Effect of Soil Temperature on Root Colonization by Mycorrhizal Fungi

Commercial products that supply mycorrhizal fungi for landscape or agriculture must be treated differently compared to traditional fertilizers. Since you are attempting to introduce living organisms into the root zone, you need to be careful to keep the fungi alive not only during storage, but also during application. This technical bulletin discusses the temperatures that will promote or inhibit mycorrhizal development. Since mycorrhizas form in soil, the temperatures we are dealing with are soil temperatures, not air temperatures. In summer, forest soil tends to be much cooler than the air, whereas in winter, the soil is usually much warmer than the air. Throughout this bulletin, author citations are provided (Authors, year) wherever facts are quoted from published literature. Full citations are listed at the end of this bulletin. Temperatures are given in both Fahrenheit (°F) and Celsius (°C).

Temperature and VAM (Endo) Fungi

The effects of temperature on the rate and extent of root colonization are complex, the responses varying with both fungus and plant (Smith and Read, 2008). The ability to form mycorrhizas at low temperatures may be related to adaptation of some fungi to the climatic conditions prevailing where they originated. (Hamel and Plenchette, 2007).

The scientific literature contains reports of root colonization of bluebells by VAM fungi at soil temperatures as low as 41°F (5°C) (Daft et al, 1980). However, in one reported case involving soybean and sorghum, VAM colonization was strongly repressed at 59°F (15°C) (Zhang et al, 1995), and completely inhibited in barley at 50°F (10°C) (Baon et al, 1994). In experimental systems, there is usually an increase in percent colonization between 50°F and 86°F (10°C and 30°C). (Note that these are soil temperatures, not air temperatures.)

Soil Temperatures for Mycorrhizal Inoculation

In light of the variability observed regarding VAM response to soil temperatures, what are the appropriate soil temperatures for successful inoculation of trees and shrubs with VAM fungi? There are significant variations, but generally, for optimum VAM colonization, soil temperatures should exceed 50°F (10°C). Setting this range as our optimum, one should avoid inoculations in soils where the temperature is below 45°F (7°C) unless the temperatures are on the increase toward a higher range within a reasonable time frame (4 weeks or less). Below that, many VAM fungi will not efficiently colonize roots. Be careful here. We are not suggesting that lower temperatures kill VAM fungi. That is not true, although VAM spores do die off rapidly after repeated exposures to freezing conditions (Kernan et al, 2000). Here we are saying that the ability of VAM spores and other propagules to colonize roots is significantly slowed, and can be halted at low temperatures. Once colonization is complete, however, mycorrhizal fungi survive throughout the winter within the root tissue of perennial host plants.

Seasonal Guidelines for Mycorrhizal Inoculation in Temperate Climates

  • Spring: Inoculate with VAM fungi when soil temperatures are above 45°F (7°C). In the spring, temperatures will be on the increase toward the preferred range above 50°F (10°C).
  • Summer and Early Fall: Soil temperatures in temperate zones should be good for VAM colonization all summer long and through the early fall.
  • Late Fall: Do not inoculate when soil temperatures drop below 50°F (10°C). In the fall, temperatures will be on the decrease away from the preferred range. It is better to wait until soil warms again in spring.
  • Winter: Avoid inoculations with VAM in cold soils. Winter is not a preferred time for VAM treatments in temperate environments where soil temperatures drop below 45°F (7°C).

Temperature and Ectomycorrhizas

Commercial ectomycorrhizal inoculum products commonly contain Pisolithus and/or Scleroderma spores. These fungi can effectively colonize ectomycorrhizal plants, and Pisolithus can prosper at significantly higher soil temperatures compared to many other ectomycorrhizal fungi. Ectomycorrhizal development for Pisolithus tinctorius was greatest at 93°F (34°C) (Marx et al, 1970), and there is published evidence that P. tinctorius can even protect its host plant from some of the stressful effects of high soil temperatures (84°F, 29°C) (Marx and Bryan, 1971). However, there is little information regarding the minimum soil temperatures needed for effective root colonization to occur. In nature, these fungi produce their spores in the fall, and continue to sporulate until the first frost. Therefore, one might expect that spores produced in this season should be effective root colonizers throughout much of the fall. But there is no hard evidence as yet, to substantiate any assumptions regarding minimum temperatures for ectomycorrhizal development. So when inoculating plants with ectomycorrhizal fungi, it is advisable to follow the same guidelines as noted above for treatments with VAM fungi, until further information is available.

Literature Cited

Baon JB, Smith SE, and Alston AM. 1994. Phosphorus uptake and growth of barley as affected by soil temperature and mycorrhizal infection. J. Plant Nutrition 17: 479-492.

Daft MJ, Chilvers MT, Nicholson TH. 1980 Mycorrhizas of the Lilliflorae. I. Morphogenesis of Endymion non-scriptus L. Garcke and its mycorrhizas in nature. New Phytologist 85: 181-189.

Hamel C, Plenchette C. 2007. Mycorrhizae in Crop Production. Haworth Food & Agricultural Products Press. NY. 319 p.

Kernan MJ, Lin MT, Marx DH. 2000. PHC VAM Fungal Spore Viability: Temperature Trials. Technical Bulletin 39. Lebanon Seaboard Corporation.

Marx DH, Bryan WC, and Davies, CB. 1970. Influence of Temperature on Aseptic Synthesis of Ectomycorrhizae by Thelephora terrestris and Pisolithus tinctorius on Loblolly Pine. Forest Science 16(4): 424-431.

Marx DH, Bryan WC. 1971. Influence of Ectomycorrhizae on Survival and Growth of Aseptic Seedlings of Loblolly Pine at High Temperature. Forest Science 17(1): 37-41.

Smith SE, Read DJ. 2008. Mycorrhizal Symbiosis, 3rd Ed. Academic Press (Elsevier), New York. 787 pp.

Zhang F, Hamel C, Kianmehr H, Smith DL. 1995. Root-zone temperature and soybean [Glycine max. (L.) Merr.] vesicular-arbuscular mycorrhizae: Development and interactions with the nitrogen fixing symbiosis. Environmental and Experimental Botany 35: 287-298.