Skip to main content

Plant pathology basics: Some plants are diseased, which ones are now at risk?

By Marissa Schuh, Integrated Pest Management Extension Educator.

Now that most of the state has gotten rain in that last week, vegetable diseases that have been held back by the drought are rearing up.  The appearance of any disease often prompts questions about how far that disease can spread and what plants are at risk.

Most Plant Diseases are Very Host Specific

For a plant disease to show up on a farm, three things need to be present: the host plant, the disease-causing pathogen, and the right environmental conditions. 

A disease can only show up on your farm if a susceptible host plant is present.  The relationships between a pathogen and plants that it can make sick are often limited to one plant family (this is why rotation helps manage soil-borne disease).

Another way to think of this in terms of other living things that get sick. Think of you and your pet.  Most illnesses a dog or cat get won’t spread to you, though there are some that can.  The same is true of plant diseases.  Most plant diseases are specific to one species or family of plants, though there are a couple of diseases that can impact multiple plant families. 

For example, powdery mildew is widespread in the Minnesota landscape right now.  You can easily find it in vine crops, ornamental plants, and even weeds. While these plants all have similar symptoms, it is different species of powdery mildew causing them.  This means that the powdery mildew you see in wild grapes isn’t about to jump into your pumpkins. Your pumpkins may still get powdery mildew, but this is a reflection of cucurbit powdery mildew spores floating in the air finding the right weather conditions.

The diseases that affect multiple plant families are often soil-borne and present long term management challenges, like white mold and phytophthora.

Phytophthora is capable of affecting vegetables in the legume, cucurbit, and solanaceous families. Photo: Marissa Schuh, University of Minnesota Extension.

Same common name, but a different pathogen.

Making the whole situation more complex is that there is a lot of overlap in the common names of many diseases.  For many vegetables, there are diseases called downy mildew and Alternaria, but these diseases are actually caused by different (sometimes related) pathogens.  

The dark, velvety spores of basil downy mildew. Photo: Rebecca A. Melanson, Mississippi State University Extension,

It is like if we called all members of the brassica family brassica at all times, even to customers.  We aren't lying to them, but we wouldn't be capturing the nuance of how cabbage, broccoli, and radishes are grown and can be used in food.

In the case of diseases, their common names (late blight, downy mildew, etc.) are easy to say and remember, but leave out some of the species-specific nuance of each pathogen and the disease it causes.

A different species of downy mildew produces dark, velvety spores on cucurbits. Photo: Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo,

Downy mildew is a case where we have multiple diseases with the same name, which can be stressful. We are now seeing one type of downy mildew in Minnesota.  The moisture has allowed downy mildew to show up in basil.  During cool, dewy times downy mildew is especially easy to identify in basil.

There is also a disease in cucurbits called downy mildew.  This is caused by a different pathogen than the ones that causes basil downy mildew.  The situation gets even more complex when you start to research cucurbit downy mildew.  Within this disease, there are two strains, one that primarily infects cucumbers and melons, and one that primarily infects winter squash and pumpkins.  This means that even if downy mildew is seen on Minnesota cucumbers, there isn’t reason for a pumpkin grower to panic.  Regardless, neither strain has been reported in Minnesota or a neighboring state this growing season, so the risk remains low.

Alternaria (species brassicicola) on chinese cabbage. Photo:Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo,

In the case of Alternaria, these are all pathogens that are closely related, but each individual species has its own biology and plants that it makes sick.  There is one species of Alternaria that is specific to brassicas that causes leaf spot and head rots, while another species of Alternaria is responsible for early blight in tomatoes.  Just because you are seeing Alternaria in one spot in your farm doesn’t mean every vegetable is going to get it.  It means at least one species of Alternaria is present, and the environmental conditions are friendly towards it.

Alternaria (species solani) in tomatoes, also known as early blight. Photo: Rebecca A. Melanson, Mississippi State University Extension,

Most diseases like the same environmental conditions.


The disease boom we are seeing now is the result of the right environmental conditions.  Host plants are established, and pathogens may be present in the air or soil as spores.

What are the right conditions? There are a lot of commonalities between diseases.

  • Moisture, be it on the leaves (for foliar diseases) or in the soil (for soil-borne diseases)

  • Lack of airflow, because this slow leaf drying, meaning more moisture 

  • Certain temperature conditions (this varies by specific disease)

Why are diseases showing up in the last week? We have water again! 

Extending the hours in a day when leaves are wet with dew and irrigation encourages disease. Photo:  Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo,

So, how much will a disease spread?

As said above, we now have the environmental conditions for more diseases. If you are seeing the disease, the pathogen is present on your farm.  How far it will spread is dependent on what the potential hosts are on your farm for that disease.  How do you figure out what plants can get sick? You have to figure out what pathogen you are dealing with.

The first step is getting a diagnosis.  For many diseases, the only way to get a solid diagnosis is via the plant disease clinic, though some diseases have very characteristic symptoms.  Natalie and I are always available to look at pictures and help you work through what you are seeing.

Seeing the pathogens themselves isn’t possible for most diseases without lab equipment. This is the spore a diagnostician would seek out if diagnosing cucurbit downy mildew. Photo: Penn State Department of Plant Pathology & Environmental Microbiology Archives , Penn State University,

Once you have an idea what the disease is, you get an idea of management tactics and how far a disease could spread.  A good general practice is removing diseased leaves and entire diseased plants.  More specific ways (including pesticides!) to prevent spread will depend on the disease.  

How do I navigate all this complexity?

Plant diseases are hard, and plant pathologists are few and far between.  Finding diagnostic tools that work well for the way you think is key.  The UMN offers basic and more in-depth webpages.  Always feel free to reach out to Extension if you have questions as you work through the complexities of what is happening on your farm. 

Take note of what diseases you are seeing now, and think about how crop planning, variety selection, and other cultural practices can help you remove elements of the disease equation for next growing season.

Print Friendly and PDF