Plant Parasitic Nematodes in Cool-Season Climates

Just last week we received a sample at the GTI Diagnostic clinic which was requested to be tested for signs of pathogens.  The sample came back negative, and so our diagnostician decided to run a nematode count in search of more answers.  Upon completion, the diagnostician had counted approximately 15,000 spiral nematodes per 100 cc of soil – a level that is almost 10x over the accepted damage threshold level!  This recent diagnosis prompted us to write an article for the OGSA, as well as produce this blog post, as more and more turf managers in cool season climates are discovering their existence and role in the season long turf stress equation.

Above: Spiral nematodes taken from a golf green in South Eastern Ontario with population levels well beyong damage thresholds. Left: nematodes in water solution after extracting them from 100 cc of soil. Right: spiral nematodes visible under microscope  Photo credit: Guelph Turfgrass Institute

BACKGROUND ON NEMATODES IN THE NATURAL WORLD AND IN TURF

Nematodes are one of the largest groups of organisms that we know of on the planet.  They have existed on Earth, relatively unchanged for millions of years and populate just about every environment we have. In fact, nematodes are so ubiquitous, it has been said that if one was to remove all the matter on earth, except for nematodes, the planet and all of its forms and inhabitants (trees, oceans, mountains, animals, humans, and even the planet itself) would still possess much of their rudimentary shapes and characteristics - just entirely made up of nematodes!  Yet, despite their numbers and prevalence in our world, we still struggle to gain full understanding of these microscopic worms.  The intention of this post is to expand on the existing discussion regarding plant parasitic nematodes, or PPNs, as a pest in cool-season turf.    

The topic of PPNs and their threat to turf in cooler climates, such as those of Canada, is an interesting one.  In the agricultural sector, PPNs are well-known and documented with conservative reports of annual damage estimates in excess of $1 billion world-wide.  In warm tropical climates, damage from PPNs to turf swards is also well documented.  PPNs thrive in warm environments and have been haunting golf superintendents, sports field managers and lawn care operators in these climates for many years. But for golf superintendents in cooler, Northern climates, the nematode puzzle is a bit foggier.  Information on life cycles, distribution and damage thresholds of PPNs in Northern climates is far less documented and researched.  In the South, nematodes are considered to be a primary damage causing pest – meaning the symptoms and threats to turf health are, in most cases, caused primarily by the nematodes.  On the other hand, in the North, it is quite rare to find cases where damage and symptoms are caused primarily by the nematodes themselves. The reason for this is that our cooler climate can restrict the potential for PPN populations to grow to high enough levels to cause significant damage.  A major contributor to this limited growth of populations we owe to our extended winters which can drastically inhibit the life and reproductive cycle of these organisms in our soil.  But this doesn’t mean they aren’t causing problems.

It is no secret to turf managers that there is a constant and aggregated barrage of various biotic and abiotic stressors ebbing and flowing on their turf throughout the season.  Good management programs are the result of considering and identifying these various stressors and managing them individually and collectively in order to reduce the overall stress-load your turf sustains – this is essentially the core of integrated pest management theory. Much time and resources are invested annually in balancing this carefully considered equation of stressors. Traffic, various maintenance practices, drought, shade, temperature, compaction, soil chemistry, thatch, drainage conditions, humidity, air flow, insect pests and disease pressure are just a few examples of the complex algorithm of stress any single grass plant endures in a growing year.  I don’t expect any of these to be new concepts for any of you. But take a moment and ask yourself how often you consider the potential populations, damage or stress caused by PPNs in your soils.  Do you have any idea how your populations expand and contract throughout the year? Do you know what species tend to inhabit your greens? Have you ever sampled a green?  If your answer is no to these questions, don’t feel bad – you are with the majority.

THE CHALLENGE WITH PPNs IN COOL-SEASON CLIMATES

The biggest challenge with PPN in cool climates is that in most cases there are no clear patterns associated with their damage.  That is to say, observable symptoms are not as recognizable as other insect pests like annual bluegrass weevil or pathogens such as brown patch or dollar spot.  In many cases damage from PPNs often appears as a general yellowing or thinning, diffuse patches, somewhat typical of a number of other potential stresses or pests – especially root pathogens. In an article written for GolfDom in 2017 by Dr. Nathaniel Mitkowski of the University of Rhode Island, Dr. Mitkowski discusses some of the frequent challenges associated with identifying nematode issues in greens. Mitkowski confirms that in most cases when root-rot style symptoms are observed in the field, PPNs are not on the turf managers usual list of suspects.  As a result, thorough drenching of fungicides are applied, and it is only later when symptoms have not improved that we tend to consider the possibility of other culprits such as nematodes.  For whatever reason, nematodes are still not part of the mainstream vernacular when it comes to cool season turf – but make no mistake, there are here in our soils – YOUR Soils.

