How to Grow Oyster Mushroom HK-35 (Pleurotus ostreatus)
How to Grow Oyster Mushroom HK-35 (Pleurotus ostreatus)
Oyster Mushroom HK-35 (Pleurotus ostreatus) is grown by inoculating pasteurized wheat straw bags with liquid culture grain spawn, colonizing at exactly 77°F for a minimum of 20 days, then fruiting at a strict 61°F with humidity held at 70–85% RH — a range most growers set too high, since peer-reviewed data confirms that running above 85% RH produces thinner, shorter-lived caps specific to this strain. HK-35 holds the fastest documented mycelial growth rate among all tested Pleurotus ostreatus strains, but it punishes fruiting temperature errors harder than any other — a drop of just 3°F below its 61°F optimum costs 30–50% of first-flush yield.
Oyster Mushroom HK-35 Equipment — Pasteurized Wheat Straw
| Item | Spec / Notes |
|---|---|
| HK-35 liquid culture syringe | Out-Grow HK-35 liquid culture — 12 cc per syringe, sufficient for multiple grain jars or bags |
| Wheat straw | Chopped to 3–5 inch pieces; best-performing substrate for HK-35 yield and mycelial growth rate across all tested substrates (Sobieralski 2011). Available at Tractor Supply or Rural King, approx. $5–8/bale |
| Wheat berries or rye grain | For making grain spawn — 1 lb dry grain per bag of fruiting mushroom substrate |
| Pressure cooker | 15 psi capable — for grain spawn sterilization only; straw uses pasteurization |
| Large stockpot or cooler | 10+ gallon; for hot-water pasteurization of straw |
| Probe thermometer | Required — must confirm internal straw bag temperature holds 160–165°F during pasteurization |
| Mushroom grow bags with 0.2-micron filter patch and self-healing injection port | Out-Grow grain bags include both; allows direct liquid culture injection without an impulse sealer |
| 18-gauge needle and syringe | For injecting liquid culture through self-healing injection port |
| 70% isopropyl alcohol | For wiping injection port before each injection |
| Room thermometer (verified accurate) | Critical — HK-35 loses 30–50% first-flush yield if fruiting drops to 58°F; verify against a second thermometer |
| Hygrometer | Required — do not estimate humidity; HK-35 cap quality degrades above 85% RH |
| Humidifier or spray bottle | Target 70–85% RH during fruiting; lower end of range produces thicker, firmer HK-35 caps |
| Indirect light source | 300 Lux for 8 hours/day required for pin formation — any window light or LED strip qualifies |
Oyster Mushroom HK-35: Pasteurized Wheat Straw Method
- 1 lb dry wheat berries (preferred by HK-35 hobbyist consensus) or rye grain
- Water for soaking and simmering
- Pressure cooker capable of 15 psi
- Quart mason jars or mushroom grow bags with 0.2-micron filter patch and self-healing injection port
Place wheat berries in a large pot, cover with water, and soak for 12–24 hours. Drain, then simmer in fresh water for 15–30 minutes until each berry is slightly tender but not split or bursting. Drain well and spread the grain on a clean surface for 20–30 minutes to let surface moisture evaporate — grain loaded into jars while still slick on the outside creates pockets that contamination can exploit. Fill mason jars or grain bags to ¾ capacity and pressure cook at 15 psi for 60–90 minutes. Allow the cooker to return to zero pressure on its own before opening — never force-cool.
If you want to skip grain preparation, Out-Grow's sterilized grain spawn mushroom substrate bags come ready to inoculate, with filter patch and self-healing injection port already installed.
- Out-Grow HK-35 liquid culture syringe (12 cc)
- 18-gauge needle
- 70% isopropyl alcohol and wipe
- Cooled sterilized grain jars or bags from Step 1
- 3–5 cc liquid culture per quart jar; 5–10 cc per larger grain bag
Wipe the self-healing injection port with 70% isopropyl alcohol and let it dry for 30 seconds. Warm the liquid culture syringe briefly in your hands and shake gently to redistribute any settled mycelium. Attach a fresh 18-gauge needle and inject the liquid culture through the self-healing injection port — the port reseals after each insertion, so no additional sealing material is needed. Inoculate at multiple points around larger bags to distribute the HK-35 liquid culture evenly. Place inoculated grain at 77°F.
