pollination are increased.