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Arbuscular Mycorrhizae (AM) in vineyards (Part 1)
Effect of AM inoculation, fungicide and rootstock on soil AM populations
André Meyer & John Wooldridge, ARC Infruitec-Nietvoorbij, Stellenbosch
Key words: Arbuscular mycorrhizal fungi, fungicide, grapevine rootstock, inoculum, spore density, vineyard soils.
INTRODUCTION
Several species of arbuscular mycorrhizal (AM) fungi have been found to have positive effects on the growth and development of young grapevines when grown in sterilized soil under greenhouse conditions (Menge et al., 1983). Other studies under controlled conditions have shown that AM fungi improve nutrient uptake, thereby stimulating vine growth (Deal et al., 1972; Karagiannidis et al., 1995; Biricolti et al., 1997). Unlike the growth media used in many trials with AM fungi under controlled conditions, commercial farm soils are usually characterized by an adequate supply of mineral nutrient elements. They also contain indigenous AM fungal species (Schubert & Cravero, 1985). These indigenous AM fungi are an integral component of plant production systems (Baumgartner, 2003), and may outperform artificially introduced AM fungi, especially where the indigenous and introduced species differ greatly in abundance or vigour, or where the presence of recently-applied phosphorus (P) in the soil reduces the effectiveness of root colonisation by the introduced AM species (Menge et al., 1978; Brundrett et al., 1996). Under certain circumstances, grapevines may nevertheless benefit from AM root colonisation, even where soil P concentrations are high (Schubert et al., 1990). In practice, little is presently known about the balance of factors that predispose grapevine roots and AM fungi to form mutually beneficial relationships, or to fail to form such relationships. This article presents results obtained from a trial in which a range of treatments, including two commercial AM inocula (CAM1 and CAM2), were applied to grapevines at planting.
MATERIALS AND METHODS
Since the layout and procedures used in this trial have already been described in detail by Meyer et al. (2004), only a brief description will be presented here. The trial was carried out in two commercial vineyards on the farm Groenland near Stellenbosch. Vineyards containing Merlot noir vines on 101-14 Mgt and 110 Richter (110R) rootstocks were planted on a ridged Westleigh soil in winter 1998, at which time another vineyard, containing Merlot noir on 99 Richter (99R) was established on an unridged Fernwood soil (Soil Classification Working Group, 1991). Five treatments were applied at planting (Table 1). Since CAM1 contains mineral nutrient elements in addition to the biological material, each non-CAM1 treatment received 50 ml of biologically inert (steam sterilised in an autoclave at 121 ºC, 100 kPa, for 60 minutes) CAM1, in addition to the other materials listed in Table 1, at planting. Each of the five treatments was replicated four times in a randomized block design. After two seasons (winter 2000) the vineyard soils were extensively sampled. Soil pH (1M KCl) and Bray II phosphorus (P) concentrations were determined at incremental depth intervals, total AM spore counts were performed and the different AM species present were identified. The data were subjected to analysis of variance using Statgraphics version seven and SAS version 6.12. Least significant differences (LSD’s) were calculated at the 5% probability level to facilitate comparison between treatment means. Treatment means that differed at P = 0.05 were considered to be significantly different.
RESULTS AND DISCUSSION
Soil phosphorus and pH
At the end of the second vineyard growth season (1999/2000), soil P concentrations ranged from 61 to 80 mg / kg in the topsoils (0-300 mm), 27 to 61 mg/kg in the 300-600 mm horizons and 6 to 50 mg / kg in the underlying, mainly Malmesbury shale-derived soil material (600-900 mm). Since fruiting perennials perform well at soil P concentrations around 30 mg/kg, the top and mid horizons were adequately to abundantly supplied with P. Soil pH levels over the trial period ranged from 5.6 to 6.1 (1M KCl), which are near-optimal for fruiting perennials such as grapevines.
