Because a sample is conceptually an abbreviated representation of a true condition, the techniques of securing representative samples should be approached with precision. In many ways, sample-taking is not merely a science, but an art as well. This is complicated by the tension between achieving the ideal sample-taking systems taught at the university level, and the practical, affordable schemes touted by experienced advisors and growers.
For example, ideal characterization of a 10-acre field for nematodes may require checkerboard mapping techniques resulting in a minimum of 25 to 30 soil samples—a rather pricey endeavor. The practical world, at present, deals with this same situation by securing as few as one sample. At what level do we find the middle ground between the ideal and the pragmatic, and how is this done?
Just as soil or tissue mineral analyses form a foundation from which to design nutrition programs, tissue and soil samples for nematode and/or disease analyses form the baseline from which to adjust nutrition, protective and/or remedial programs. This article attempts to describe salient considerations required of the sampler, and the effective techniques to secure accurate, representative selections.
Essential Considerations for Accurately Sampling Aerial and Foliar Diseases
- Always attempt to sample for prevention” of disease, rather than reaction to a disease. That is, try to anticipate, through characterization of disease potential or tendency. As much as possible, do not place yourself in a position where you have to sample a diseased condition.
- Sampling for characterizing disease potential requires knowledge of the overwintering stage, the reservoir hosts and tissues, and the mode of transport. (For example, brown rot or Monilinia overwinters in blighted shoots, aerial and ground mummies, and blighted buds. It is typically transported by wind and can be transported into clean fields from distant infection sites.)
- What is the host range of the disease, and is the pathogen more or less aggressive on the species and/or cultivars in question? (Example: Downy mildew of grapes is specific to Vitis species where it is an obligate parasite without an alternate host outside of the genus, Vitis; the American species, labrusca and V. aestivalis have moderate resistance while the wild species, V. rupestris, V. rotundifolia, and V. cordifolia have acquired strong resistance to downy mildew.
- How many generations does the disease produce in one season? (e.g. Verticillium wilt occurs once per season per host, whereas powdery mildew infections can occur repeatedly for many generations during the season.)
- What type of weather is conducive to the disease, and what are the predominant weather patterns in the area in question (e.g. wind patterns, eddying swirls that may concentrate inoculum, moisture periods, dry periods, etc.)?
- What are the cropping or wild growth patterns in your area of similar and alternate hosts?
- Can the disease be controlled with chemicals and/or by other means, and is the disease explosive or slow moving and chronic?
Sampling Soil-Borne Pathogens and Nematodes
- Soil-borne pathogens and nematodes have a nonhomogeneous and clumped distribution. That is, their populations will be uneven throughout the field.
- Plant-parasitic nematodes are obligate parasites which rely upon a living host for securing sustenance. Thus, the likelihood of detection is always highest on or close to roots.
- Soil-borne plant pathogens generally overwinter as resting bodies, and/ or as actively growing organisms on undecomposed plant refuse. Thus, their populations will tend to be highest where a previous host has been colonized.
- Plant-parasitic nematodes will complete a life cycle from egg to egg in approximately 35 to 45 days. Thus, a typical species may go through 3 to 4 generations during a growing season. Peak population levels, then, will often be found in the fall. This latter point is critical, as the standard extraction efficiency runs about 40%. Thus, higher nematode populations are more readily detected.
- Most soil-borne pathogens will be found within the first 6 to 8 inches of soil where oxygen levels are sufficient.
- The majority of plant-parasitic nematodes and soil-borne pathogens tend to live and proliferate in moist soils.
- Both plant-parasitic nematodes and soil-borne pathogens have a lower survival rate and rate of multiplication in microbially-active soils.
- Both plant-parasitic nematode infestations and/ or suboptimal nutrition can predispose root tissues to water mold infections.
The Art and Practice of Sampling Aerial and Foliar Diseases
- Secure possible overwintering organs or tissues (e.g. undecomposed debris, cankers, mummies, reservoir hosts, etc.) to determine estimated density of disease sources.