Healthy Oyster Mushroom HK-35 (Pleurotus ostreatus) liquid culture appears as a suspended white to off-white cloudiness in the syringe. If the liquid looks yellow, brown, or has settled completely without any cloudiness remaining, the liquid culture may have degraded and should not be used.
- Inoculated grain jars or bags from Step 2
- Space holding 77°F — do not exceed 86°F; yield drops 25–30% above 82°F and colonization stops entirely above 88°F
- No light required during colonization
Place inoculated grain in a location that holds consistently at 77°F — a dark shelf, closet, or cabinet all work. Oyster Mushroom HK-35 (Pleurotus ostreatus) holds the fastest documented mycelial growth rate of any tested Pleurotus ostreatus strain: expect noticeably aggressive colonization, with rope-like rhizomorphic strands binding the grain into a dense, unified mass as the mycelium matures. Shake or break up the grain gently once the mycelium has covered roughly one-third of the jar or bag, then let it finish undisturbed. HK-35 was specifically documented as producing "denser and thicker" spawn run than all other strains in comparative trials — this is normal and correct behavior for this strain.
Watch for color other than white: bright blue-green or yellow-green powder is Trichoderma mold and the bag must be removed from the grow area immediately, before spores disperse. Gray-brown sliminess with a foul smell is bacterial contamination — same action. Both usually result from inoculating before the grain cooled fully, or from grain that was not pressure cooked long enough.
Ready to start growing? Out-Grow carries HK-35 liquid culture — the world's most studied commercial Pleurotus ostreatus strain.
Start with this culture — Pleurotus ostreatus HK-35- Wheat straw chopped to 3–5 inch pieces — 5 lbs dry straw per fruiting bag (do not substitute oak or birch sawdust alone; Sobieralski 2011 documented significantly slower HK-35 mycelial growth and lower yield on these hardwoods versus wheat straw)
- Large pot, turkey fryer, or clean cooler
- Probe thermometer
- Mushroom grow bags with 0.2-micron filter patch
- Enough water to fully submerge the packed bags
Pack straw loosely into mushroom grow bags, then submerge the packed bags completely in water heated to 160–180°F. Weight the bags down so they stay below the surface. Insert your probe thermometer inside a bag and confirm the internal temperature holds 160–165°F for a full 40–60 minutes — the water temperature outside the bag is not what matters. After 60 minutes, remove bags and drain them in a clean location until the internal temperature drops to 85°F or below. Inoculating above 85°F will kill HK-35 grain spawn on contact.
Lime cold-soak alternative: Dissolve ⅛ cup hydrated lime (calcium hydroxide) per gallon of cold water. Submerge straw fully and soak for 2–4 hours — the elevated pH (above 12) suppresses competitors without heat. Drain completely and inoculate immediately; no cooling wait required.
Do not use oak or birch sawdust as a sole mushroom substrate for HK-35. Peer-reviewed trials confirmed that HK-35's enzymatic activity is not sufficient to efficiently break down dense hardwood lignin without supplementation, reducing both colonization speed and first-flush yield. Use wheat straw or a 4:1 wheat straw/hemp shive blend — the hemp shive blend produced higher first-flush yields than wheat straw alone in Sobieralski et al. trials.
- Colonized HK-35 grain spawn from Step 3
- Pasteurized wheat straw from Step 4
- Spawn rate: 3% by weight is the peer-reviewed Somosné Nagy figure for HK-35 specifically; for first-time growers, use 15–20% (roughly 1–1.5 lbs colonized grain spawn per 5 lbs prepared straw) — the higher rate gives HK-35's mycelium a colonization speed advantage over any surviving competitor organisms
- Large mushroom grow bags with 0.2-micron filter patch
- Clean, alcohol-wiped work surface
Wipe your work surface with 70% isopropyl alcohol. Open the colonized HK-35 grain spawn bag and break the colonized grain apart into loose pieces. Layer grain spawn and pasteurized wheat straw alternately into each mushroom grow bag — a layer of straw, a layer of grain spawn, another layer of straw — until the bag is full. The goal is grain spawn distributed throughout the entire straw mass, not concentrated at the top or bottom. Seal the top of the mushroom grow bag with an impulse sealer, or fold and clip it tightly. The 0.2-micron filter patch handles all gas exchange — do not puncture the bag walls.