Soil AM counts
Despite the adequate to high soil P concentrations, soil AM counts in winter 2000 were high (in the thousands) in the vineyards containing the three rootstocks and five treatments. Spore counts ranged from 1075 to 1925 (average 1492 / 100g air-dry soil) in the 110R vineyard, 2242 to 3779 (average 2915 spores / 100g air-dry soil) in the 99R vineyard and 1000 to 1458 (average 1200 spores / 100g air-dry soil) in the 101-14 Mgt vineyard. Spore counts did not differ significantly between AM treatments in the block occupied by the 110R and 99R rootstocks. In contrast, in the 101-14 Mgt vineyard, spore counts in the fungicide treatment (1458 spores / 100g air-dry soil) exceeded those in the CAM1 (1000 spores / 100g air-dry soil) and CAM2 (1013 spores / 100g air-dry soil) treatments. Spore counts in the 99R vineyard soil therefore greatly exceeded those in the 110R and 101-14 Mgt soils. Precisely why the spore counts did not differ between treatments in the Richter rootstocks is unclear. Neither is it clear why the spore count in the fungicide-treated 101-14 Mgt soil exceeded those in inoculated treatments (CAM1, G1054 and CAM2). It is possible that the effects of the fungicide faded fairly quickly after application, creating a fungus-depleted zone that later became abundantly re-colonised by AM species from the surrounding soil (Menge, 1982).
AM species identified
Analysis of the AM species present in the soils after two seasons (winter 2000) showed that seven species (Acaulospora spinosa, Gigaspora gigantia, Glomus mossae, Glomus sinuosum, Scutellospora calospora, Scutellospora dipurpurascens and Scutellospora fulgida) were present in the soils of each of the five treatments in all three rootstocks (Table 2). These results, which bore no apparent relation to either treatments or rootstock, imply that these species may be common, if not indigenous to Western Cape vineyards or, at least, to those having similar soils, climatic conditions and cultural histories to those at Groenland.
Gigaspora decipiens was present in the soils of all five treatments in the101-14 Mgt vineyard, but was not observed in the soils from the 99R and 110R vineyards. This observation suggests that there may be an affinity between Gigaspora decipiens and 101-14 Mgt that is not shared with the Richter rootstocks. Further data is needed to confirm or disprove this possibility.
Glomus intraradices was present in the CAM1 inoculum, and was identified in the soils of the CAM1 treatment, in all three rootstocks. Glomus intraradices was also observed in the control and fungicide treatments of the 101-14 Mgt and 99R vineyards, but not the 110R vineyard. Conceivably Glomus intraradices is not uniformly distributed in the soil.
Glomus etunicatum was consistently present in the control and CAM1 treatments, but absent from the Fungicide treatment, in all three rootstocks. This may indicate that Glomus etunicatum is indigenous (as well as being present in CAM1), but does not readily recover after once-off fungicide applications of the type applied in this trial.
Glomus sp.1054 was not detected in any of the treatments. This finding may nevertheless be anomalous. Glomus sp. 1054, an apparently newly recognised species, was identified in soils from Western Cape apple orchards, by Prof. Joe Morton of INVAM and subsequently bulked to form the inoculum used in this trial (Wooldridge, 1999). However, because G1054 is not easily identified by locally-available techniques, it is likely that any G1054 present was included with other species of the genus Glomus. Other species that were apparently present in the inoculums, but which were not specifically identified in the soils of the respective treatments in any of the rootstocks were Glomus fasciculatum (in CAM1), and Glomus fasciculatum, Glomus caledonium and Glomus versiforme (in CAM2). These species may not be compatible with vineyard soils that are well supplied with P.
Several other species of AM were identified, but these were inconsistent and sporadic in their occurrence. Notable amongst these were Glomus clavisporum, Acaulaspora scrobiculata, Glomus tortuosum, Scutellospora cerradensis and Scutellospora erithropa, as well as isolates of Scutellospora spp., Sclerocystis spp. and Gigaspora spp. (data not shown).
FIGURE 1. Structures typical of arbuscular mycorrhizal fungi. Auxiliary cells (arrowed) of Scutellospora (A) and Gigaspora (B), both indigenous.
CONCLUSIONS
Results from this field trial indicate that a range of AM fungi are naturally present in vineyard soils at Groenland despite soil P concentrations that are adequate to high for grapevines. Under the prevailing conditions the native AM fungi competed strongly with introduced AM species. The effects of these fungi on the vines will be discussed in the second article in this series.
For further information contact André Meyer at meyera@arc.agric.za.
REFERENCES
Baumgartner, K. 2003. Why and How. Encouraging beneficial AM fungi in vineyard soil. Practical Winery and Vineyard 14, 57 - 60.
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