Risicover: Adequate control of soilborn diseases in strawberries (Risicover)

Current problems:

For a good cultivation of strawberries a healthy, balanced soil and pathogen free planting material is needed. Due to the shortage of new, uncultivated soils, the growers are forced to plant year after year strawberries on the same soil. As such problems with soil pathogens, like fungi, bacteria and nematodes arise. One technique to control problems with soil pathogens is to perform a soil fumigation. However, these treatments encounter at this moment some problems due to governmental, environmental and public concern leading to the fact that some treatments are not registered any more in Belgium. As a result Because of that, more and more problems with soil pathogens occur. This leads to high losses and high costs for the strawberry growers. Nevertheless, the soil is not the only problem. Soil pathogens can also already be present on the ordered planting material. Furthermore, the available products to manage these soil pathogens, do not reach an efficacy of 100%. As such it is almost impossible for the strawberry fruit growers to start their cultures on healthy soils and with healthy plant material. Not only the strawberry cultivation in soil, but also the cultivation on substrates suffers from high losses caused by Phytophthora sp..

Recently also a new pathogen, causing wilting of strawberry plants was identified, i.e.nl. Pestalotiopsis. This pathogen is already causing big problems in the cultivation of planting material and no research is yet performed regarding this pathogen. Furthermore, new strawberry varieties also seem to be very susceptible for soil-borne diseases.

Based on all these findings, it is clear that strawberry fruit growers need good support and advise concerning ‘whether or not to plant a specific variety on a specific soil/substrate’ and ‘which control strategy they need to include’.

By performing a risk-evaluation of the planting material and the soil, the grower can obtain very important information on which he can base his decision. Such risk evaluations can be performed by a fast and reliable molecular technique (for example qPCR). This technique makes it possible to estimate the seriousness of the problem and to identify action thresholds. Furthermore, as the technique of soil fumigation is under discussion, there is also a need for other chemical or alternative techniques to manage soil pathogens or for an adaptation in the management strategy. 

The main goal of this project is to make the work of strawberry fruit growing more cost-effective for the grower by limiting the occurrence of big losses due to soil pathogens. This will be reached based on a reliable risk evaluation of planting material and soil and based on an optimization of the disease management strategy.

Sub-goals:

-      Optimalisation and implementation of a fast, reliable molecular detection and quantification technique for the selected pathogens.

-      Optimalisation of the sampling technique for soil and plant samples.

-      Development of a bio-assay which can be used in the future to screen new varieties for their susceptibility towards soil-borne pathogens.

-      Determine the risk of a certain degree of infestation (on plants, in soil or a combination of both) for the cultivation of strawberries of a specific variety on a specific soil/substrate

-      Optimalisation of the disease management strategy (chemical/biological/alternative)

-      To set up a decision support system easily interpretable for the strawberry fruit growers.

Focus is on 4 important soil pathogens in strawberry fruit growing, including Verticillium,Phytophthora sp., Pestalotiopsis and Colletotrichum.

Looking at the planting material the focus will be on the commercial varieties Elsanta, Sonata and Portola, and the new upcoming varieties Elegance and Malling centenary.

Within WP2: Optimisation of rapid and quantitative diagnostics, qPCR assays were optimised or developed for the detection of Phytophthora cactorum, P. fragariae, Pestalotiopsis longisetula and Verticillium dahliae. In addition, standard curves have been established for the three pathogens to allow quantification of pathogen DNA in field samples. The detection limit for the three qPCR assays was 1 fg of DNA and the quantification range between 1 ng and 1 fg for V. dahliae and between 1 ng and 10 fg for the other two pathogens. The development of qPCR methods for detection and quantification of P. cactorum was published in European Journal of Plant Pathology (Verdecchia et al., 2021). For the detection of P. fragariae, several already published qPCR assays were tested and validated. The qPCR assay developed by Bonants et al. (2004) gave good detection of P. fragariae in plant material. Because P. fragariae could not be added to the fungal collection until June 2020, validations around this pathogen are rather limited.

In addition, sampling procedures were also finalised in WP2 for soil samples. For the detection of lower amounts of Phytophthora in soil samples, a technique using baits (Rhododendron punches) was designed to attract Phytophthora zoospores. For the detection of microsclerotia of Verticillium in strawberry plots, a technique using "density flotation" was developed.

A final task within this WP was to map any "co-occurrence" patterns of the selected soil pathogens. 

In WP3, bioassays were finalised and damage and action thresholds determined. These are obtained for P. cactorum/fragariae and V. dahliae. In the biotests with Pestalotiopsis species, additional stress factors, e.g. presence of nematodes, are needed to obtain failure.

In this WP, damage and action thresholds were also determined in soil for P. cactorumand V.dahliae. Figure 2 shows the results for P. cactorum for several varieties.

