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  • Writer's pictureBart Acres

Mushroom Growing Troubleshooting

Updated: May 14, 2023

While mushroom cultivation can be a fun and rewarding hobby, it doesn't go perfectly all of the time. Mushrooms are unique organisms with complex biology, and when we humans cultivate them, it is our attempt at mimicking their ideal conditions for healthy cropping.

Sometimes one or more factors will be slightly 'off', resulting in problems with the mushroom growth and difficulty in obtaining a healthy and bountiful crop. So, we have compiled this list of some of the more common problems seen in home mushroom cultivation, their possible causes, and some options for solutions. If you want to jump to a specific question, just click on the question below.

Let's start off with one simple tip, and this is, 'think like a mushroom!'. Many problems with mushroom growing can be greatly helped by simply imagining a kind of environment where you might stumble across a healthy crop of mushrooms in the forest... They have a good nutritious substrate to grow from, plenty of fresh air, medium to medium high humidity, a moderate temperature, and gentle dappled light. This is the kind of conditions which will make mushrooms thrive! This tip is so fundamental, it's how we got our name - 'Myco' (pertaining to mushrooms) 'Logic' (a particular way of thinking).

Now, lets get into some specific problems commonly encountered by mushroom growers, their possible causes, and solutions. Click on a topic to jump to it or just have a browse!

Mushrooms are wet, soggy or discoloured

We have a tendency to think of mushrooms as loving as much humidity and moisture as possible. This leads to many growers applying 'generous' amounts of mist / sprayed water onto their mushrooms. In reality, the best quality mushrooms can be grown in 'moderate' to 'moderate high' humidity.

It can often be better to focus on providing a humid environment, rather than spraying the mushrooms with water directly, which can result in them becoming water logged. It's also very important to make sure that mushrooms are growing somewhere with a decent amount of air movement and air exchange. Mushrooms do not like to be 'cooped up' with stale or still air. A bit of air movement and freshness can help to evaporate surplus moisture from mushroom fruitbodies.

Solution: Mist mushrooms less, decrease humidity, increase airflow

Mushrooms are dry, crinkled, or yellowing at the edges

Mushrooms exposed to dry conditions
Mushrooms exposed to dry conditions

These are classic signs of mushrooms drying out. Exposure to direct sunlight, or strong wind can cause mushrooms to dry out rapidly. Also if the general humidity is very dry at the time (e.g. in hot, dry areas during summer) it can be a challenge to keep mushrooms hydrated. If mushrooms have only dried out a little bit, then they can often recover from that, if they are substantially dried out, its best to harvest those and adjust to better conditions for the next flush.

Solution: Increase the general humidity in the area, move mushrooms away from direct sunlight or strong wind, but be conscious of maintaining adequate airflow for healthy mushroom growth.

Mushrooms are spindly and elongated with small caps

Mushrooms grown without enough fresh air become spindly

This is one of the most common problems seen in the mushroom growing world, so much so that it has spawned a mantra in mushroom communities of 'More FAE!' This stands for more fresh air exchange. Spindly, elongated mushrooms are caused by too high of a concentration of CO2 in the growing environment. Mushrooms and mycelium excrete CO2 (the same as humans) as part of their metabolism. So, if the CO2 that they produce is not ventilated by a healthy amount of fresh air movement, fruits will become spindly, as they are stretching in search of oxygen in a similar way to how a plant might stretch in search of light. Oyster mushrooms in particular require much more fresh air than people imagine.

Solution: Increase fresh air exchange and air movement in the grow space

Mushrooms are large, ruffled and releasing lots of spores

Over mature oyster mushrooms
Over mature Oyster Mushrooms

Mushrooms grow exponentially fast. As in, in the first couple of days of growth, they may not increase in size by much. And then nearer the end of their growth, they can rapidly increase in size in just one day or even a few hours. This means that its quite easy for mushrooms to go from 'ready to pick' to 'over mature' in a fairly short window of time, especially if the weather is warm (and therefore the mushrooms are growing faster). Mushrooms that are over mature can start to degrade in quality by becoming oversized, ruffled at the edges, releasing large amounts of spores, and gaining a pithy texture. Further to this, mushrooms which are allowed to grow to a large overmature stage will rapidly deplete the energy and moisture reserves of the substrate, 'robbing' potential yield from the second or third flush.

Solution: Pick mushrooms earlier, before they become over mature

There are small flying insects hovering around the mushrooms and gills

Fungus gnats

There are a few types of insects which are known to enjoy eating mushrooms, the main ones that you will encounter are called fungus gnats (also known as sciarid flies). Many mushroom growers consider fungus gnats to be their #1 enemy! An outbreak of fungus gnats in a mushroom grow space can be very frustrating, as once they become established, they can be hard to get rid of.