- Inoculated mushroom grow bags from Step 5
- Space holding 77°F — do not exceed 86°F
- 20–26 days colonization time — the 20-day minimum is non-negotiable for HK-35 (Somosné Nagy 2010); moving to fruiting conditions before 20 days are complete delays or prevents pinning
- No light required
Place sealed straw bags in the 77°F space and leave them undisturbed. Track the date of inoculation and do not move to fruiting conditions before the 20-day mark has passed — peer-reviewed data on HK-35 (Pleurotus ostreatus) specifically establishes this minimum colonization period for optimal first-flush yield. Watch the bag exterior for the color change from the natural golden-tan of wheat straw to full, uniform white coverage. Metabolic moisture droplets ("sweating") appearing on the interior bag walls are a normal, healthy sign of active HK-35 mycelial growth. If any portion of the bag develops bright blue-green or yellow-green discoloration, isolate that bag immediately and remove it from the grow space before the contamination releases airborne spores.
- Fully colonized straw blocks from Step 6
- Fruiting room holding exactly 61°F ± 1°F — a cool basement, unheated garage in spring or autumn, or temperature-controlled grow tent; HK-35 is specifically documented as a spring/autumn cultivar and is not suited to summer production without active cooling
- Hygrometer — target 70–85% RH (not higher; above 85% RH produces thin, fragile HK-35 caps per peer-reviewed data)
- Humidifier or spray bottle
- Indirect light: 300 Lux for 8 hours/day — the documented HK-35 fruiting initiation protocol (Somosné Nagy 2010); any window light or LED strip at this level qualifies
- Fresh air exchange source: fan, open window, or tent intake — target CO₂ below 1,000 ppm with 4–6 complete air exchanges per hour
- Scissors or knife
Move the fully colonized straw blocks into the fruiting space and allow the room to settle at 61°F. This temperature drop — from 77°F colonization down to 61°F — is the environmental trigger that initiates primordium formation in Oyster Mushroom HK-35 (Pleurotus ostreatus). The 61°F target is not a range; it is the peer-reviewed optimum for this specific strain, and running even 3°F lower to 58°F costs 30–50% of first-flush yield (Somosné Nagy 2010). Confirm your room thermometer is accurate by checking it against a second unit before committing to this step.
Cut a 4–6 inch X on the side of each bag, or fold back the top, to open a fruiting surface. Maintain 70–85% RH — using a humidifier targeted at the room air rather than misting directly onto the straw surface or developing pins. Water sitting on HK-35 (Pleurotus ostreatus) cap surfaces for more than 4 hours invites Pseudomonas bacterial blotch, which has been specifically documented as a commercial HK-35 problem. Run at least 4–6 fresh air exchanges per hour to keep CO₂ below 1,000 ppm; elevated CO₂ produces elongated stems with small, underdeveloped caps.
- Pinning straw blocks from Step 7
- 61°F fruiting space maintained throughout development
- 70–85% RH
- 300 Lux indirect light, 8 hours/day
- 4–6 fresh air exchanges per hour
- Cool, non-chlorinated water for block rehydration between flushes
Once pins appear, maintain all conditions without interruption. Oyster Mushroom HK-35 (Pleurotus ostreatus) caps develop from pins to harvestable size in 3–10 days depending on temperature — faster at the warmer end of the 61°F range. Harvest when the cap edges are still slightly inrolled downward and the cap has not yet flattened fully to horizontal. Cap diameter at harvest is typically 2–5 inches. Do not wait for caps to flatten: HK-35 caps that go fully convex release a heavy white spore cloud that covers the grow space, shortens shelf life, and deposits spores onto remaining substrate — increasing contamination pressure on subsequent flushes.
To harvest, grip the entire cluster at the base of the stems where they contact the straw and pull firmly with a slight twist. Pulling is preferable to cutting — cutting leaves stipe tissue embedded in the substrate that serves as a mold entry point. Harvest the entire cluster at once when at least half the caps are at prime maturity; individual caps within a cluster mature at slightly different rates, but leaving a partial cluster causes the remaining mushrooms to stall and the disturbed tissue to attract contamination.
After harvest, remove all residual stipe tissue from the fruiting surface. Allow the block to rest 7–14 days at fruiting conditions. If the block feels noticeably lighter than it did after inoculation, submerge it in cool, non-chlorinated water for 4–12 hours — do not exceed 12 hours, as bacterial contamination risk increases markedly beyond that point. Drain completely before returning to fruiting conditions. Proper rehydration recovers 40–60% of the first-flush yield in the second flush. HK-35 (Pleurotus ostreatus) typically produces 3–4 productive flushes on wheat straw before the block is spent. A spent block turns yellow-brown throughout rather than staying creamy white, fails to pin after 14 or more days at correct fruiting conditions, or shows green mold colonizing the interior of the mushroom substrate rather than just the outer surface.
