Contemporary Dutch mushroom growers can choose from an enormous range of mushroom strains. This article describes the various strains and the practical experiences of Dutch growers who harvest manually. Choosing is difficult, as the ideal mushroom strain does not exist.

The original strains were white, intermediate or brown strains. The white strain was crossed with the intermediate and brown strains, resulting in the hybrid strains. These hybrids had the natural characteristics of their parents, which lead to their classification into large hybrids, intermediate hybrids and small hybrids. As the name suggests, large hybrids strains are mushrooms with a large cap diameter and a high piece weight. The best-known example is the Horst U1. Small hybrids have a smaller cap diameter and a lower piece weight. Intermediate hybrids are somewhere in between the other two.

The term large hybrids is sometimes associated with difficult fruit body formation, intermediate hybrids with easier fruit formation and small hybrids are known for their spontaneous fruiting properties.

In the distant past mainly white strains were grown for manual harvest and after the crossing and creation of hybrids, the large and small versions. Intermediate hybrids came into the picture with the arrival of mechanized harvesting methods.

At the present day only large and intermediate hybrids are cultivated in the Netherlands. Small hybrids fruit too easily and produce many small mushrooms which makes labours costs in our country too high. These strains are mainly seen in warmer countries where fruit body formation using large and intermediate hybrids is more difficult.

Characteristics per group

The final characteristics of a strain harvested from a bed are party determined by compost quality, the structure and humidity of the casing soil, climatic conditions, the harvesting method and techniques etc. Comparing an identical strain on compost from two different suppliers will show distinct differences in production, extent of maturity, colour and so on. If this strain is only grown on one or two types of compost, the compost quality will partly influence the appearance and characteristics of the strain. However, a general indication can be given for traits within a certain group as well as the differences between strains in the group.

Large hybrids

In the Netherlands, the large hybrids U1, 608, X8 and Alpha are grown. These strains are really suitable for manual harvesting. They fruit somewhat difficultly which means the final number of mushrooms is easy to control in Dutch growing conditions. At blow down the air temperature must be quickly reduced by a further 0.5 to 2 degrees Celsius to obtain the same number of pieces compared to an intermediate hybrid. On average the mushrooms are larger, mature less quickly, have a higher piece weight and should be whiter in colour. The disadvantage of the large hybrids is their sensitivity to compost quality. In concrete terms, this means that if there are problems with compost quality, production and quality levels will fall faster than with the strains from the intermediate hybrid group.

Intermediate hybrids

Intermediate hybrids are becoming increasingly popular due to their strong growth pattern and good performance on less than optimal composts. The production level is often slightly higher than that of large hybrids. Their rapid growth and easy fruit body production means they are better suited to tight growing schedules. The first picking day can often be brought forward by between one to three days. Intermediate hybrids usually produce more mushrooms, are slightly more sensitive to scaling and  have a greater tendency to veiling than large hybrids. Various intermediate hybrid strains have certain characteristics of the large hybrid strains. A combination of their fast growth and less reliance on excellent quality compost makes these strains highly requested at the moment.

In The Netherlands the following intermediate hybrids are grown: A15, 512, 737, Delta and 2200. Predominantly A15, 737 and Delta used for the fresh market.

The perfect strain

The perfect strain for manual harvesting does not exist and if growers were asked to describe their idea of the perfection they would all give different interpretations of the various characteristics. Asking this question now or ten years ago would show a shifting accent on certain features. At the present time we expect more importance to be placed on colour, shelf life and piece weight than in the past. The perfect strain can be picked five days a week with a maximum daily production of 4 kilos per square metre, is pure unblemished white, smooth, has an attractive rounded shape, a high piece weight, is damage resistant, is impervious to bacterial blotch and bubble and can easily be kept for five days. Plus it produces 40 to 45 kilos per square metre in three uniform flushes. The list could doubtless be extended, but the majority of growers would be more than satisfied with this! Unfortunately none of the strains can fulfil all these wishes using the current raw material available. This explains why choices still have to be made. In the second section of this article, several mushroom growers will share their thoughts on the strains they use.

