March: The Irish – Good Luck in Life, Bad Luck in Plants

As March arrives, being in part Irish by ancestry, my thoughts tend towards St. Patrick’s Day and as a plant pathologist, I imagine what havoc plant disease might cause for the holiday.


ShamrocksA major symbol of St. Patrick’s Day is the shamrock.  While several plants can be called shamrocks, the most common plant to be so-named is white clover (Tifolium repens).  This plant was once a common component of lawns (in combination with grasses such as Kentucky bluegrass) and served the important function of enriching soil with nitrogen.  Interestingly, it’s not the clover plant itself that is instrumental in this nitrogen enrichment process.  Actually, the credit goes to the bacterium Rhizobium which colonizes the roots of clover (along with the roots of other plants in the pea family) and causes formation of nodules (swellings) on the roots.  Inside the pinkish, elongate nodules, Rhizobium takes nitrogen gas (which is very common in the air) and converts it to a form of nitrogen that is more easily used not only by the bacterium, but by the clover plant it colonizes.  In exchange for this ready supply of nitrogen, the clover plants provide Rhizobium with sugars (produced through photosynthesis) that it needs to grow and reproduce.

Root-knot nematodes cause swollen, distorted roots that can interfere with movement of water and nutrients within a plant.
Root-knot nematodes cause swollen, distorted roots that can interfere with movement of water and nutrients within a plant.

This interesting symbiosis between clover and Rhizobium, can be disrupted by the plant pathogenic nematode Meloidogyne, more commonly known as the root-knot nematode.  Nematodes are small (typically microscopic) worm-like organisms.  Many nematodes are beneficial, but root-knot nematode infects the roots of a variety of plants (including clover) causing damage.  Root-knot nematode females tunnel into roots and set up feeding sites.  In the process of feeding, they secrete saliva that stimulates root cells to grow larger than normal, grow faster than normal, and divide like crazy.  This uncontrolled growth leads to a tumor-like swelling on the infected root (called a gall or knot).  Formation of the galls can interfere with root function (i.e., movement of water and nutrients to leaves and stems above ground) and can also interfere with proper nodulation by Rhizobium.  Thus plants with root-knot nematode often look stunted and discolored due to nutrient deficiencies caused by the presence of the pathogen.  You’re not going to find a lot of four-leafed clover leaves on plants with root-knot.



The food that comes to my mind as a symbol of St. Patrick’s Day is corned beef and cabbage.  While I can’t say too much plant pathological about beef, cabbage is another matter.  The primary disease that I can think of that would prevent you from enjoying your cooked cabbage is black rot.  I have seen an amazing increase in the incidence of this bacterial disease over the past five years or so.  The disease not only affects cabbage, but virtually all types of brassicas, the group of plants that includes cabbage, broccoli, cauliflower, kale, rutabaga and turnip, as well as weed plants such as shepherd’s purse and wild mustard.  Often the causal bacterium (Xanthomonas campestris pv. campestris) comes into a garden on contaminated (but asymptomatic) seed or transplants.

Black rot causes V-shaped yellow and brown/ dead areas in affected leaves. (Photo courtesy of Amanda Gevens)
Black rot causes V-shaped yellow and brown/ dead areas in affected leaves. (Photo courtesy of Amanda Gevens)

Eventually wedge-shaped yellow, then dead areas develop on leaves or other plant parts leading to deterioration of the plant.  Black rot can be followed by soft rot (another bacterial disease), leading to even more extensive damage.  It’s not a happy day when cabbage with black rot and soft rot arrives in my clinic.  The stench is overpowering!  Good debris clean up, decontamination of gardening tools, proper weed control, proper vegetable rotation, and hot-water seed treatments can all help in managing this disease.


And no discussion of the Irish would be complete without a mention of late blight, the cause of the Irish potato famine.  This devastating disease wiped out the Irish potato crop for several years in the 1840’s and 1850’s.  For a variety political and social reasons, potato was the primary food of the Irish during this period.  Loss of the crop due to late blight led to the starvation of over 1 million Irish and the emigration of over 1 million more Irish, many of them to the US.  I am sitting, writing this article in Madison, WI due to this disease.

Late Blight on Potato Tubers, photo courtesy of Prof. Amanda Gevens
Late Blight on Potato Tubers. (Photo courtesy of Amanda Gevens)

Even today, late blight can have a huge negative impact on both commercial and home garden potato (and tomato) production.  Without proper treatment the disease can wipe out entire potato and tomato patches/fields in a matter of a few days.  It is critical therefore to identify any occurrences of the disease in Wisconsin as early in the growing season as possible and also identify which variant(s) (and there are many) of the pathogen is(are) causing problems.  For that reason, my clinic provides free diagnoses for late blight for anyone growing potatoes and tomatoes in Wisconsin.  All you need to do to get the free diagnosis is send in a potato or tomato sample and invoke the words “late blight” and the diagnosis is free.  Even if you don’t think your potato or tomato problem is late blight, send in a sample, mention “late blight” and I’ll provide a diagnosis and management recommendations for free.  You can send samples to:

Plant Disease Diagnostics Clinic
Department of Plant Pathology
University of Wisconsin-Madison
1630 Linden Drive
Madison, WI  53706-1598

As always, if you have questions, feel free to contact me at (608) 262-2863 or

With that, go forth, wear green, drink green beer, think about the contributions that the Irish have made to US culture and of course, don’t forget about the all-important plant diseases.  Happy St. Patrick’s Day!!

Shamrock and Claddagh