Wheat straw is the substrate on which all peer-reviewed Oyster Mushroom HK-35 (Pleurotus ostreatus) yield data was generated, and it remains the most economical and widely available option. Growers who prioritize cap firmness, cluster aesthetics, and consistent flush timing may prefer the sterilized supplemented hardwood sawdust method below — it requires pressure sterilization equipment not needed for straw, but it produces denser, more photogenic clusters with firmer caps. The same strict HK-35-specific fruiting parameters — 61°F, 70–85% RH, 300 Lux, 20-day minimum colonization — apply to both methods.
Oyster Mushroom HK-35 Equipment — Sterilized Supplemented Hardwood Sawdust
| Item | Spec / Notes |
|---|---|
| Hardwood sawdust pellets | Oak, maple, or other true hardwood — do NOT use pine, cedar, or fir; softwood resins inhibit or prevent HK-35 colonization entirely |
| Wheat bran | 15–20% of total substrate weight; this supplement requires full sterilization — do not pasteurize supplemented sawdust |
| Gypsum (optional) | 1–2% of substrate weight |
| Kitchen scale | Required for accurate substrate ratio |
| Pressure cooker | Must hold 15 psi; standard 23-quart stovetop unit works for 1–2 bags; sterilize for 2.5–3 hours — longer than grain requires |
| Polypropylene autoclave mushroom grow bags | Must be rated for pressure sterilization; with 0.2-micron filter patch and self-healing injection port |
| HK-35 liquid culture syringe | Out-Grow HK-35 liquid culture — 5–10 cc per block |
| 18-gauge needle, isopropyl alcohol | Same inoculation setup as Method 1 |
| Room thermometer, hygrometer, humidifier | Same HK-35-specific fruiting parameters apply: 61°F, 70–85% RH, 300 Lux 8 hrs/day |
Oyster Mushroom HK-35: Sterilized Supplemented Hardwood Sawdust Method
- 4 lbs dry hardwood sawdust pellets (oak or maple) — rehydrate by adding water gradually until pellets break into fine sawdust
- 12–16 oz wheat bran (15–20% of mushroom substrate weight by dry weight)
- 1–2 tbsp gypsum (optional)
- Water to reach 60–65% substrate moisture
- Polypropylene autoclave mushroom grow bags with filter patch
- Pressure cooker at 15 psi
Add water to hardwood sawdust pellets gradually, mixing until all pellets break into a uniform fine texture. Add wheat bran and gypsum (if using) and mix thoroughly. Check moisture using the squeeze test: grip a firm handful and squeeze — only a few drops should emerge; a steady drip means the mushroom substrate is too wet. Pack into polypropylene autoclave bags to about two-thirds full and seal. Pressure cook at 15 psi for 2.5–3 hours. The longer sterilization time compared to grain is required because wheat bran creates a high-nitrogen environment where Trichoderma and competitor organisms colonize faster than Oyster Mushroom HK-35 (Pleurotus ostreatus) mycelium if any viable competitor spores survive. Allow to cool completely — 6–12 hours for large bags — before inoculating.
Out-Grow's wood-based inoculate and wait mushroom substrate is a pre-sterilized alternative that skips this step entirely.
- Out-Grow HK-35 liquid culture syringe — 5–10 cc per block
- 18-gauge needle
- 70% isopropyl alcohol and wipe
- Cooled sterilized sawdust blocks from Step 1
Wipe the self-healing injection port with 70% isopropyl alcohol and wait 30 seconds. Shake the liquid culture syringe gently, attach a clean 18-gauge needle, and inject 5–10 cc into the sawdust block through the self-healing injection port at multiple angles to distribute the Oyster Mushroom HK-35 (Pleurotus ostreatus) liquid culture throughout the block. The self-healing port reseals after each insertion. Place inoculated blocks at 77°F.