This showed that Sonata was the most sensitive variety with an action threshold of less than 60 oocytes per ml of substrate. The action threshold for the other varieties (Elegance, Malling Centenary, Elsanta and Portola) was between 100 and 300 oöspores per ml of substrate.

Damage and action thresholds were also determined for V. dahliae by dipping the roots of plants of the Elsanta variety in different concentrations of conidia suspensions. This showed that the damage threshold is at 100 conidia per ml and the action threshold is at 103 conidia per ml.

 Biological, alternative and chemical control methods of these soilborne pathogens were investigated in WP4. Trials showed that agents based on either potassium phosphonates (Soriale), Trichoderma or Gliocladium had moderate to good efficacy against P. cactorum. 

Regarding chemical agents, the products Ranman Top and Infinito, both registered in potato cultivation against Phytophthora, showed very good efficacy.

In a trial of Verticillium control, two biological products were able to reduce the percentage of infestation to the same extent or even more than a chemical reference agent (50% and 69%). These were agents based on Trichoderma whether or not enhanced with algae extracts. In terms of chemical agents, Beltanol showed a better effect than the chemical reference agent Topsin.

During the project, we also tested the effect of using different alternative cuttings soils and substrates to prevent root diseases in the root-prone variety Sonata. In the trials, we worked with different fractions of peat, sphagnum moss (the top layer of peat that can renew itself faster), garden turf, coir, perlite and wood fibre. Sphagnum can be a valuable alternative of peat in substrates. For instance, the water-holding capacity, stability and inertness of this substrate is sufficient to pass a full crop. Moreover, in several trials we observed better rooting of plants in both the cutting soil and production substrate. This better rooting results in fewer failures due to root diseases. Incorporation of sphagnum up to 50% in the cutting soil and up to 40% in the production substrate was tested in the trials without any problems. Wood fibre can also be a valuable alternative to peat or coir in the cuttings soil. Important to note here is that we are talking about fine wood fibres and not chopped wood. The fibres fix part of the nitrogen present, so we need to mix an extra nitrogen source into the substrate. When we mix in up to 20%, we see less failure compared to standard cutting soils. The fibres fix some of the nitrogen present so we need to mix an additional nitrogen source into the substrate. When we mix in up to 20%, we see less failure compared to standard cutting soils. One explanation for this is the better drainage due to the addition of wood fibre. Less failure has a positive effect on production. In production crops, we still need to further investigate the use of wood fibre. But here, too, we see opportunities for wood fibre incorporation. 

Another alternative control method investigated is the purification of plant material by temperature treatment in the plantsauna. With the plant sauna, we try to remove latent infections with the respective soil pathogens through temperature treatment (up to temperatures of 44 °C) and start with healthy planting material. Plants infected by fungi on the tray field thus no longer enter the greenhouse and cannot cause spread and failure. Latent infections with P. cactorum in the planting material were eliminated by the plantsauna treatment in the majority of trials. In some trials, we did see an adverse effect on crop development. If the plants are kept for continued cultivation, we saw little added value of the plantsauna treatment in spring. During the trials, it immediately became clear that a treatment prior to frigo-storage is not a good idea. A plantsauna treatment after cold storage manages to reduce the losses to 3%. The technique is already in practice but scaling up the treatment infrastructure is definitely still needed.

Another alternative control strategy investigated is the use of plant enhancers in the propagation phase. Plant enhancers can at best add value as a component in an overall strategy. From the tests carried out, potassium phosphonates and the drug Accudo seem to offer the most potential as a component in such strategies with even a reduction of fallout by P. cactorum. However, the mixtures of Napagro and Hesco also showed good formation of roots on the tray field with very homogeneous crop stands, which could provide a useful contribution in the strategy.

Finally, screening for antagonistic microorganisms with potential for use in biocontrol towards soil fungi was also carried out. In order to identify new biocontrol agents, a selection of ~300 bacterial strains isolated in previous research from different ecological niches was made, with a focus on Pseudomonas spp., Bacillus spp. and Paenibacillus spp. as these are often associated with biocontrol activity.

Based on their antagonistic activity, identity (determined down to species level), and biochemical properties, a representative selection of 8 promising candidate biocontrol strains was made. These strains were tested for their biocontrol activity in a bioassay ("detached leaf assay") and in planta in small-scale greenhouse trials with the sensitive cultivar Sonata. All results obtained indicate that a promising candidate BCO was identified, and that this BCO is compatible with cultivation techniques and conditions in Flanders.

 The results described above concerning detection techniques, action thresholds and control strategies were compiled into new strategies for the control of soilborne pathogens and this for both field and substrate cultivation.

 Additional information can be obtained from An Ceustermans (an.ceustermans@pcfruit.be); Stef Laurijssen (stef.laurijssen@proefcentrum.be) and Hans Rediers (hans.rediers@kuleuven.be).