Dr. R. Paul Schreiner, Research Plant Physiologist, USDA-ARS
Renewed interest in vineyard soil health driven in part by advances in microbiome research provides a rationale for reviewing what we know about the foremost component of the root microbiome in grapevines, the arbuscular mycorrhizal fungi (AMF). While other soil bacteria and fungi play important roles in vineyard health and productivity, AMF are unique because of the broad range of benefits they confer. These benefits include improving nutrient uptake from soil (particularly phosphorus (P) and other less mobile ions), increasing soil carbon storage, maintaining soil aggregate stability, and increasing tolerance to drought and pathogens. In the red hill soils of western Oregon, grapevines cannot obtain enough P to grow beyond a few nodes if AMF are absent. They are an integral component of grape and wine production here, and how we treat our soils and vines influences their abundance and the benefits they can provide. There are a few basic issues for viticulturists to consider in managing AMF to get the most from our below-ground fungal partners. These fall under pre-plant and post-plant considerations.
Pre-plant AMF Management. The key pre-plant issue is whether or not the population of AMF is ample enough to ensure that vine roots are quickly colonized. In most cases the answer to this question is yes. AMF are naturally present in almost all soils worldwide because over 80% of all plant species form this type of mycorrhizal association. However, in modern farming systems certain practices can destroy or greatly reduce AMF in soil. While their use is rare in viticulture, pre-plant soil fumigants (methyl bromide, metam sodium, dichloropropene/chloropicrin, and dimethyl disulfide) typically used to control nematodes and soil-borne fungal diseases can wipe out AMF populations. AMF can also be reduced if host plants are absent for an extended period prior to planting a new crop. This can result from long term fallow periods or from the cultivation of non-host plants. Work in Australia to understand the phenomenon of “long fallow disorder” showed that a fallow period of 1 year or more reduced AMF propagules in soil resulting in poor AMF colonization and P deficiency in subsequently planted crops. Soils from long fallow plots could be rescued by adding AMF back to the system from recently cropped soils. Weeds can also maintain AMF populations in soil and may be important in some cases. For example, my lab showed that soil solarization conducted in the summer reduced AMF populations the following spring in western Oregon because solarization suppressed weeds over the fall and winter that acted as bridge to maintain AMF. Growing cash crops or cover crops that are not hosts for AMF can also reduce AMF propagules in soil. A number of plant species do not form mycorrhizal associations of any type or form other types of mycorrhizas that will not maintain AMF propagules in soil. Common ones used as cash crops or cover crops in agriculture are the mustards (Brassicales) including numerous vegetables, rapeseed, and meadowfoam, as well as spinach, buckwheat, amaranthus, and lupine. A new vineyard planting that follows these crops may benefit from adding AMF at planting or boosting the native AMF population by growing a host plant cover crop before planting. Planting a vineyard after hazelnuts is the most likely scenario where adding AMF will be needed in western Oregon because hazelnuts are ectomycorrhizal and because the orchard floor is kept bare for many years (not allowing host plant weeds or cover crops to maintain AMF).
Exactly when the AMF population is too low for healthy vine establishment is not clear. I conducted numerous AMF inoculation trials when I first began working on grapevines over a decade ago in nurseries and new vineyards. Results from the vineyard trials showed that inoculation with AMF (produced in my lab) enhanced root colonization and improved vine growth in only one of five experiments conducted in the Willamette Valley. By year 2, however, the non-inoculated control vines no longer differed from inoculated ones, and in no case in the nursery or vineyard was vine survival significantly altered by inoculating with AMF. Viable AMF were present at all the sites where we conducted inoculation trials so that the control vines became colonized at every site to at least a small degree.
Post-Plant AMF Management. Even though grapevines rely heavily on AMF to obtain ample P and often other nutrients, they also can reduce the extent of AMF colonization within their roots when nutrient status (particularly P) is high. Therefore, avoiding fertilizer applications unless a nutrient is demonstrated to be low or deficient is a good practice to reduce negative impacts on AMF. For example, AMF colonization of Pinot noir roots was reduced in vineyards receiving foliar P fertilizer sprays. Root colonization was also negatively correlated to leaf P and leaf nitrogen (N) concentrations across a survey of 31 Chardonnay and Pinot noir vineyards in the Willamette Valley. There is evidence from other farming systems that organic forms of nutrients are less harmful to AMF than synthetic fertilizers, but even organic sources including manure can reduce AMF and potentially reduce other benefits they provide if applied at high rates.
Soil applied fungicides will obviously harm AMF, but what about foliar fungicides? At this time, there is no evidence that the fungicides used in our spray programs to control powdery mildew and grey mold have a negative impact on AMF. However, reducing tillage can benefit AMF because tillage breaks up their hyphal networks in soil. Indeed, we showed that in-row cultivation reduced AMF colonization in Oregon vineyards as compared to herbicides (mainly glyphosate) used to suppress in-row weeds. Finally, in separate studies both east and west of the Cascades, AMF colonization in grapevine roots was lower in vines at wetter sites (west) or in vines that received more irrigation water (east). Therefore, applying less water will also enhance AMF in vineyards. Since AMF provide other benefits beyond their key role helping grapevines obtain P, choosing management options that enhance their abundance (or at least do the least harm) also improves other aspects of soil health.