What Kind of a Toad is a Nematode?

No. 6 at Keller Golf Course in St. Paul Although our random probing with a Hoffer tube of 142 greens on 36 golf courses back in 1969 and 1970 did work to provide documentation that just about every established green (139/142) that we sampled was infested with plant nematodes, I am not necessarily comfortable in recommending such "randomness" with regard to sampling. Plant pathogens (organisms that cause plant disease) of all types (and plant nematodes really can be the cause of plant disease) rarely are evenly distributed across any piece of "plantscape" including a green. Soil cores possibly collected from "hot spots" as well as "cold spots" across a green may provide an "average" that may mask a significant potential stress with which the turfgrass manager may have to deal. Or, in other words, averages include "extremes" which, in the case of plant nematodes, may be growth-limiting or "predisposing" populations. That fact became very evident back in 1995 when two soil samples cut with a cup-cutter arrived at the Plant Disease Clinic (PDC) in Stakman Hall. The samples were taken from two of the USGA greens at Keller Golf Course that had been rebuilt in 1991 and sodded in either the fall of 1991 or the spring of 1992. The sod that was used had been grown on sand and theoretically should have grown well when laid on the sand of the USGA greens. By the summer of 1995 several but not all of those 12 greens had become essentially unplayable during the heat of a summer that Charlie Pooch (Superintendent at Les Bolstad course) described as being "a beast of a year for growing turf grasses." The Keller superintendent at that time felt that he had explored all of the chemical and cultural options other than application of a nematicide that existed to him. And so in early September he collected a soil sample from each of the two worst greens to learn if plant nematodes could be "at the root of his problem." Ms. Sandee Gould, Chief Diagnostician of the PDC at that time, asked for advice about how she should proceed to process those samples for plant nematodes. Essentially she was asking: "How do I proceed with this bag of loose sand topped with a very thin disc of sick plants with unhealthy roots?" We decided that she should try to get 100 cm-3 of sand from each "thin disc" and process it for nematodes. One sample in particular turned out to be from what some plant pathologists would call a very "hot spot" on Green #8. It contained in excess of 5,000 plant nematodes. Most of which were stunt nematodes, (tylenchorhynchus spp.) which is the most common plant nematode inhabiting Minnesota's golf greens. No other sample from Keller has ever been found to contain anywhere close to that many nematodes. The other cup-cutter sample from Green #18 also came from a "bad" area because it contained over 1,100 stunt nematodes. How did the superintendent find such "hot spots"? Was it luck, skill, chance? Could he have repeated his sampling prowess if he had gone out a second time? I believe that the "hot spot" sample came from the left rear portion of Green 8 where the turfgrass may have been a bit more healthy than it was in the center of the green. But why there? If the superintendent had a specific reason for sampling there that reason has been lost. At any rate the plant nematode populations present in those two samples from greens that were visually "in trouble" dramatically re-aroused our interest in the plant nematodes that can parasitize turfgrasses. It seemed to be a classic example of a disease that could be caused by plant nematodes: 1) the grass started off OK in spring when plant stresses that adversely affect the growth of turfgrasses tend to be minimal and plant nematodes in the cool soil tend to be sluggish. 2) The amount and quality of plant growth declined as the various plant stresses including nematode activity built-up with the heat of June. 3) Nothing that the experienced superintendent did with fungicides and watering had any beneficial effect on the diseased turf. 4) The grass started to come back in September with cooler conditions that are more favorable for the growth of turfgrasses and which slow down the activities of plant nematodes. And 5) the entire recently published issue of TurfGrass Trends (Volume 4, Issue 10, October, 1995) was devoted to "Nematode Disorders of Turfgrasses: How Important are They?" by Eric B. Nelson. Table 4 in that article listed "Damage Thresholds for Various Nematodes on Representative Cool-season and Warm-season Turfgrasses". The threshold for damage by the "Keller Nematode," a stunt nematode, was listed as 300/100 cm-3 of soil. The stunt nematode populations detected by two "cupcutter" samplings thus exceeded a published threshold for damage by a factor of at least 3 to about 17. Our interest in plant nematodes in putting greens probably would not have been "jump-started" if the Keller superintendent had done his collecting by combining 10 randomly-selected one-inch diameter "Hoffer tube" cores in one bag. More intensive sampling (one sample from each of the 18 greens) in the fall of 1995 revealed that the average Keller "Hoffer tube" sample contained about 409 stunt nematodes/100 cm-3 of soil which could be considered by some as being sort of a "Ho-Hum" population. And the randomly collected Hoffer tube samples from those two worst greens (#8 and #18) only contained 831 and 233 stunt nematodes/100 cm-3 soil, respectively. In defense of the cup-cutter sampling technique, it did provide data documenting what could happen in terms of multiplication and survival of plant nematodes in two Keller greens as affected by the environment, the management practices and amount of play that existed in 1995. And