There are two main kinds of fungus gnats seen on mushrooms in New Zealand - a smaller species which looks very similar to a sandfly, and a species with longer wings, legs and abdomen which looks very similar to a mosquito or small crane fly. The lifecycle of both is basically the same - adult gnats sniff out mushrooms, and lay eggs into the mushroom tissue. The eggs hatch into small larvae (these look like miniature maggots) which then tunnel through the mushroom stems and cap, eating it as they go to fatten themselves up and prepare to hatch into adults.

Generally, gnats are more prevalent at warmer times of year. In areas with cool frosty winters they can disappear entirely for the colder six months of the year. Mushrooms with a small amount of gnat larvae damage (a few tunnels visible when the mushroom is cut) is no problem for home consumption, however after a while the larvae will eat the mushrooms to the point that they begin to fully disintegrate, which is not ideal at all! And it is considered unacceptable for commercial growers to sell mushrooms with any sign of gnat tunelling at all.

So, its in the best interest of mushroom growers to keep fungus gnats to an absolute minimum in the grow space. Unfortunately, there is no single 'silver bullet' solution for the issue of gnats. Most experienced growers employ a range of methods which contribute to a 'multi pronged approach' for reducing and managing gnat populations. Some of these include:

  1. Physical exclusion. It's certain that prevention is better than the cure when it comes to fungus gnats. An established population can be difficult to eliminate without entirely clearing out the grow space and starting fresh from scratch. So, keeping gnats out in the first place is a great thing to try for. Check for gaps where gnats can get into your growing area, and use shade cloth or other breathable mesh material to block these gaps off so that adult gnats can't just fly in from the outside world to lay eggs on your mushrooms. If you're just growing one or two mushroom kits, our mini mushroom tents are completely gnat proof and provide a great environment to grow mushrooms in.

  2. Sticky traps or beer / vinegar traps. Often times, a few gnats will inevitably find their way into the grow space. It's a good plan to have a strategy in place to trap as many gnats as possible, before they get a chance to lay eggs in your mushrooms or mycelium. The most common trap that mushroom growers use is yellow sticky traps, a yellow card which gnats are attracted to and is covered in a sticky film that they get stuck to. You can hang one or two of these in the grow space, which will serve not only to monitor the gnat population, but also to trap them. Replace the cards about once a month. Some growers also use bowls or cups full of beer or vinegar, the concept behind this is much the same as sticky cards - lure the gnats in, and then drown them.

  3. Removing infested grow blocks. Once a mushroom substrate has become infested with fungus gnat larvae, its pretty hard to get rid of them. Leaving an infested grow block, bag or bucket in the grow space means that these larvae will soon hatch into adults and only serve to make gnat problems much worse. So it pays to closely observe the grow space, any blocks which have a large number of gnats hanging out on or near them should be removed from the grow space and disposed of, or isolated somewhere far away.

  4. Interrupt the gnat life cycle. From the day a fungus gnat egg is laid until the day that the larvae hatches into a fertile egg laying adult is around 30 days. In good growing conditions, most mushroom species should be able to have two flushes after a one month period in the grow space. Many commercial mushroom growers are in the firm habit of only keeping grow blocks for two flushes, especially if fungus gnats are present, because an old block hanging around for a long time in the hopes of a (usually small) third flush is likely to cause more problems than it is worth.

  5. Predatory insects and carnivorous plants. Some growers have found that natural 'biocontrol' of gnats can be quite effective. This includes keeping carnivorous plants like venus fly traps, sundews, and pitcher plants in the grow space. Sundews seem to be the best at catching gnats. Predatory insects like spiders and praying mantis can also be on full time patrol for gnats. Some growers have even allowed tree frogs to live in their grow space to help keep gnats at bay.

  6. Insecticides. Some mushroom growers and especially large scale commercial mushroom farms do employ chemical insecticides for the control of fungus gnats. We do not recommend this approach, as many of the insecticides available have potentially adverse health effects for consumers, and this is made difficult by the fact that sprays or mists can land directly on the mushrooms, and mushrooms are not commonly washed before being cooked and eaten. Any pesticides or insecticides used in a mushroom grow space should be thoroughly researched beforehand to make sure that they are safe to use and are food safe.

