Inoculation of potting soil mixtures with the mycorrhiza Rhizophagus irregularis, available from the EU project REFERTIL, improved plant health of young strawberry plants. A greenhouse experiment carried out by researchers Joeke Postma and Marieke Förch from Wageningen UR has demonstrated this. Unrooted strawberry cuttings planted in a substrate with mycorrhiza became less infected by Phytophthora cactorum than plants in the absence of mycorrhiza. This is a promising way of preventing crown rot in strawberries.

Strawberry is a high value crop which is grown in soil and soilless growing media. In Europe, in total 160,000 ha (source: FAOSTAT) are used for strawberry production. Part of the strawberry production in Europe is produced in protected systems with potting mixes (peat, coco-peat) supported on structures 1.5 m above the ground (see photo’s). Also part of the propagation plants are grown in potting mixes.
These potting mixes have been adopted to create a pathogen-free start. However, during plant growth pathogens can infect through water, dust, or human activities. Phytophthora cactorum is an important disease in strawberry cultivation, both in plant propagation and fruit production. It is a devastating disease causing crown rot and wilt of plants.

The objective of our research was to reduce disease pressure of P. cactorum in strawberry cultivation by the addition of organic compounds or beneficial micro-organisms in the potting mixes. Unfortunately, none of the amendments (several composts, animal bone char and chitin) elevated suppressiveness of the potting mixes against P. cactorum. Also several antagonistic micro-organisms (Trichoderma, Pseudomonas) were ineffective.
In 2014, two mycorrhiza species were mixed through the potting mix. These species (Rhizophagus irregularis and Glomus etunicatum) were delivered by Dr. H. von Alten from the University of Hannover. The mycorrhiza Rhizophagus was able to reduce infection with P. cactorum by around 50% when compared with the control plants without mycorrhiza. There were fewer plants with brown roots and also fewer plants with internal symptoms of crown rot. The other mycorrhiza strain was not really effective.

Mycorrhiza species have a number of beneficial properties, such as making phosphorus available to the plant or reducing pathogen infection by means of competition. However, potting mixes and other substrates in which no plants have grown before, are free of mycorrhiza. Therefore the inoculation of mycorrhiza in such growing media can have beneficial effects. The control plants growing in the un-inoculated potting mixes had roots free of mycorrhiza. After treatment of the substrate with the mycorrhiza Rhizophagus, 40% of the strawberry roots contained mycorrhiza. In the Glomus-treated plants, only 10% of the roots contained mycorrhiza.
This greenhouse experiment demonstrated that inoculation of potting mixes with mycorrhiza can have beneficial effects, reducing the infection by plant pathogens. However, the experiment should be repeated, testing the efficacy in different substrates, cultivars and with varying environmental conditions.