This article discusses recent research papers. Most of this type of research was conducted by Asian scientists, as exotic mushroom cultivation is widely practiced there. The first subject is how electric stimulation improves both the number of fruit bodies and the total yield. However, there is no equipment on the market to introduce this technique on a commercial scale, yet. The second subject, Super Water Absorbents, can easily be put into practice by commercial growers.
Tohoku Electric Power Co, Japan
This energy company on the Japanese countryside surprisingly performed research on shiitake too. Less surprising is the subject: electric stimulation of shiitake fruit body formation. Unlike oyster mushrooms, enokitake (Flamulina velutipes) and Hypsizygus, the production of shiitake on small sterilisable units has not developed rapidly in Japan. An important reason is that shiitake cannot be cultivated in bottles, as the mycelium forms fruit bodies on all sides. The spawn run time (often 90 days) and fruiting periods are also quite long, compared to other mushrooms. A third reason is that fruiting requires low temperatures, and cooling is prohibitively expensive for a crop with a long fruiting period.
The Regional Technology Promotion and Development program therefore developed techniques which hasten the fruiting of shiitake on artificial media. The R& D Centre had already found that an electric stimulus in the order of kV helped fruit body formation in wood logs, but these are heavy, irregular in size and therefore difficult to handle. The new research (published in March this year) was dedicated to substrate in sterilized bags. A mesh shaped electrode was placed against the substrate blocks, and a 50 Hz AC electrical stimulation was given at various potentials ((200, 400 and 600 V).
The electric stimulation system consisted of a simple 100 V AC source, a variable #spoel# which could be switched to 3 positions (200, 400 and 600 V), a voltmeter (parallel), ampere meter in series and the mesh electrode.
The treatment was given for 30 seconds, prior to the submersion of the blocks in water, at intervals of 20 days. The number of mushrooms increased with 40% with the 600 V treatment, compared to the control which lacked electrical stimulation. However, at lower voltages, there was no significant increase. Strikingly, the reported increase was found in the first flush only, in contrast to the study by Ohga e.a. in the following paragraph.
The effective current is in the range of 150 – 400 mA, at a voltage of 600 V. This means the electric resistance of the substrate fluctuates in the same order, as V (voltage) = I (current) * R. (resistance). Tohoku is currently developing a prototype of a stimulating device.
Pulsed power for exotic mushrooms
A well detailed study by S. Ohga e.a. was presented in April this year.
Four types of shiitake strains were tested. The first flush was inducted spontaneously, the second and third by submersion under water for 16 and 20 hours respectively.
The substrate was given a pulsed power treatment with a 100-170 kV Yamabishi instrument, 1.0 kWs. The first mushroom flushes appeared within two weeks after the pulsed power treatment. The treatment resulted in a higher production for all species. The last strain, KS-12, is a winter strain and 90 days spawn run was not sufficient for this species. Interestingly, the effect of the pulsed power treatment is greatest for a shiitake strain in this condition. It suggests that spawn run may be shortened in combination with the pulsed power.
The pulsed power treatment also increased the yields of the other investigated mushrooms. Pleurotus production most notably increased. Pleurotus abalonus was the most sensitive with a yield increase of 73%; Pleurotus ostreatus reached 68% and Pleurotus eryngii 50%. Yield increase is defined here as the increase in comparison to the controls; e.g. from 180 gramms per 1.2 kg bag to 270 grams for P. eryngii.
The results of this study cannot easily be transferred into practice, as no suitable equipment is available for the treatment on a large scale. Yet another factor is important: how to retain moisture? European growers dislike submerging the substrate because of the expensive labor involved. They try to retain humidity by misting and spaying, which leads to lower humidity levels.
Water Super Absorbent
Ohga e.a. also published a study what effect Water super absorbent (WSA) has in shiitake substrate. WSA’s are widely used in horticulture and in tree nurseries, e.g. when bare root plants are transferred. WSA has a water holding capacity which is 100-1000 times higher than its dry weight.
The moisture content of sawdust-based substrate is an important factor in the fruiting process of the shiitake. Normally, the moisture content decreases much after each flush; the blocks of substrate are submerged under water for several hours to retain their moisture. Absolute water content and water potential are important factors for fruiting initiation.
To study the effect of WSA, the standard 1,2 kg blocks of substrate were used, with a substrate of sawdust and wheat bran (ratio 5:1), WSA and a moisture content of 63%. The controls of course did not contain WSA.
The spawn run was not influenced by the extra WSA. Biological efficiency of the 1st flush was superior on substrate containing WSA compared to the control: depending on the strains the improvement for the first flush was 13 – 20%, 26% - 46% for the second flush and 38 – 42% for the 3rd flush. WSA seems to improve the water availability for fruiting. Thus the conclusion is that it seems worthwhile to investigate further in this respect.
The key question for growers is of course: how much do I earn extra with it? Obviously, this depends on the costs of the WSA. Some WSA’s are prohibitively expensive. One type of WSA, Watersorb, costs 170 US$ for 50 lbs including shipping in the US. A small amount of WSA is sufficient, as its water uptake is a factor 100 – 1000 its dry weight. Added in a 0,1% ratio (dry weight of the WSA versus wet weight of the substrate), 1 kg of WSA would have to be added to one ton of substrate. If the grower picks only 3 kg of shiitake extra from per ton, he already makes a profit.