Solution: Utilise one or several of the methods mentioned above to control and manage fungus gnat populations

Green, black or other coloured mold is visible on the substrate or mushrooms

Mushroom substrate is chosen because it's good, nutritious food for mushroom mycelium. Unfortunately, this also makes it great food for other fungi, including molds! If mold spores happen to land on mushroom substrate before it's fully colonised with mycelium, there is a good chance that a patch of mold may grow there. The same can happen on actual mushroom fruitbodies, especially if they are growing in areas of high humidity and/or with lack of airflow. Contamination by mold can occasionally happen in even the best, highest tech mushroom farms, but one goal of the mushroom grower is to keep incidence of contamination as low as possible by following good hygienic practice, and removing any visible mold from growing areas as soon as possible. If you are growing a mushroom grow kit and notice mold growing, it can possibly be salvageable. Coin sized patches of contamination are often no problem at all - they can either be cut out, or even if they are left as is the mushroom mycelium will commonly fend them off. If contamination is widespread or actively spreading, it is likely that the grow block is not salvageable and should be isolated away from other grow blocks, ideally disposed of or add the substrate to the compost heap. Growing environments such as tubs, tents or grow rooms which have had badly contaminated blocks in them should be cleaned after all of the contaminated substrates have been removed.

Solution: Don't worry about small patches of mold. For widespread contamination, dispose of blocks and clean the growing area with a mild bleach solution.

Mushrooms are growing slowly

Mushrooms, being living organisms, have a certain degree of natural variability in the way that they grow - even when all conditions are the same, they don't necessarily grow like clockwork, so some variation in the speed of mushroom growth can be expected. It's also important to keep in mind that, contrary to popular belief, mushrooms do not spring up 'overnight'. Mushroom fruitbodies will commonly take several days to several weeks to develop to a harvestable size, depending on the species and the growing environment.

The number one biggest influence on the speed of mushroom growth is temperature. Mushrooms will generally grow very slowly when it is very cold, and temperatures which are very hot can also slow down or stall growth. Most mushrooms grow the fastest at around 20 to 25°C. Temperatures below about 12°C will result in slow fruitbody formation. However, mushrooms growing as fast as possible is not necessarily the main goal. Mushrooms grown fast in warmer conditions can have inferior texture and flavour, and will be quick to 'blow out' and over mature, so it can be hard to harvest them at the right time. Mushrooms grown slowly in he cold may require a bit more patience, but the texture and flavour is usually superior. In our grow rooms, we aim for a growing temperature of around 15 to 20°C for premium quality mushrooms that are easy to harvest at the right time.

Solution: Ensure that temperatures are in the correct range and that the substrates are not contaminated

Mycelium is taking a long time to colonise substrate

There are a few different factors that can influence the speed of colonisation. These include temperature, spawn rate, substrate hydration, substrate aeration, mushroom species, and substrate sterility. Under good conditions, mushroom substrate should colonise in one to three weeks, depending on the species and what proportion of spawn was used.

Most mushroom mycelium grows fastest around 20°C. If it's below 15°C or above 25°C you will notice that the growth rate slows down. Extremes further outside of these ranges can cause colonisation to stall completely.

Substrate hydration also has a massive influence on the speed of colonisation. Substrate should be hydrated to the point that when a handful of it is squeezed firmly, it feels moist and maybe a drop or two of water comes out. If multiple drops of water come out when the substrate is squeezed, it is over hydrated, which will drastically slow down the speed of colonisation. Grow bags containing over hydrated substrate will usually look very wet at near the base if the block is flipped over to view the bottom layer.

Over hydrated substrate can also be more prone to bacterial contamination, which may not present as visibly obvious contamination, but will hamper progress of mycelium through the substrate if this occurs (and can cause further problems, like bacterial blotch, during fruiting). If you gently squeeze the grow bag and sniff the air coming out of the filter patch, it should smell woody, fresh and fungal. 'Funky' or sickly sweet smells can be a sign of bacterial contamination.

The other reason that over hydrated substrate can be problematic relates to aeration of the substrate. Since mycelium is alive, it needs to respire (breathe) fresh air in order to keep growing. Over hydration can result in water blocking the microscopic air channels in the substrate, hampering the myceliums ability to breathe. Other restrictions to air flow, such as the top of a bag being folded over so that the filter patch is blocked off from the substrate, or incubating in a crowded space without adequate ventilation, can also slow down the speed of colonisation significantly.

It's also important to remember that different species of mushroom colonise at different speeds. Oyster mushrooms, for example, are very fast and vigorous growers, spreading a layer of dense white mycelium across the substrate very quickly. Whereas other species, like Enoki or Hericium, may have slower growth, and the mycelium may appear much less vivid white with some species (for example, the mycelium of Pekepeke-kiore / Hericium is very thin and wispy, sometimes requiring a very close inspection to even see it!)

Solutions: Ensure that temperatures are in the right range, check that substrate is not over hydrated, check for bacterial contamination, ensure that mycelium can breathe, be patient!