- Determine proximity of overwintering sources of disease and relationship to wind patterns.
- Examine crown and root tissues for signs of attrition or disease as a predisposing factor to aerial symptoms.
- Secure soil and tissue mineral analyses to define another possible predisposing factor to aerial symptoms.
- Explore projected climatological trends conducive to disease.
- Secure pertinent data related to associated cultural practices (e.g. irrigation schemes and frequency, preventative spray programs used, etc.).
- Secure asymptotic target tissues for incubation as to expression of disease in a quiescent or latent state (e.g. blossoms, leaves, shoots, fruit, etc.).
Reactive Sampling Following Infection and Expression of Symptoms
- Secure 3 stages of the diseased tissues, (a) in incipient stages of disease, (b) in moderate expression of symptoms, and (c) showing severe symptoms of disease.
- Examine the crown and roots tissues for signs of attrition or disease as a predisposing factor to aerial symptoms and secure symptomatic tissues, along with healthy perimeter tissues.
- Secure soil and tissue mineral analyses to define possible predisposing factors to aerial symptoms.
- Secure pertinent, historical climatological data associated with both predisposing stress as well as specific weather patterns conducive to infection.
- Secure pertinent data related to associated cultural practices (e.g. previously used preventative programs, type of irrigation and frequency, etc.).
- Secure or at least observe possible predisposing, biological agents (e.g. insect vectors, rodent pressures, insect feeding, etc.). Note: Place all samples in ½ to 1 gallon Ziploc bags. Keep refrigerated and out of direct sunlight.
Soil-Borne Disease (SBD) and Plant-Parasitic Nematode (PPN) Sampling
Open Field Sampling
- Wherever possible, secure pre-characterizations of the field that may aid in delineating possible epicenters of disease (e.g. areas of weak growth for the previous crop(s), swales, sand streaks, compaction zones, aerial photographs, nature of previous crops, etc.).
- If possible, examine the projected direction of irrigation run and tractor work and other factors which may aid in radiating the disease from the epicenter.
- What is the source of irrigation water (e.g. canal, reservoir, tail-water, well, etc.)?
- Realize that a trouble spot or area with high inoculum levels will serve as a nucleus of disease spread.
Thus, based on the specific delineated zones listed under (1) above, secure representative samples from each defined zone:
- In your defined zone, inscribe an equilateral triangle with arms of about 100 yards each.
- Secure subsamples from each corner of the triangle, one for PPN and disease, one for minerals.
- Combine the subsamples and list them under the defined zone (e.g. PPN / Sand Streak-1).
- For PPN, it’s preferable to dig in a site previously occupied by a host plant. If possible, attempt to find a zone of moist soil and roots, and secure samples of each.
- For SBD, secure a sample from the top 6 to 8 inches of soil, preferably near a previous host plant.
Planted Field Sampling
- If possible, work off of aerial photographs, and/ or walk the field to define troubled areas.
- If the troubled area can be delineated, secure three subsamples from within this zone by taking samples from the points of an equilateral triangle. Inscribe a larger triangle in the area outside the affected zone, securing another three subsamples.
- When securing samples from a troubled area, avoid taking from the base of host plants showing severe symptoms. Concentrate on plants with light to moderate symptoms.
- If a distinct troubled site cannot be defined, inscribe an equilateral triangle (as before with arms of about 100 yards) and secure a biological sample from each point while taking a one-third subsample for a soil mineral analysis, combining the 3-for-1 mineral sample (i.e. three biological and one mineral sample).
- For PPN, use a shovel (not a probe) and dig down to an area with moist soil and fibrous roots.
- Secure the ball of soil surrounding the roots and the fibrous roots.
- For SBD, use the same soil secured for PPN. Note: Place all samples into a 1 quart to ½ gallon Ziploc bag. Keep refrigerated and out of direct sunlight.