April 2024: Dodging Plant Disease Bullets at the Greenhouse

Dodging IconOur early warm weather has me chomping at the bit to visit local greenhouses to spend money on my favorite annuals and perennials.  Greenhouse growers do their best to produce plants that are healthy and vigorous, but even with the best of care, plants can sometimes end up with unintended disease issues.  As consumers, we need to be diligent and watch for potential issues to make sure we don’t bring diseases home to our ornamental and vegetable gardens.  Here are a few tips on what to look for when buying plants to avoid problems. 

Select plants that are vigorously growing, but aren’t overly leggy. 

Stunting in plants can be caused by several types of diseases including root rots and a variety of plant viruses (more on virus diseases in a bit).  To get a better sense if plants have a root rot problem, carefully pop plants out of pots and look for root discoloration.  Healthy roots should be white, but if you see roots that are brown or black, there’s a good possibility that root rot pathogens are present.  That said, root discoloration can also be caused certain non-disease issues such as heat or salt injury, and plants with these sorts of injuries should be avoided as well.

Leggy plants may be healthy, but often this leggy growth is wimpy and easily injured, particularly as plants are moved and transplanted.  Whenever you injure plants, the wounds you create can provide entry points for a variety of plant pathogens, particularly bacterial pathogens. 

Avoid plants with leaf browning. 

Browning around the edges of leaves typically indicates that plants have been stressed for water.  This marginal browning could mean that the plants weren’t watered enough during production but can also be another symptom of root rots

Brown leaf spots can arise due to a variety of disease-causing organisms.  If leaf spots are roughly circular, then the most likely cause is some sort of fungal pathogen (and there are a myriad of fungi that can cause leaf spots and blights).  If you see spots with concentric rings (i.e., looking like a target), then you could be dealing with certain viral pathogens like Impatiens necrotic spot virus (INSV) or Tomato spotted wilt virus (TSWV)These viruses are common in greenhouse settings and are easily moved around by thrips (a type of insect). 

If you notice angular leaf spots (i.e., spots bordered by veins, giving the spot a straight-edged look), check to see if there is a yellow halo bordering the dead tissue.  If so, you’re likely dealing with a bacterial disease of some kind.  If the angular spots don’t have a halo, then you could be seeing symptoms cause by foliar nematodes

Whether spotting is caused by a fungus, a bacterium, a virus, or a nematode, don’t bring these symptomatic plants home.  All of these pathogens can potential spread to other plants in your garden. 

Avoid plants with odd leaf coloring.

Some plants naturally have variegated foliage and this is a highly prized horticultural feature.  However, if you notice plants with unexpected blotchy light and dark-colored leaves, yellow lines or yellow ring patterns, avoid these.  These sorts of symptoms are often caused by plant viruses such as Tobacco mosaic virus (TMV), Hosta virus X (HVX), Cucumber mosaic virus (CMV), and Tobacco rattle virus (TRV).  Once introduced into a garden, plant viruses can easily be spread to other plants by touch (TMV), gardening tools (virtually any plant virus), insects such as aphids (CMV), and naturally occurring soil-borne nematodes (TRV). 

Avoid plants with fuzzy growth on the leaves.

Fuzzy growth typically indicates a fungal or water mold infection of some kind. 

White fuzzy growth on upper and lower leaf surfaces can indicate a powdery mildew problem.  Powdery mildews are ubiquitous and readily develop in humid conditions that are common in greenhouse settings.  These diseases are often relatively cosmetic, but on certain plants (e.g., cucumbers, squash, pumpkin, bee balms, phlox), powdery mildews can cause substantial leaf browning and loss. 

White, gray, or purplish fuzzy growth on the undersides (and only the undersides) of leaves is characteristic of downy mildew diseases.  I routinely find downy mildew on basil wherever this plant is sold.  The first symptom of the disease that I notice is typically leaf yellowing.  When I see this symptom on basil, I then flip the leaves over to look for typical fuzzy gray/purple growth on the undersides.  Downy mildews tend to be more destructive than powdery mildews, so it’s best not to bring these diseases home on your newly purchased plants. 

Don’t panic. 

Most plants available at your local greenhouse and nursery will be healthy and disease free.  With just a little care, you can avoid those rare diseased plants that may pop up, and end up buying healthy, pathogen-free plants that will provide months, if not years, of gardening enjoyment. 

Need help?

For more information on plant diseases and their management, check out the UW-Madison PDDC website (https://pddc.wisc.edu/).  Alternatively, if you have disease questions, feel free to call (608) 262-2863 or email pddc@wisc.edu for advice and guidance.  To keep up to date on clinic activities and resources, follow the PDDC on Facebook or Twitter (@UWPDDC), or subscribe to the clinic’s listserv, UWPDDCLearn (by emailing or phoning the clinic to subscribe). 

Happy shopping everyone!