Yield was smaller than expected

Yields of mushrooms is an inherently variable thing, given that mushrooms are living organisms, and there are always going to be subtle differences between different substrate blocks, made or fruited at different times or in slightly different conditions. Outside of this natural variation, there are a few factors that can influence yield, some to fairly significant degrees. Some of these factors include:

  1. Substrate nutrition can have a big influence on yield. Substrates supplemented with a portion of higher nutrient material (like soybean hulls, beet pulp flakes etc) are known to produce higher yields than sawdust or straw on its own. It pays to either follow standard substrate recipes, or have a good idea of the types of things you want to add and at what proportions to make a good substrate. You can read about substrate mixes in our grow guides section.

  2. Substrate density can also play a role in yields. Remembering that the natural substrate of most edible mushrooms is either logs or soil (both of which are quite dense) it pays to make sure that you pack your substrate nice and tightly so that it has a good concentration of water and nutrients to pump out big crops of mushrooms.

  3. Growing environment is obviously a key factor here as well. Just remember to focus on the basics. Temperature, humidity and air exchange. If any of these three factors are outside of the optimal range then it can greatly reduce yields for one reason or another.

  4. Contamination can hamper yields of mushrooms as well. While it is quite easy to see visually if a block is contaminated by molds, what is less easy to see is bacterial contamination, which is subtler in appearance but can still waste the myceliums energy in fighting it off, rather than growing big crops of healthy mushrooms!

  5. Previous cropping is a big influence too. It's normal for the size of subsequent flushes of mushrooms to be smaller than the first flush. However if mushrooms from previous flushes had been allowed to become very large or overmature, its going to have a negative impact on subsequent crops. Whereas harvesting mushrooms at the correct stage of maturity for each flush will leave energy behind for the next mushrooms, and even out the yields across multiple flushes.

Normal yields for oyster mushrooms would be around 1/3rd of the original weight of the substrate block (so, a 3kg substrate block should yield around 1kg of mushrooms over 2 or 3 flushes). For most other species about 1/4 of the weight of the substrate before fruiting is a good estimate. Experienced growers can often obtain higher yields than this under good conditions.

There's a powdery substance on surfaces near the mushrooms

Sporulating mushrooms
Sporulating mushrooms

As we all know, mushrooms release spores in order to reproduce. Spores are microscopic in size, but mushrooms release them by the millions, if not billions, when they are mature. It's not uncommon for mushrooms to release visible amounts of spores - these can often be seen collecting as a dust-like substance on surfaces near the mushrooms, the colour of which depends on the mushroom species but usually white or brown (or pink for pink oysters). and in the right lighting sometimes spores can be seen floating directly out of mushroom gills, appearing like thin wisps of smoke trailing through the air. Where there is not much air movement, sometimes spores can stick together via electrostatic charge and form 'icicles' dangling off of the edges and gills of mushroom caps.

Excess spore release can become a messy problem, especially if spore dust becomes wet and can then start to go moldy in the grow space. Spores can also clog up exhaust fans, humidifiers, and other equipment in the grow space. Further to this, its not great for human health to be breathing in too many spores - so its always a good idea to kee this in mind, don't grow mushrooms in your main living areas and wear a mask in areas with high spore load.

Mushrooms release the most spores in the later stages of their development. If mushrooms are harvested at the correct stage which would be considered 'optimal' for eating or for sale, they should not yet be releasing 'mega' loads of spores. Harvesting at the correct time is the best way to prevent spore buildup in the growing area.

Solution: Harvest mushrooms earlier, ensure adequate ventilation and exhaust in the grow space, don't grow mushrooms inside your house, wear a mask in areas with high spore load.

Other mushrooms grew in my outdoor bed or mushroom log

When we're growing mushrooms outdoors, whether that's logs or outdoor patches, we're essentially placing nice, fresh, nutritious mushroom food in an outdoor environment. We inoculate the substrate with spawn or dowels in order to give the mushroom that we want to grow an upper hand in being the first one to colonise the substrate. But, of course, there are always going to be spores of wild mushrooms floating in the air which can also land on and grow in our substrates. It's not uncommon for other species of mushrooms asides from your intended species, to grow in outdoor mushroom patches or on mushroom logs. Unless the infestation by wild / 'weed' mushrooms is severe, this won't usually impact the growth of your intended species too much - mushrooms can often happily coexist in relatively close quarters. But it does mean that its always a good idea to positively identify your mushrooms grown outdoors, before you eat them, because the mushroom might not be what you inoculated with.

Solution: Precede as per the original plan! If infestation of wild fungi is severe, it may be time for a second attempt at the project, discarding original material. Be sure to identify all outdoor grown mushrooms before consumption.

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