- Inoculated sawdust blocks from Step 2
- 77°F colonization space — minimum 20 days before moving to fruiting conditions; the same 20-day minimum that applies to straw blocks applies here
- Then: 61°F fruiting space, 70–85% RH, 300 Lux light 8 hrs/day, 4–6 fresh air exchanges per hour
Colonize sawdust blocks at 77°F for at least 20 days, until the entire block surface is uniformly white with no dark or brown patches remaining. Once full colonization is confirmed and the 20-day minimum has passed, move blocks to the 61°F fruiting environment — the same strict 61°F ± 1°F target that applies to straw blocks. Cut a 4–6 inch X on the side of the bag or fold back the top. Maintain all HK-35-specific fruiting conditions: 70–85% RH (not higher), indirect light, and sufficient fresh air exchange to keep CO₂ below 1,000 ppm. Oyster Mushroom HK-35 (Pleurotus ostreatus) on sawdust tends to produce denser, more uniform clusters with firmer, thicker caps than straw — a result of the supplemented mushroom substrate and more controlled fruiting surface. Harvest, rest, and rehydrate the sawdust block using the same method described in Method 1, Step 8: pull entire clusters at the base, rest 7–14 days, submerge in cool water for 4–12 hours if the block is light, and return to fruiting conditions. Expect 3–4 productive flushes before the mushroom substrate is spent.
Oyster Mushroom HK-35 Troubleshooting — Common Problems
Contamination during colonization of Oyster Mushroom HK-35 (Pleurotus ostreatus) almost always has one root cause: substrate that was not adequately pasteurized or sterilized, or straw that was inoculated before it cooled below 85°F. Trichoderma — the competitor mold that appears first as white mycelium nearly indistinguishable from healthy HK-35 mycelium, then rapidly turns bright blue-green or yellow-green — shares HK-35's colonization temperature optimum of 77°F exactly. The conditions that produce the fastest HK-35 spawn run also produce the fastest Trichoderma growth. The only reliable prevention is accurate pasteurization (confirm the inside of the packed straw bag reaches and holds 160–165°F, not just the surrounding water) and complete cooling before inoculation. If contamination appears before day 10 of colonization, inadequate pasteurization or hot inoculation is almost certainly the cause. If it appears between days 10 and 26 in a bag that looked clean at the start, Trichoderma likely entered through the filter patch or a micropuncture in the bag — isolate and remove the bag immediately before spores spread to neighboring blocks.
Failure to pin, or significant yield loss on the first flush of Oyster Mushroom HK-35 (Pleurotus ostreatus), is nearly always a temperature or timing error rather than a humidity problem. HK-35 is unique among commercial Pleurotus ostreatus strains in how severely it responds to fruiting temperature errors: a drop of just 3°F below the 61°F optimum to 58°F costs 30–50% of first-flush yield — a penalty quantified specifically for this strain in peer-reviewed trials (Somosné Nagy 2010) and not documented at this magnitude for any other P. ostreatus cultivar. If fruiting temperature is correct and blocks still fail to pin, check colonization time — moving to fruiting conditions before the 20-day minimum means the mycelium is not physiologically prepared to fruit and will take significantly longer or abort entirely. Elevated CO₂ above 1,000 ppm is the third cause: if pins form but abort quickly, or caps develop as elongated columns with tiny underdeveloped surfaces, inadequate fresh air exchange is the likely culprit.
Bacterial blotch — tan to dark brown sunken spots on Oyster Mushroom HK-35 (Pleurotus ostreatus) cap surfaces — is a documented commercial HK-35 problem caused by multiple Pseudomonas species that were specifically isolated from diseased HK-35 fruiting bodies in the Somosné Nagy study. Blotch requires standing water on cap surfaces for 4 or more hours to establish. The counterintuitive correction is to reduce humidity below what most growers instinctively run: peer-reviewed data on HK-35 confirms that 70% RH during fruiting produces thicker, denser, better-storing caps than 80–90% RH — and also dramatically reduces blotch incidence. If bacterial blotch has occurred, drop fruiting humidity to 70–75% RH, stop misting directly onto the fruiting surface, and confirm that air exchange is delivering at least 4–6 complete room volumes per hour. Harvesting at the inrolled-cap stage rather than waiting for caps to flatten also reduces water contact time and spore accumulation on cap surfaces.