Alpha versus U1 and 608
Manager Patrick Dingenen and Bert Boonen, Breechamp, Bree:

We only grow large hybrids; the first reason is their quality and the second their better controllability. On our farm fruiting is quite spontaneous on average. We grow the U1, Alpha and 608 on CNC, Verschoor and Walkro compost in succession. Of course, compost quality has an obvious influence on production and quality of the end result. These three large hybrids show a clear line. For speed (the moment of blow down up to the start of harvesting) Alpha is the fastest, followed by the 608 with the U1 bringing up the rear. The strains also react differently to changes in compost quality. This has something to do with the strain's vitality. The Alpha is the least sensitive to changes in compost quality and the U1 the most sensitive. The number of picking days in the first flush is longest with the U1 reaching 4 to 5 days, the Alpha with 3 to 4 days, while the 608 is usually finished after 3 days. There is hardly any difference in colour between the three mushroom strains. The piece weight of each mushroom varies from heavy to less heavy from U1, Alpha to 608. A higher piece weight doesn't mean better production, as there is little difference in performance between the three strains grown on our farm. If we take the percentage of small-grade mushrooms harvested, the U1 gives 5 to 6 percent, the Alpha 8 to 10 percent and the 608 about 15 percent.

The Alpha is the quickest of the three, as well as being the least compost sensitive, and concerning the number of picking days and size between the U1 and the 608. There are no obvious variations in colour and production.

U1 to A15
Mushroom grower Michel Willé, Zichen Zussen Bolder:

At the end of autumn 2000 our entire farm switched to growing U1 on CNC compost. Previously we always used U1 in one room and filled the second room with S100, 608 or U1. We stopped using the S100 as there were problems with quality and the 608 because of clustering. We then filled just U1 for a while. Production snags in the third flushes and problems with the colour in the first flushes of the U1 meant we changed to alternating U1 and A15 in May 2001. In the beginning the yield given by the U1 was about 1 kilo per square metre lower than with the A15. A year later this figure had risen to 5 kilos per square metre. And this is with compost from the same supplier, the same farm, climate installation etc. The difference in yield was the main reason to change to fully cultivating the A15 in May 2002. The better colour in the first flushes showed in the huge increase in the percentage of grade I mushrooms.  The A15's more spontaneous fruiting usually gives more mushrooms, but this is no problem, as we want to harvest a large percentage as small grade.

Trial X8
Maurice Koppen, Oakfield Mushrooms, Stramproy:

We currently cultivate mainly strain A15. In the past we chiefly grew the U1 and the A15. We are running a few trials at the moment with spawn strain X8. Results from Switzerland with this strain were hopeful, so we are experimenting with a few growing rooms on Verschoor compost. Production levels of the X8 are a bit below the A15, but the first results show that mushrooms are whiter in colour at harvest. Even when the mushrooms have been in cold storage for four days they have an exceptionally good appearance. However, it's a little early to draw any conclusions yet. In the coming weeks a few more rooms will be put into production and then we can judge properly if this strain performs better than the A15 and see if we will be using it more in the future for its possibly whiter colour and better shelf life. (In the meantime we have heard that trials at Oakfield with the X8 will not be continued -editor.)

From A15 to U1
Mushroom grower Dirk de Vries, Kerkdriel:

About a year ago I replaced the A15 by the U1 on CNC compost. The reasons then were namely quality problems and low yield particularly in third flushes. The A15's start was good, but at the moment the quality of the second and third flushes is not as it should be. This is possibly partly  caused by steps I took to reduce the number of pieces, such as high air temperatures, not introducing too much fresh air between flushes and compacting the casing soil too firmly. This has probably been counterproductive, because it's still difficult to constantly get the right number of mushrooms and correct spread over the beds in the second and third flushes. In the first flush it's not a problem to pick larger medium grades but in later flushes I have more medium grades of 45 - 55 millimetres.

The time between the first and second flushes is also increasingly shorter, which creates more peak labour periods on my farm. The A15 is of course less compost sensitive, but compost quality recently has been relatively constant so I don't expect many problems with the U1. Anyway, the first and second flushes look perfect after the first round. Growing a large hybrid again takes some getting used to, but I hope we can continue harvesting large, heavy, white mushrooms. Hopefully this will help improve the competitive position of the trading organisation.

A15 against 737
Mushroom grower Willy Geurts, Milheeze:

We grown strains A15, 737and 608 on incubated compost from Nooyen. The problem with the A15 is controlling numbers in the second flush. Reducing numbers by raising the air temperature after the first flush doesn't always give the same results. This means that we sometimes have to pick higher quantities of finer grades if there are too many mushrooms on the beds. With the 737 the number of pieces in the second flush is easier to control, which is a big advantage of this new strain to the Netherlands. In other aspects the 737 closely resembles the A15. Controlling the first flushes is not problematic in either strains, but alertness is required. The yields given by both strains are comparable. So the first experiences are positive. It we remains satisfied, we intend to fill more rooms using the 737.

I still have the 608 in production on my farm, also on Nooyen compost. This mushroom is better in quality, however with a lower yield of around 3 kilos per square metre. And that makes a big difference in financial terms. The 608 will be given a rest before the coming holiday period. We have to bring the flushes forward and that's easier with the faster intermediate hybrids.

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