In addition to samples that are under severe heat stress, we have been receiving many samples with high nematode populations. I talked about this in another post and Dr. Tomaso-Peterson also wrote a great synopsis on nematodes. We have been receiving some questions on how nematodes cause damage to plants so I’ll attempt to explain a bit using creeping bentgrass as the host of interest.
Nematodes cause damage on creeping bentgrass by feeding on the roots. Feeding by these tiny animals (roundworms) can injure and impair root functions such as water and nutrient uptake. Nematodes may feed externally or internally. Utilizing a specialized spear-like feeding structure (stylet), nematodes puncture plant cells in the roots, causing discolored areas (lesions) or swellings. Nematode activity also can predispose the grass to attack by other pathogens such as fungi and can cause the plants to be more prone to heat-related stress. Some of the most common plant pathogenic nematodes we find in putting greens in the southern U.S. include lance (Hoplolaimus spp.), ring (Cricinemella sp.), spiral (Helicotylenchus sp.), stunt (Tylenchorhynchus spp.), sting (Belonolaimus spp.), sheath (Hemicycliophora spp.), and root knot (Meloidogyne spp.). Female nematodes can lay many eggs (up to 500 eggs), which in turn hatch into juveniles. These juveniles will then mature through a series of four molts then becoming adults. Nematodes are motile in the soil for very short distances. Long distance spread occurs through surface water runoff and movement of infested soil from location to location. In non-tilled soil environments such as on a golf course, movement of infested sod is a primary method of dispersal. Generally speaking, sandy soils are more likely to have nematode problems than soils with a high clay content. Therefore, sand-based putting greens are a great environment for nematodes to flourish.
Above ground symptoms of nematode induced diseases will often resemble drought stress, general lack of vigor and little to no response to irrigation, fertilization or even fungicide applications. Symptom expression is directly related to the number of feeding nematodes. Nematode populations can increase very quickly in the summer months. The length of the nematode life cycle is partially regulated by temperature. As temperature increases, life cycles get shorter and the population of nematodes can increases at a faster rate. In addition, if we consider creeping bentgrass plants, as the heat increases, root and shoot growth can slow or stop. If roots have stopped growing (and maybe dying) due to high temperatures and there is a lot of nematode feeding, it is easy to see how creeping bentgrass plants can die very quickly without much warning. Further complicating matters, is the presence of multiple groups (genera) of plant pathogenic nematodes, which can have a synergistic effect on disease severity and symptom expression. The best way to confirm a nematode problem on a stand of creeping bentgrass is to have a soil sample evaluated. These assays are usually expressed in numbers of individuals of different nematode genera per volume of soil. Various treatment threshold numbers have been established for some nematode genera on certain turfgrass types.
Few chemical fumigants/nematicides exist as options for controlling plant pathogenic nematodes. Therefore, superintendents should practice methods to promote a vigorous root system on creeping bentgrass plants in the spring before stressful growing conditions prevail in the summer months. I have written before about managing nematode-induced stress. However I will reiterate here. Cultural management should include raising mowing heights (.160 in. or more would be advised), adjustment of the frequency of mowing and rolling to reduce the amount of stress applied to the turf stand, and continued frequent, light applications of fertilizer and water are recommended. Any practice that reduces stress and damage to turf plants will assist the plants in surviving the additional damage that nematode feeding causes. Regular soil assays should be done if nematode activity is suspected.
Dr. Nathan Walker and myself have various experimental chemical treatments that we are evaluating for efficacy toward nematodes. In future posts I hope to comment and show some data. Hopefully we will have something positive to present. Until next time…