As with most pest and disease problems in agriculture, the control of plant-parasitic nematodes is viewed in the mindset of poisoning and eradication. Near the turn of the century in 1871, Julius Kuhn demonstrated the nematicidal qualities of carbon disulfide. Mrfac. J.D. Mathews in 1919 followed with pioneer studies on the nematicidal effects of chloropicrin. Dichloropropene dichloropropane (D-D) found its place among nematicidal fumigants through the efforts of Walter Carter in 1943. Dow Chemical Company followed through introductions of such fumigants as ethylene dibromide (EDB), Nemagon (DBCP) and methyl bromide.
Accustomed to the substandard growth of crops in nematode-infested soils, the ensuing contrasts following fumigation were nothing short of a dramatic improvement. Fumigation further, expressed the magnitude of crop debilitation rendered by plant-parasitic nematodes (PPN). In the 1970s and 1980s several non-fumigant nematicides such as Temik, Mocap, Vydate, Furadan and Nemacur found their way into commercial agriculture. However, as we gained more knowledge and awareness of health, environmental and crop productivity considerations, the fumigants and non-fumigants have slowly and progressively faded from agronomic use.
This article summarizes over 38 years of personal research on deciphering key biological characteristics of plant-parasitic nematodes and beneficial microflora, which can serve as a foundation for developing effective and safe management-control regimes.
Key Biological Characteristics of Plant-Parasitic Nematodes
Plant-parasitic nematodes (PPN) represent the pinnacle stage of development in the dynamic progression of species from the ancestral to the derived, or what some mistakenly refer to as primitive to advanced. A PPN, then, wherever the particular species resides on the branch tip of the evolutionary tree, has become completely dependent on the plant host for its sustenance, a condition known in biology as ‘obligate parasitism.’
In contrast to obligate parasitism are the ancestral organisms, which can secure their food from multiple nonliving sources, a condition known as ‘saprophytism,’ with such organisms referred to as ‘saprophytes.’ Midway in the progression from the ancestral saprophytism to the derived, obligate parasitism, reside species that can secure their food from both nonliving and living sources, a condition known as ‘facultative parasitism,’ which literally means ‘conditional parasitism.’ In review then, we have the following dynamic, biological, evolutionary progression:
Saprophyte → Facultative Parasite → Obligate Parasite
The significance of these 3 developmental groups is their differential capabilities in successfully adapting to changing environmental and/or biochemical stimuli. The ancestral saprophyte is endowed with the most expansive and flexible genome for adapting to numerous and varied stimuli. The intermediate facultative parasite is also intermediate in its adaptability, while the group most specialized and with limited adaptability is the obligate parasite.
This phenomenon of ancestral, expansive adaptability transitioning to facultative, intermediate adaptability, and finally to obligate, specialized, limited adaptability is characteristic of the dynamic progression of all living entities in their transitional development from the ancestral to derived. Literally the progression can be described as “complex to simple.” That is, organisms not only develop unique physical and physiological adaptations in response to the particular niche they occupy, but in many cases the guiding force is a search for reducing competition.
The ancestral niche represents one of intense competition from numerous saprophytes vying for sustenance and space. Facultative parasites have minimized competition for food and space by being endowed with unique enzymes that allow them to also secure an occasional meal from a living host. The obligate parasite limits competition for food and space even more as its entire digestive machinery has become specialized to feed only on living hosts. In this transition from the complex to the simple, then, organisms gain an advantage through specialization.
Further, coinciding with the biological laws of an organism’s efficiency, then, it would be energy expensive for the derived species to attempt to hold onto both derived and its corresponding ancestral characteristics. Thus, obligate parasites with their specialized, derived characteristics, also host a limited flexibility for adaptation to multiple environmental and biological stimuli.
Key Developmental Characteristics of Plant-Parasitic Nematodes and Adaptations to Plants in Nature
The course of development of the PPN has occurred over hundreds of millions of years. Further, this progressive development and adaptation to host and niche has occurred in nature in parallel with plant hosts that can at best be rated as mediocre to substandard in physiological processes. Characteristic of plants operating at suboptimal physiological efficiency is a simple carbon compound constitution of the sap. Thus, PPN have for hundreds of millions of years progressed in their development within the confines of a simple carbon compound, sap constitution. This means that their digestive and enzymatic systems have evolved within the realm of this physiological status of host plants.
In working with various plant species and adjusting their water and nutrient requirements to near perfect balance and overall status, I have found some key physiological changes in these physiologically superior plants. Plants hosting superior and efficient physiology are capable of:
- Engaging their defense response within a shorter period of time.
- Developing a complex, as opposed to simple, carbon compound sap constitution.
