Managing the Dark Days of Spring

Preventing last minute disease issues in spring bedding crops

It is often said that “April showers bring May flowers,” but for ornamental greenhouse operations, the arrival of dark, wet weather for an extended period can mean trouble from common greenhouse pathogens like Botrytis, downy mildew and powdery mildew. Greenhouses are filled to capacity, and plants are maturing to a saleable size. Space is at a premium this time of the season and most crops have formed a canopy that creates an ideal breeding ground for these destructive diseases. The common denominator between these diseases is that high relative humidity for a period of several hours allows for spread and development of the pathogen.

There are two important measures for moisture in the greenhouse:
Relative humidity (RH) is the ratio between the weight of moisture in the air and the total moisture-holding capacity of that air at a specific temperature and pressure. It is relative because warm air has a higher moisture-holding capacity than cooler air. As the temperature of air increases, the relative humidity decreases even though the amount of water remains constant. Air at 70°F will hold twice as much moisture as air at 50°F.

Dewpoint temperature indicates the temperature at which water will begin to condense out of moist air. Condensation or “free moisture” forms on surfaces, including leaf surfaces that are at or below the dewpoint temperature. The higher the RH at the same temperature, the closer the dewpoint moves to the ambient greenhouse temperature. When the greenhouse is 60°F and 85% RH, condensation occurs when leaf temperature is lower than 55°F. At 95% RH at the same temperature, condensation occurs when leaf temperature is only one degree lower than ambient temperature, or 59°F.

Scouting, knowing, and recognizing these diseases are the keys to success:
Botrytis thrives in the free moisture that condensation leaves on the foliage, RH of 85% and especially when those conditions persist for four hours or more. All plant parts above ground can be infected. Look for gray, fuzzy spores on buds, flowers, leaves and stems (Fig. 1).



Downy mildew is another pathogen that spreads and develops when there is free moisture on the leaves and RH of 85% for four hours or more. Fuzzy sporulation occurs on the underside of the leaves, but a yellowing, chlorotic patterning can be seen on the upper surface (Fig. 2) Often this will appear to be a nutritional issue. As infection becomes more severe, it moves into the stems of the plants. Downy mildew does not move between plant families, as the species are host-specific, but there are species that infect a wide range of greenhouse ornamental and edible crops. Coleus and impatiens are well known targets for this pathogen.



Powdery mildew requires warm days of 68°-80°F, cool nights, high relative humidity and it forms on dry foliage. This is another host-specific pathogen, but infects many ornamental and edible crops, especially dahlia, gerbera, phlox, zinnia and popular species used for hanging basket production. Powdery mildew is easily recognized by the white, powdery dust on upper or lower leaf surfaces, stems and petioles (Fig. 3).



Reduce humidity levels and increase air movement to protect foliage:
As difficult as it sounds, we must keep the greenhouse as dry as possible going into the night, as temperatures drop. Water early in the morning or afternoon to avoid puddling water on the greenhouse floor, wet foliage and overly wet growing media surfaces.

Use a combination of ventilation and heating to reduce humidity. Closing the greenhouse and heating the air increases the capacity of the air to carry moisture, thus avoiding condensation. Ventilation allows the exchange of moist greenhouse air with cooler, drier air from outdoors. Complete air exchange through the heating and venting cycle two or three times when closing in the evening, and early in the morning at sunrise, even if it is cool or raining. Remember, air at 50°F and 100% RH (raining) contains only half as much moisture as the greenhouse air at 70°F and 95% RH. Each full exchange can reduce the RH by half or more.

As mentioned previously, once the canopy has formed, it is difficult to get sufficient air movement to the plants. Adequate air movement around the plant occurs when the leaves move slightly. Moisture does not get a chance to condense on the leaf surface, resulting in less Botrytis.

Use fungicides preventatively to prevent damage:
Scouting and good cultural practice are important, but only part of a program to control Botrytis, downy mildew, and powdery mildew. Preventative sprays of effective chemistry must be applied proactively when conditions favor these pathogens, but prior to infection.

Products that have shown moderate to good efficacy for diseases marked with an “X” as preventative or curative spray are included in the table below. All recommendations are for foliar sprays. Most foliar sprays listed will perform better with a surfactant such as CapSil added at ¼ to ½ tsp per gal (4 to 8 oz per 100 gal) to increase coverage and reduce visible residue. It is important to read pesticide labels to ensure plant sensitivity concerns as well as compatibility with surfactants.

FungicideMOABotrytisDowny MildewPowdery MildrewItem Number
Affirm WDG19XX71-1131
Astun7X71-1190
Avelyo3X71-1195
BotryStop WPNCX71-12601
Broadform7 & 11XX71-12901
CeaseBM02XXX71-13301
CompanionBM02XX71-1372A2
EcoSwingBM02XX71-1455
Fenstop11X71-14800
FosphiteP07X71-1522
GuardaBM01XX71-1610
Heritage WG11X71-1408
Medallion WDG12X71-16502
MilStop SPNCX71-2878
Mural7 & 11XXX71-1690
Micora40X71-1655
Orkestra Intrinsic7 & 11XXX71-2205
Pageant Intrinsic7 & 11XXX71-26801
Palladium9 & 12XX71-2685
PerCarbNCXXX71-8687
PhostrolP07X71-2725
Postiva3 & 7XX71-2400
Protect DFM03X71-2748
Regalia CGP05XXX70-2841
Segovis49X71-3100
Segway O21X71-31102
Seido50X71-1320
Sil-Matrix LCUNXX71-2939
Spirato GHN12X71-2948
Stature SC40X71-14652
StargusBM02XXX71-2950
Triathlon BABM02XXX71-3040
ZeroTol 2.0NCXXX71-35501
Note: Not all products are registered in all states. Some pesticides are restricted use in some states or regions and not others. It is the responsibility of the applicator to read and follow all label directions. Other products may be safe and effective. Consult the 6th Edition GGSPro Reference Guide for a comprehensive list including rates and use patterns.