By Dr. Thomas T. Yamashita
Vineyards, like many permanent crops, oftentimes experience a steady decline in vigor, yield and quality. This appears to be an increasingly common occurrence. Many of us have heard of entire vineyards being pulled out due to phylloxera or nematodes, and replaced with high density plantings to combat the suspected foe through numbers. The question arises as to whether the context of these problems is being weighed along too narrow of an evaluation. Is there a broader picture we should be examining? This article attempts to explore additional possibilities.
While there are many causes of vineyard decline, the common result is a reduction in carbon and energy being produced to support vine metabolism and growth.
For example, a plant-parasitic nematode infestation reduces the mineral and water harvesting capacity of the root system. They also reduce the production sites for important growth-promoting cytokinins, which are normally translocated to the canopy, helping among others to create a source-sink relationship in buds. In this example, like others, the final results stem from the effects of the pest on reducing photosynthate production.
Let’s take another example which is even more graphic. Table grape growers not only spray with gibberellic acid but further enhance berry size through ‘girdling’ of the trunks. Inspection of these girdles will oftentimes reveal they are excessively deep and difficult to heal over. Girdling interrupts the phloem tissues, which are instrumental in translocating photosynthates back to the root system, which relies on delivered food for their functioning. Thus, repeated girdling operations, the disrupted hormonal balances inducing more carbon partitioning to berries (caused by gibberrellic acid applications), and absence of compensatory nutrition all add up to starvation of the root system.
With the next spring’s push, minimal photosynthates are sent from the roots (the primary storage organ) to help set and carry buds. Again, the primary deficiency developed is with photosynthates, carbon and energy.
The traditional approach to remedy phylloxera and nematodes pose a common and formidable problem in vineyards. For one, vines appear to be a preferred host for a variety of plant-parasitic nematodes (PPN). At the University of California, Davis, for example, my major professor grew many of our pure cultures of PPN on grapevines. Phylloxera can run rampant on grapevine roots as well.
Just killing pests is no longer sufficient to return grapevines to optimal health. Nutritional assistance is also needed.
The traditional approach has been to strictly target the pest and reduce its numbers. This is a carryover from the use of extensive fumigation practices, including DBCP. However, as our limited arsenal of materials continues to be reduced due to environmental concerns, we will need to change how we combat these soil-borne pests.
We have to understand that we are working with old, tired soil, and the intensity of our demands on output exceed the standards of a few decades ago. Thus, merely killing a soil-borne pest may not produce the results it once did on more fertile ground. When a pest such as a nematode debilitates and reduces the effective root area, we can’t expect to achieve optimal results by simply killing the nematodes. A reduction of the pest must be accompanied by a ‘growth-promoting response.’
This growth-promoting response must be comprised of:
- Increasing the mineral-release characteristics of the soil;
- Increasing the soil’s natural ability to suppress further increases in the nematode population, as well as soil-borne disease organisms;
- Improving physical parameters conducive to root development (such as water infiltration rates, mellowness, and textural ease of growth).
If we were, for example, to markedly reduce the pest but contribute nothing to the growth-promoting parameters, recovery of the vineyard is greatly minimized. We have repeatedly seen this, for example, with a number of popular non-fumigant nematicides. If we were to reduce a portion of the pest population while improving growth-promoting parameters, our activities would be equivalent to tipping the scale towards more root surface area to compensate for that taken away by the pest. It would be akin to ‘inarching’ or grafting another root to the main stock, a commercial practice.
Of all the commercial, permanent crops grown, grapevines host one of the highest potentials for root system regeneration, if the right assistance is provided.
Returning to the example of non-fumigant nematicide, we have a classic case where the pest is targeted, but the growth response is left up to the plant’s natural ability to recover in an environment with fewer nematodes.
Yet, in light of this example, and the failures observed, we have forgotten that recovery is a rate-dependent response. That is, if recovery does not happen quickly, surviving pest populations will again shift the proportions to where their numbers equal and eventually exceed the slowly recovering, available root system.
This takes us to a point of reflection and examining our mindset. Is it the flaw in the whole system? Our demand for ‘magic bullets’ has allowed us to take a quantum leap in productivity. Yet, we are constantly trying to improve our production in the context of environmental and regulatory pressures. That is, we are being forced to become innovative. But, in order to become innovative, we must leave behind the past and the magic bullet approach to farming.
Innovative technologies demand that we define and characterize the field with which we are dealing, and only following this detailed characterization can we design a tailormade program for a specified set of complex goals (e.g. not only kill the pest, but also impart growth-promoting factors). For example, how often have we seen a vineyard manager approach a phylloxera problem with the single focus to eradicate the pest? Has he examined the soil nutrition banks, existing texture, soil microbial activity, nematode pressures, degree of natural suppressiveness in the soil, possibilities for recovery supplementation via foliar nutrition, and other factors?
Years ago I rode with a very qualified PCA, who had done well and was part of an investment group developing many acres of almonds. The take on a 500-acre planting was beautiful, but in riding past the field I noticed low-lying areas with signs of run-off collection. I immediately suggested to him that we examine this area in detail to define the status of water mold pressures, potentially capable of damaging his beautiful planting. Being typically clumped in their distribution, I rarely take less than 3 to 4 samples spaced some distance from one another.
To this latter practice, I was severely questioned as to the need, and was told that a single, less expensive sampling should suffice as a representation of the entire area. What he failed to comprehend was that I not only intended to define the inoculum density of water molds, but also their distribution in space and the level of natural suppressiveness present in the field as well. He saw the target, water molds, but failed to see the intricacies surrounding this parameter that lends itself to a panoramic definition of the overall picture.
This larger perspective of securing a more thorough definition of a field complements the acceptance of the fact that the greatest losses to growers comes not from pathological, but from physiological diseases. Once this is grasped, the inherent need to define the issue in its entirety should come naturally. This means soil mineral analysis, tissue analysis, calculating a mineral release potential, defining disease pressures, characterizing natural suppressiveness, examining irrigation efficiency, etc.
There is too much of an emphasis on identifying and eliminating pests, and not enough emphasis on ensuring proper nutrition in agricultural soils.
Current PCA standards emphasize pest management, including insects, mites and weeds as primary foci. Yet, as a plant pathologist, in the thousands of calls I have responded to, I can honestly say that more than 95% of the maladies were traced back to a physiological imbalance precipitated by suboptimal nutrition.
Proper nutrition, as many of you know, is a result of responses to definition of soil and tissue mineral levels, examining microbial activity of the soil, characterizing nematode and/ or disease pressures, examining irrigation efficiency, and many more parameters.
Let us get back to the girdling example. Loss of vigor and productivity ensues after repeated girdling. Here is a case where we as farmers need to take a closer look at the quality of girdling. Is it too deep for rapid healing, so important to recovery of the vine? This is a common error, as examination of the resulting blown-up berries is judged as sufficient to gauge the quality of the girdling.
Watching your girdling crew with a discerning eye (just as an almond grower follows his shaking crew to avoid barking) is a must. In addition, many growers need to use narrower knives to facilitate more rapid healing and recovery. They must also disinfect knives between vines to avoid transmitting crown gall. In addition, many a girdler is under the mistaken impression that the desirable effects result from scraping off a large chunk of the wood with the bark. This issue is not a difficult one to remedy, but typifies an all-too-common mindset of going through the motions of established practices, without defining their quality and ramifications.
The ultimate goal of these often-neglected analyses must be to decipher, interpret, and influence relevant factors to ensure maximum production of carbon and energy in the grapevine, and not mere elimination of a pest or disease. It is a simple matter of paying greater attention to the details.