Why Botrytis Prevention Programs Fail, and How to Run One That Works

Ask a room of greenhouse growers which disease costs them the most sleep, and gray mold will win most years. Botrytis does not announce itself with a dramatic outbreak. Instead, it starts as a few water-soaked spots on a senescing leaf, a blighted flower nobody noticed on Friday, and by the following week a bench of finished plants is unsalable. The losses are rarely one catastrophic event. They shrink, week after week: culled pots, downgraded baskets, labor spent picking out infected plants before a shipment, and the occasional crop that has to be dumped entirely days before it was due to ship. Most of these losses happen in houses that already have a prevention program on paper. The sprays go out, the fans run, the checklist gets signed. However, the problem is that the program is usually built around the calendar and around habit, while the pathogen runs on conditions. When the two stop matching, Botrytis wins quietly.

The Infection Conditions Growers Underestimate

Botrytis is an opportunist with precise requirements, and it is worth stating them plainly because most prevention failures trace back to one of them. The fungus sporulates heavily when relative humidity stays above roughly 93%, and its conidia need free moisture, or near-saturated air, on the plant surface to germinate and penetrate. Given leaf wetness of 8 to 12 hours at temperatures between 59 and 73 °C, infection is the expected outcome. Cooler nights slow it somewhat; warmer days above775 °C slow it more, but the moderate range is exactly where most spring and fall crops are grown.

Three features of this biology are consistently underestimated.

The first is leaf wetness duration. Growers manage air temperature carefully and humidity casually, yet the variable that decides infection is how long the leaf stays wet. A canopy that dries by 9:00 a.m. and one that stays wet until 12:00 p.m. live in different disease worlds, even in the same house at the same setpoints.

The second is the dusk humidity spike. As the sun drops and air temperature falls faster than the heating system responds, relative humidity climbs sharply and can sit above 95% for the first hours of the night, which is precisely when plants are still wet from the day’s last irrigation.

The third is condensation. On clear cold nights, leaf temperature can fall below the dew point of the surrounding air even when a sensor at head height reads a safe humidity level. The crop gets wet from the air itself, no irrigation required, and the grower checking a climate computer in the morning sees nothing wrong in the averages.

Three Program Failures That Keep Showing Up

The first failure is calendar spraying: a fungicide every seven or 10 days regardless of what the weather, crop stage, or infection pressure actually looks like. It feels disciplined, but it wastes applications during dry, low-risk weeks when they do little, and worse, it can leave a gap exactly when a stretch of wet nights makes protection essential. It also drives resistance hard, because the same chemistry often gets repeated simply because it is what is on the shelf, exposing generation after generation of the fungus to one mode of action.

The second failure is poor air movement, specifically the dead zones nobody maps. Horizontal airflow fans get installed, curtains get managed, and everyone assumes the air is mixing. In practice, corners, the ends of benches under hanging baskets, and the interior of a dense canopy can be nearly still. Still air means the thin layer of humid air at the leaf surface never gets stripped away, so leaves in dead zones stay wet hours longer than leaves 2 meters away. This is why Botrytis so often appears in the same pockets of a house crop after crop.

The third failure is sanitation treated as a cleanup task rather than a disease control measure. Botrytis survives on crop debris, fallen petals on the floor and in the canopy, senescing leaves left on the plant, and algae-covered surfaces, and it sporulates on that material generously. A single blighted flower head left in a basket can produce inoculum for everything around it. Wounded tissue is the other quiet entry point: cuttings, pruning stubs, snapped stems from spacing and shipping, each one a ready infection court for a fungus that colonizes damaged tissue far more easily than healthy tissue.

A Condition-Based Prevention Program

A program that works starts with the environment, because environmental control is the only tool that acts on the infection conditions themselves. The core strategy for the night is to keep the canopy dry: heat and vent in the late afternoon to drive moist air out before dark rather than after humidity has already peaked, and be willing to spend a little heat to buy dry air, since cracking the vents while heating is far cheaper than a blighted crop. Heating pipes or unit heaters run briefly before sunset raise plant temperature above the dew point and prevent the condensation wetting that catches growers out. Horizontal airflow should be checked where the crop actually is, at canopy height and in the corners, with ribbon or a smoke test, not assumed from fan specifications. Spacing matters for the same reason: a bench packed a week too long is a humidity trap, and moving plants on schedule is disease control, not cosmetics. Irrigation timing is the simplest fix of all. Water early in the day, avoid overhead watering late in the afternoon entirely, and where the crop allows it, shift to subirrigation or drip so foliage stays dry from the start.

On top of that sits a sanitation protocol with the status of a spray schedule, not an afterthought. Remove senescing leaves and spent flowers as part of routine crop work, bag infected material at the bench rather than carrying it through the house, and clear floors and bench surfaces of debris between crops, every crop, not just after an outbreak. Control algae on floors and under benches, because it holds moisture and harbors fungus gnats and shore flies that move spores around. Keep pruning tools and hands in mind when moving from a suspect area to clean stock: work the clean houses first and the problem houses last.

Fungicides still have a role, but as protection timed to risk, not as ritual. Protectant chemistry belongs on the plant before high-risk periods, ahead of a forecast stretch of wet nights or a crop entering its susceptible flowering stage, not after lesions are showing, because most of these materials protect surfaces and do not cure established infections. Rotation across FRAC groups is non-negotiable with this pathogen, which has a long record of developing resistance to single-site fungicides: alternate modes of action between applications, use multi-site protectants as rotation partners and tank-mix anchors where labels permit, and never run the same single-site group back to back through a crop. Keep records by FRAC group rather than by product name, since two different brands can be the same chemistry wearing different labels.

Biologicals earn their place in the preventive slots of the program. Trichoderma-based products colonize the root zone and plant surfaces and compete directly with Botrytis for space and nutrients, and Bacillus-based biofungicides produce compounds that suppress germination and growth of the pathogen on treated foliage. Used regularly during low to moderate pressure, started early while plants are clean, they reduce the number of chemical applications a crop needs and give the rotation more breathing room. What they will not do is rescue an outbreak in a wet house. Growers who trial a biological during the worst week of the season and conclude it does not work have tested it under the one condition it was never designed for.

Scouting: Catching It in the First 48 Hours

Everything above works better when someone is looking. An effective scouting routine for Botrytis is short, scheduled, and focused on the places the disease starts. Twice a week at minimum, and more often during high-risk weather, walk the same route: the known dead zones first, the shaded ends of benches, the oldest crop in the house, and any bench where a case was found last cycle. Look at the lower canopy and the oldest leaves, check flower clusters in susceptible crops, and turn over a sample of leaves rather than scanning from the aisle. Gray-brown lesions with the characteristic dusty gray sporulation, blighted petals stuck to leaves below them, and stem lesions at pruning wounds are the early signs worth acting on. Flag finds with tape so the spot gets rechecked, and log them against the weather and irrigation of the previous days. That log is what turns scouting into forecasting: after a season, patterns emerge, certain crops, houses, and weather sequences will identify themselves, and the prevention program can be tightened exactly where it is actually failing.

None of this requires new technology or a bigger chemical budget. It requires treating Botrytis as a conditions problem first and a spray problem second, and being honest about where the wet hours and the inoculum are hiding. Growers who make that shift usually find the fungicide bill falls on its own, because the chemistry they do use is finally being asked to do a job it can actually do.