Above: Classic ambiguous stress patterns caused by PPN which can often resemble a number of other stressors such as root pathogens, heavy traffic, heat or drought.  This difficult to identify characteristic of PPN symptoms make them more challenging to accept as a legitimate pests in cooler-season climates.

Over the years, a number of studies have been conducted on PPNs affecting turf in Ontario. Particularly of interest was a study done by Dr. Katerina Jordan’s research team in 2013 funded in part by the Ontario Turfgrass Research Foundation.  Here is a quick highlight of some of the findings from that study:

• 99% of samples taken from Canadian golf courses possessed the presence of PPNs
• 5 predominant genera of PPNs were identified.  They were: Spiral, Ring, Stunt, Root-knot and Cyst nematodes
• Populations of PPNs were highest during summer periods
• Samples from greens between ages 50-100 years had the highest populations
• Samples taken from coastal regions appeared to have highest populations (Vancouver especially due to warmer winter periods)
• Rolling frequency and percentage of poa annua populations tended to be correlated with nematode populations
• Soils more likely to hold moisture through available pore space was also linked to nematode populations

The significance of Dr. Jordan’s study was that it was specific to Canadian golf courses and confirms not just the presence of PPNs in our soils but also supports the growing recognition of PPNs as a potential variable in the stress equation of cool season turf. Until recently, PPNs were not even considered to be a pest to cool-season turf.  After being involved in a number of different research projects regarding PPNs in both the agriculture and turfgrass industries, Dr. Katerina Jordan now firmly believes that PPNs have always played a significant role in the struggles our turf endures, we just did not recognize it until now.  As cases of confirmed PPN damage continue to rise, Dr. Jordan hypothesizes that the recent increase of cases is a result of the pressures to reduce heights and speed up greens. As grasses continue to be pushed to the edge or survival, their risk and susceptibility to all stressors, including nematodes will increase.  As a result of her research findings and the growing interest from the golf community, Dr. Jordan was inspired to expand the GTI Diagnostics Clinic services by offering a nematode population sampling program.  It will be through the nematode sampling services, continued education and the increasing number of cases of PPNs being observed that will help this pest become more of a recognized threat to cool season grasses and hopefully generate additional funding to continue the needed research in this area.

TESTNG FOR NEMATODES SHOULD BE IN YOUR VERNACULAR

The GTI Diagnostic Clinic recommends that all golf superintendents consider testing their weakest or most susceptible greens for the presence of PPNs on a regular basis.  The goal of sampling is not about confirming their presence in your soils but to determine the diversity of species that populate your soils and gauge their numbers. Over time, PPN population data (especially at different points of the year) can help to paint a picture of how populations expand and contract - We call this benchmarking.  Developing thresholds with PPNs is a bit complicated. There are too many variables influencing a turf’s susceptibility to PPNs to develop a standard set of thresholds.  According to both Dr. Jordan and Dr. Mitkowski details like host grass species, soil pH, soil bulk density, soil physical properties, soil moisture content, shade, mowing heights, traffic, water quality, cultural practices, fertility etc… all play a role in how your turf may be affected by PPNs.  Dr. Mitkowski uses this example in his 2017 article in GolfDom: “While 3,000 stunt nematodes per 100cc of soil likely place significant stress on Poa annua plants growing in a push-up soil with only one-half inch of root in the middle of July, the same number of nematodes is unlikely to have any effect on a sand-based creeping bentgrass (Agrostis stolonifera L.) green with a rooting depth of 5 inches in the fall.” The point being that each property, green, or even sections of a green can have varying levels of susceptibility with regard to PPN. Beginning the testing program now will help you to better understand how your own PPN populations fluctuate throughout the season on various sites, as well as helping to determine how they may play a role in the overall stress equation of you turf. According to Dr. Jordan, it is only through regular sampling and testing that quality benchmarking can be determined. More so, it is only through regular benchmarking that individual site thresholds can begin to take shape.

For your interest, we have collected a range of accepted damage thresholds for PPNs and placed them in a helpful chart below.  It is important to recognize these figures are still oly approximations due to the extreme variation of tolerance based on location, growing conditions, host species, soil types, and additional abiotic and biotic stressors.

If you wish to know more about sampling for nematodes on your property, please refer to the GTI Diagnostic Clinic page for FAQs, sampling techniques and additional information. Feel free to reach out to the Guelph Turfgrass Insitute if you have additional questions or concerns regarding PPNs.