May: Heinous Hitchhikers – Purchased Plants as Pathogen Providers

As we get into May, many gardeners begin thinking about buying perennials to replace plants that have died over the winter, or annuals to fill decorative pots and hanging baskets.  Being the optimist that I am (NOT), whenever I’m visiting my local nursery or garden center, I’m always evaluating plants as potential sources of plant pathogens.  For me, having diseased plants can be good (demo plants for classes and workshops, anyone?).  For sane, rationale gardeners however, avoiding potentially diseased plants is a must.  Here on some pointers on what you can do to try to minimize the likelihood that you will bring home unwanted guests as you garden.

  • Buy plants from a reputable business. Most businesses want to sell a good product.  The livelihood of a nursery or greenhouse depends on the quality of the products that it sells.  If a business sells poor plant materials, this reputation will get around and the business will likely not last long.  One way to decide on where to shop is to check with friends or colleagues on where they have purchased high quality plants in the past.  Word of mouth is often the best recommendation for a business.  That said, keep in mind that sometimes even the best, most conscientious plant producers/sellers can have disease problems.  In the past, diseases like Ralstonia wilt and impatiens downy mildew have been serious, and economically devastating diseases, for producers/sellers and no one has been immune to these disease issues.
  • Buy locally, when possible. Locally produced plants are often better adapted to the local climate, which can translate into better survival long term for perennial plants.  For annuals, plants grown in southern regions are often more likely to be exposed to pathogens earlier during production, with more opportunities for infections to occur.  These pathogens can travel north with plants as they are shipped into Wisconsin for sale.  Introductions of the Southern blight fungus and late blight pathogen have occurred in this manner in the past.
  • Avoid plants showing disease symptoms. Look carefully for any abnormalities in plant size, growth, or color that might indicate disease issues.  Common disease symptoms can include necrotic (i.e., dead) areas on leaves that might indicate a fungal, bacterial, nematode, or even viral  Also watch for lightning bolt-like line patterns, ringspots, or just blotchy light and dark patches on leaf tissue.  These symptoms are typical of viral diseases such as tobacco rattle, cucumber mosaic and hosta virus X.  Be sure to pop plants out of their pots to inspect the roots.  Roots should be plentiful and white.  If the roots are few and far between, or even worse, brown, then root rots could be a problem.  If you see abnormalities of any kind, DO NOT buy the symptomatic plants.
  • Avoid plants showing signs of disease-causing organisms. Some types of pathogens, particularly fungi and water molds, can produce spores on plant surfaces that will be visible to the naked eye.  Typical diseases where pathogens might be visible include powdery mildews and downy mildews, including the infamous impatiens downy mildew and basil downy mildew.  If you see any indication of this sort of growth, again DO NOT buy the plants.
  • Avoid plants with insects. Insects can cause damage to plants on their own through their feeding activities, so it is important not to bring home these pests with your plant purchases.  Insect pests can spread to other plants in your garden and cause substantial damage on their own.  From a disease standpoint, insects are important because they are plant pathogen vectors, moving disease-causing organisms from plant to plant as they feed.  There are insects that are known to move fungal, and bacterial pathogens in the environment, but where insects tend to have their biggest impact is through movement of viral and phytoplasma  In particular, aphids and thrips are important in moving viruses such as Cucumber mosaic virus and Impatiens necrotic spot virus from plant to plant.

With a little bit of effort and by using good observational skills, you can minimize the risk of bringing diseased plants into your garden.  However, even if you follow the advice outlined above, purchasing plants is not totally risk-free.  Sometimes plants harbor disease-causing organisms with nary a symptom nor sign in sight.  These pathogens may rear their ugly heads and start to cause problems once you’ve begun growing the plants in your garden.  Even if you dodge the bullet and successfully avoid purchasing infected plants, know that Mother Nature has tricks up her sleeve to bring plant pathogens to you.  So expect at least a little bit of disease, no matter how careful you are.  In the end though, plant diseases tend to be the exception and not the rule, so remember that most of time when you look at your garden, what you will see will be healthy and happy plants.  KEEP ON GARDENING AND LOVE EVERY MINUTE OF IT!

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