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Shop mushroom substrate at Out-Grow — Pleurotus ostreatusHow to Grow Pleurotus ostreatus HK-35
Questions and Answers About Pleurotus ostreatus HK-35 Cultivation
Q. What makes Oyster Mushroom HK-35 (Pleurotus ostreatus) different from other oyster mushroom strains?
A. Oyster Mushroom HK-35 (Pleurotus ostreatus) holds the fastest documented mycelial growth rate and highest yield irrespective of substrate type of any tested Pleurotus ostreatus strain (Sobieralski 2011). It is also the single most widely used control strain in peer-reviewed oyster mushroom cultivation research globally. The trade-off is a uniquely narrow fruiting temperature window: HK-35 specifically loses 30–50% of first-flush yield if fruited even 3°F below its 61°F optimum — a sensitivity not quantified at this magnitude for any other documented P. ostreatus cultivar. It is best suited to spring and autumn production when ambient temperatures naturally fall in the correct range.
Q. Why does Oyster Mushroom HK-35 (Pleurotus ostreatus) need exactly 61°F to fruit?
A. The 61°F figure is the peer-reviewed primordium formation and fruiting optimum established specifically for HK-35 by Somosné Nagy (2010, Corvinus University of Budapest) in controlled variety-specific trials. At 64°F and above, fruit bodies develop quickly but with reduced cap quality. At 58°F, yield drops 30–50%. At 55°F and below, yield loss is severe. This temperature sensitivity is what defines HK-35 as a cool-season, spring/autumn strain. Warm-weather or summer Oyster Mushroom (Pleurotus ostreatus) cultivation without active cooling should use a different strain with a higher fruiting optimum.
Q. Why does higher humidity produce worse caps with Oyster Mushroom HK-35 (Pleurotus ostreatus)?
A. This is the most counterintuitive HK-35-specific finding in the peer-reviewed literature. Somosné Nagy (2010) documented that 70% RH during fruiting produces thicker, denser, better-storing Oyster Mushroom HK-35 (Pleurotus ostreatus) caps than 80–90% RH — even though 80–90% RH is the commercial standard that maximizes flush volume. At higher humidity, HK-35 caps grow faster but thinner, with poorer texture and shorter shelf life. This result does not necessarily apply to other oyster mushroom strains — it is an HK-35-specific finding. For HK-35 mushroom cultivation, running humidity at 70–75% RH produces caps that are commercially superior in quality even if total flush weight is slightly lower.
Q. What is the best mushroom substrate for Oyster Mushroom HK-35 (Pleurotus ostreatus)?
A. Wheat straw is the best-performing mushroom substrate for Oyster Mushroom HK-35 (Pleurotus ostreatus) mycelial growth rate and yield across all tested materials (Sobieralski 2011). A 4:1 wheat straw/hemp shive blend produced higher first-flush yields than wheat straw alone in the same trials. Oak and birch sawdust — without supplementation — produced significantly slower HK-35 mycelial growth because HK-35's enzymatic output is insufficient to efficiently break down dense hardwood lignin without added nutrients. If using hardwood sawdust, supplement with 15–20% wheat bran and pressure sterilize at 15 psi for 2.5–3 hours — pasteurization is not adequate for supplemented mushroom substrate.
Q. Why does Oyster Mushroom HK-35 (Pleurotus ostreatus) need a 20-day minimum colonization before fruiting?
A. The 20-day minimum colonization time is the peer-reviewed parameter established specifically for HK-35 by Somosné Nagy (2010). Moving Oyster Mushroom HK-35 (Pleurotus ostreatus) straw blocks to fruiting conditions before 20 days at 77°F are complete means the mycelium has not fully colonized and physiologically prepared the mushroom substrate for reproduction. The result is significantly delayed pinning, reduced first-flush yield, or complete failure to produce primordia. The full HK-35-specific colonization window is 20–26 days — blocks at the longer end of this range that show full white coverage are equally ready to fruit as blocks at the shorter end.
Q. What causes the brown spots on my Oyster Mushroom HK-35 (Pleurotus ostreatus) caps?
A. Brown, sunken spots on Oyster Mushroom HK-35 (Pleurotus ostreatus) cap surfaces are bacterial blotch, caused by Pseudomonas species — including P. tolaasii, P. fluorescens, and P. putida — that were specifically isolated from diseased HK-35 fruiting bodies in commercial production environments. Blotch develops where caps remain wet for 4 or more hours after misting, particularly at cap-to-cap contact points. Prevention: hold fruiting humidity at 70–75% RH rather than 90%+, eliminate direct misting onto developing clusters, and ensure 4–6 fresh air exchanges per hour so cap surfaces dry within 1–2 hours of any moisture exposure. Once blotch appears on a cluster, harvest immediately — affected fruiting bodies will not recover.