Bekæmpelsesmiddelforskning fra Miljøstyrelsen, 93 – Bæredygtig produktion af småplanter i forstplanteskoler

Sustainable Production of Small Plants in Forest Nurseries

Summary and conclusions

Root diseases and weeds are causing severe problems in forest nurseries. Control of these plagues is traditionally achieved by using chemical soil disinfectants (Basamid) and herbicides. However, due to heath issues and environmental concern there is a demand for alternatives.

Therefore, the main objective of the present project was to develop a sustainable, alternative growth system in forest nurseries without use of chemical soil disinfectants and, based on this knowledge, to put forward prevention and control strategies for root diseases and weeds in Danish forest nurseries. As alternative methods to root diseases and weeds, we focused on seedbed steaming and green manure crops alone and in combination.

Field experiments were conducted at two Danish forest nurseries in selected "problem areas" with four different forest plants; Caucasian Fir (Abies nordmanniana), Noble Fir (Abies procera), Sitka Spruce (Picea sitchénsis) and Grey Alder (Alnus incana), which are all having problems with pre and post emergence damping-off. The experiments had 11 treatments: 1) untreated, 2) steam autumn, 3) Steam spring, 4) Basamid autumn, 5) Basamid spring, 6) Turnip (Brassica rapa var rapa) as green manure, 7) Oat (Avena stiva) as green manure, 8) combination of steam and Turnip, 9) combination of steam and Oat, 10) combination of Basamid and Turnip and 11) combination of Basamid and Oat. Each treatment had four replicates and the experiments had a complete randomised block design. Each plot was four metres long and 1 metre wide with four 1m²-subplots with each of the four experimental plants. The experimental areas were established in June 2002, with subsequent establishment of green manure crops in August 2002. Steam and Basamid treatments were performed in July 2002 and April 2003. The green manure crops were incorporated into the soil in April 2003, and one month later the seeds were sown and the experiment initiated. The experiments were completed in October 2003.

In this project we examined the influence of the above-mentioned treatments on plant parameters (emergence, mortality, total number of plants harvested and plant biomass at harvest), presence of weeds and root pathogens, formation of ecto-mycorrhizas and arbuscular mycorrhizas and the composition of soil microbial communities.

The effects of the different treatments on root diseases (measured indirectly using the plant parameters: seedling emergence, mortality of emerged seedlings and number of plants harvested) were crop dependent. No significant treatment effects were observed in Caucasian Fir, Sitka Spruce and Grey Alder, but all treatments except green manure alone increased emergence and survival of Noble Fir resulting in significantly more plants harvested, especially in the combination treatments.

Isolation of root fungi other than mycorrhizal fungi with subsequent test of pathogenicity showed that especially Fusarium oxysporum, Fusarium avenacium and Pythium ultimum are causing major damage in forest nurseries. Surprisingly, the composition of populations of root fungi other than mycorrhizal fungi did not differ between roots of healthy plants and plants with root rot symptoms.

As expected, steam and Basamid markedly suppressed emergence of weeds, while Turnip unexpectedly gave a similar, marked suppression of emergence of Annual Bluegrass, which was the dominating weed in both nurseries. The reason for the suppression of Annual Bluegrass with Turnip as green manure is most likely due to allelopathic compounds released during the decomposition of Turnip prior to sowing. Turnip as a green manure crop also suppressed emergence of Grey Alder and Sitka Spruce but had no effects on Noble Fir and Caucasian Fir. In contrast to Turnip, Oat as a green manure had no effect on weeds and root pathogens.

No long-term effects of steam and Basamid on the general soil microbial communities were measured, while incorporation of green manure crops increased the microbial biomass in the soil in one of the forest nurseries with low initial biomass compared with the other nursery where green manure incorporation did not result in an increased microbial biomass. However, 10 months after steam and Basamid treatments the biomass of soil fungi as measured using the signature fatty acid 18:2ù6,9 was still markedly lower than in the untreated control. On the other hand, 14 months after steam and Basamid treatments, formation of arbuscular mycorrhiza in Grey Alder and ecto-mycorrhiza in Caucasian Fir was unaffected by the various treatments.

The results from the present project do not allow a general strategy for prevention/control of root diseases and weeds in forest nurseries, as the effects of the tested methods were plant dependent. However, the results can be used as a first step towards development of alternatives to chemical soil disinfection, showing that steaming of seedbeds seems to be as good as Basamid at preventing weeds and fungal root diseases. Turnip as a green manure looks very promising for control of Annual Bluegrass. This should be investigated further.