- Developing numerous carbon compounds and their derivatives, many of which can be classified as direct resistance compounds.
- Engage physiological and physical healing of damages to their tissues within a short period of time.
Once a plant is raised to a superior physiological status, then, the PPN experiences much difficulty in the efficient digestion of this complex sap. The phenomenon equates to the once successful PPN species attempting to feed on a host foreign to its system. In numerous field, greenhouse and laboratory studies, I have observed that host plant species which were previously susceptible to PPN develop into resistant hosts. Evidence for this evaluation comes from numerous tests in which the PPN populations extracted from the rhizosphere of plant hosts with superior physiology have the following characteristics:
- The intestinal tract, once replete with fat globules, begins to develop islands of clear zones which are absent in fat globules, with the intestinal tract being completely cleared in advanced cases.
- Whereas healthy, feeding and reproducing populations of PPN hosted a proportion of various juvenile and adult stages (J2, J3, J4 and A; modeled after migratory ectoparasitic species). The compromised PPN populations host a preponderance of J4 and A stages. In other words, PPN populations in resistant hosts are comprised almost entirely of adults.
Accelerating Natural Attrition and the Demise of Plant Parasitic Nematode Populations
When PPN populations reach a state of debilitation exemplified by partial to total clearing of the intestinal tract of fat globules and a preponderance of J4 and A stages, their natural resistance to antagonistic microbes typically becomes compromised. If an integral part of the program to maximize balanced moisture and fertility also encompasses selective microbial activation of the rhizosphere, the microbial community will begin to successfully exert pressure on PPN.
Indeed, in extractions of PPN from the rhizosphere of physiologically superior plants, we not only finds specimens with partially to totally cleared intestinal tracts and primarily J4 and A stages, but also specimens infested with fungal hyphae and/or protozoa and/or bacteria. That is, when a superior plant host physiology creates a foreign environment that is difficult to for otherwise successful PPN species to parasitize, the PPN becomes increasingly vulnerable to the complex competitive environment from which it was derived and escaped millions of years previously.
Precise Evaluation of Plant Parasitic Nematode Pressures
Coinciding with previous work by various nematologists, I have found that PPN species are capable of surviving in soil, in the absence of a host, for in excess of 24 months. Thus, the traditional quantitative evaluation of PPN species oftentimes overstates the true magnitude of PPN pressures. Thus, it’s important to conduct a qualitative evaluation in conjunction with quantitative analyses. The qualitative analysis should minimally examine the:
- Health status of the PPN populations.
- The population distribution of various stages.
- Overall rating of damage potential to a specific crop.
Converting a Susceptible Plant to Resistance Status
My confidence in our ability to convert susceptible plants to resistant status comes from my previous studies in plant pathology. I was able to take a VFN line of tomato plants (with resistance to Verticillium wilt, Fusarium wilt and root-knot nematodes) and compromised their physiological status through nutritional imbalances, thereby creating susceptibility to all 3 organisms.
Conversely, I was able to take Rutgers tomato seedlings that were rated susceptible to all 3 organisms and enhance their physiological status through meticulous nutritional balances, thereby creating resistance to all 3 organisms. It became evident that while genetic makeup was important, the overall physiological status of the host plant was at least as important, if not more so.
Since these studies, I have restructured the physiological efficiency of various plant species under greenhouse and field conditions, successfully imparting resistance to a variety of plant-parasitic nematode species, resulting in a gradual, steady decline in PPN populations and their damage potential. It is a service now provided by the SUNBURST Plant Disease Clinic—Fusion 360’s research and development arm—under the “Complete I,” “Complete II,” “Complete III,” and “Complete IV” analysis services.
It is possible to control nematodes without the use of nematicides.
Traditional theory with PPN control has revolved about eradication attempts through use of fumigant and/or non-fumigant nematicides. However, health, environmental and crop productivity considerations are pushing for a phasing out of these materials and technologies.
By investigating the biological characterizations of PPN species, it has become evident that management and control of PPN can be successfully attained through alternative and safe technologies. PPN species are obligate parasites, and obligate parasites lack a measure of adaptability to changes in the environment and/or biochemical balances. PPN species have evolved over hundreds of millions of years in parallel with hosts in nature that are at best mediocre to substandard in physiological status. Characteristic to plants with suboptimal physiology is simple carbon compound constitution of the plant sap. Plants that are physiologically efficient, however, can host a varied, complex carbon compound constitution in the plant sap.
In an efficient physiological status, the once successful PPN experiences much difficulty in digesting this foreign, complex sap. Weakening of the PPN ensues and difficulties with reproduction follow. In this compromised state of health the PPN loses resistance to microbe populations occupying the same rhizosphere, many species of which can now colonize the PPN cuticle and body proper.